A COMET consists of two parts which fall under our observation, viz. a solid, spherical, opaque body, which, like the planets, shines by reflecting the light of the Sun, and no way differs in appearance from a planetary globe; this is by astronomers called the nucleus: Also a very extensive atmosphere; which, from the form it usually exhibits to us, is called the tail, and commonly exhibits a faint beam of light, which diverges as it recedes from the head, and often appa­rently extends thro' distant constellations, when it nearly resembles those momentary corruscations which shoot upwards from the horizon towards the zenith, during the appearance of an aurora borealis, or northern light. This tail increases in length as the Comet approaches the [Page 10]Sun, et vice versa; and its direction is always nearly in opposition to the Sun. Sometimes indeed the cometic atmosphere assumes a different form, sur­rounding the nucleus equally on every side like a thin cloud or mist, or, as some have fancied, like a bush of hair; whence the Comet has been denomi­nated crinite or hairy. The latter is the usual appear­ance of a Comet when first discovered in its descent toward the Sun, provided the Sun and Comet be in opposite hemispheres; and may take place in some other situations, agreeable to the rules of optics, even when to spectators in other parts of our System, the Comet may exhibit a tail of an enormous length.

IT has been sufficiently demonstrated by Sir Isaac New­ton, Dr. Halley, and others, that these bodies are, in com­mon with the other globes of the solar system, subject to the law of mutual gravitation, and that they regard the Sun as their common center of gravity, and con­sequently move round him in conic-sections, carrying their atmospheres or tails along with them. Their orbits differ widely from circles on the one hand; on the other, since the discovery of the Newtonian method of computing their trajectories from observations, they have never been found to deviate into hyperbolas; and though their observed places, in the small parts of their orbits in which they are visible to us, agree with their computed places, upon the supposition that they move in parabolas; yet, as the periodical revolutions of some of them have been ascertained by their regular returns after certain intervals, it is agreed by astronomers, that their paths are truly elliptical, though very excentric; [Page 11]such ellipses, near the extremities of their transverse axes, differing but insensibly from parabolas. All that is attempted in this Essay, is,

• 1. To account for the phaenomena of the tails of Comets, upon philosophical principles: And then,
• 2. To point out some ends to which they seem adapted, and for which they are probably designed.

SIR Isaac Newton has sufficiently proved that these tails consist of a fluid matter, extremely rare, emitted from their heads upon their approach to the Sun, which is rendered visible by reflecting his rays^*: But what cause there may be existing in nature, capable of projecting the cometic atmospheres through such im­mense spaces, is a question which still remains to be solved, no satisfactory account having hitherto been offered to the publick. In order therefore, to a ra­tional solution of this curious phaenomenon, the tail of a Comet, the following propositions, observations, &c. are submitted to the candid perusal of the reader: First premising, that when a subject, under consider­ation, is in its nature purely physical, it is to be pre­sumed that strict, mathematical demonstration will not be expected.

WE shall now endeavour to prove the following propositions.

FIRST, THAT the primary Planets, the Comets, and the Sun, are all surrounded with atmospheres.

[Page 12] SECONDLY, THAT these atmospheres consist of the same fluid with the atmosphere of the earth, viz. Air.

THIRDLY, THAT they are mutually repellent to each other; as the globes they surround are in a state of mutual attraction or gravitation.

1. AS the proof of the existence of the at­mospheres of the Sun, Planets and Comets depends upon a variety of astronomical observations, it is ne­cessary to be particular as to each of them.

AND, 1. That the Earth we inhabit (which is a Planet vastly inferior, both in bulk and situation, to some others) has one, we have the best evidence possi­ble, viz. the testimony of our senses; besides, we all know, that the air, which every where surrounds the Earth, is essential to the breath of life.

2. THAT Mars has an atmosphere, and that very extensive, has been demonstrated from the occultation of a fixed Star by that Planet^*; as the Star vanished at a distance from, or without ever arriving at a visible contact with his limb; as was observed by Cassini, October 1st, 1662. The like was observed after the same occultation, by M. Roemer, at Rome, the Star not being visible after the transit till at a distance from his limb.

3. JUPITER's belts are in a fluctuating state, as they frequently vary their form, size, and situation;^† which cannot be accounted for, unless we suppose them [Page 13]to be clouds and exhalations, floating in an atmosphere which surrounds his globe.^†

4. SOME belt-like appearances were discovered in Saturn by Mr. Hadley through his five-foot reflecter, and by Mr. Pound through Hugenius's glass, though they appeared very faint, as he is so remote^‡; which belts are probably in the same fluctuating state as those of Jupiter, and arise from similar causes.

5. THAT Venus has an atmosphere may also be inferred from the variable spots which have been ob­served upon the face of that Planet,^‖ which are pro­bably of the same kind with the belts of Jupiter, or the clouds which float in our own atmosphere, and, consequently, have a similar one to sustain them^†. But for proof of its existence we need go no further back than the late transit of Venus over the Sun's disc, anno 1769; when the atmosphere itself was visible, and that in different situations, to observers in distant parts.^*

6. MERCURY is too near the Sun to favour us with such observations; but if we suppose his globe inhabited, he doubtless stands as much in need of an atmosphere as any other Planet, and has as important purposes to be served thereby.

[Page 14] 7. THAT the Comets are surrounded with at­mospheres is already taken for granted, because self-evident. Sir I. Newton, from various observations, concludes that their diameters are, one with another, at least equal to ten diameters of their nuclei or solid globes.^†

8. THAT the Sun has an atmosphere, and that proportioned to his amazing magnitude, is rendered highly probable by the macular appearances, or spots frequently discovered upon his disc: Which, as they often suddenly break out, and as suddenly disappear, and sometimes vary their shapes, even when under the eye of the observer; can be no other than huge clouds of smoke, or other vapours floating in such atmosphere; huge indeed! as they frequently ex­ceed the whole superficies of the Earth. Mr. Derham, who was peculiarly assiduous in observing them, has assigned a cause, to which their phaenomena accurately agree. He supposes them to be immense volumes of smoak, belched forth by volcanoes or fiery erup­tions, which are frequently breaking out upon his surface^‡ These spots, when large, sometimes con­tinue during a whole revolution of the Sun round his axis, or about twenty-five days; and it is from the regular returns of such spots to the same part of his disc again that this revolution has been determined. But such clouds, were they not supported by an at­mosphere, would tumble down again upon the Sun presently after the explosions which raised them were [Page 15]over, as we see vapors emitted in an exhausted receiver, for want of air to sustain them, sink down to the bot­tom of the receiver.

II. WE shall now endeavour to prove that the planetary, cometary, and solar atmospheres, consist of the same elementary fluid with the atmosphere of the Earth, viz. AIR.

AIR is a fluid, which when pure is transparent, and in the highest degree elastic, being indefinitely com­pressible and dilatable; which qualities are truly cha­racteristic of it, as we know of no other fluid to which they belong.

IT may be necessary to observe here, that upon our globe, which (as already observed) is itself a Planet, the presence of air is necessary, both to the preservation of animal life, and to the existance of flame: For in a glass receiver, small animals die, and candles go out, immediately upon the air being exhausted; upon air also seems to depend the explosive power of enkindled vapors; for gun-powder itself, one of the most powerful agents hitherto invented by the art of man, if fired by an hot Iron in vacuo, will consume away, but never flash, nor explode; and it is generally agreed by those who are acquainted with experimental philosophy, that this vi­vifying, inflammating quality of air, depends upon its elasticity, or the active, contrifugal power of its par­ticles, to be considered under the next general head. This being premised, we proceed to prove, First, THAT the atmospheres of the several globes [Page 16]of the System consist of transparent fluids: The truth of which will be sufficiently evident, if we consider the several phaenomena by which they were discovered.

IT has already been shewn that Mars is surrounded with a very extensive atmosphere; which, had it not been transparent, must have escaped the notice of as­tronomers during the time of the occultation of the star before mentioned; for were it opaque, it would, by reflecting the Sun's rays, as well as by its want of transparency, have hid the Planet itself, and by the ob­servers have been confounded with it^†; in which case the moments of the occultation of the star, and of its visible contact with the limb of the Planet, would have been the same; whereas the disappearance of the star before any such contact could take place, is a demon­stration of the refraction of its rays in a transparent medium, with which the Planet is surrounded.

THE changes which are observable in those fluctu­ating collections of heterogeneous matter, of which the belts and spots in the other Planets, and the Sun consist, for the same reason, could never have been discovered, were they not sustained in transparent fluids above the surfaces of their respective globes.

THE transparency of the cometic atmospheres in un­deniable, as their nuclei are frequently seen thro' them, although they occupy spaces equal to many diameters of the Earth; and the smallest stars are visible through [Page 17]the tails which proceed from, and are only expansions of them; in which tails we have ocular demonstration of a most surprizing dilatability; from whence their recompressibility and elasticity may be justly inferred.

Secondly, ALTHOUGH there are no observations in the records of astronomy which prove this elasticity in the planetary atmospheres; yet, as the Planets are with the highest reason supposed to be inhabited Worlds, the presumption is at least very strong, if short of a demonstration, that their atmospheres are designed to answer purposes every way similar to those which are ef­fected by the atmosphere of the Earth, and that they are endued with all those properties, which are, with us, ne­cessary to the preservation of animal Life, consequently that they are elastic, as well as transparent, and alto­gether like our air.

THE atmosphere of the Sun is, by his excessive lustre, hid from our view; but if we consider him as an im­mense flaming globe, kindled up to warm and en­lighten the whole system, we may well suppose that he has a large share of that fluid, without which (as before observed) scarcely any flame can subsist with us for a moment. Some Authors indeed are of opinion, and not without reason, that the Sun is not a body of fire, as commonly supposed; Doctor Knight in particular, in his curious treatise of at­traction and repulsion, as two universal principles, by which he endeavours to solve all the phaenomena in nature, seems to be of opinion, that the inhabitants of the Sun (if inhabited) are in as much danger of [Page 18]suffering from cold, as from excessive heat^*. But of whatever substance the body of the Sun may consist, Mr. Derham has, by his observations, put it beyond all reasonable doubt, that the maculae and faculae, or the darker and brighter spots observable at times upon the Sun's disc, are really [...]ing to the bursting of vol­canoes; ‘the faculae being only the appearance of the flames, after the dense smoke attending the ex­plosions is dissipated, or removed to a distance from them^†.’ Now there is no reason to doubt but that, upon our globe, air is as necessary to the flames of Aetna and the eruptions of Vesuvius, as to the smaller blaze of a candle, or to the explosion of gun­powder; may we not then rationally conclude, that it is equally necessary to those astonishing volcanoes in the Sun?

IT is now proved, as far as the proposition is in the nature of it capable of proof, that the celestial bodies are surrounded with atmospheres, and that these at­mospheres consist of transparent elastic fluids, like the air surrounding this Earth. It remains that we prove.

[Page 19] III. THAT the atmospheres of the Sun, Planets, and Comets are mutually repellent to each other; as the solid globes they surround are in a state of mutual attraction or gravitation.

IT must here be observed; that in reasoning upon gravitation, the great Author of the present phi­losophical system of the Heavens argues downward from the greater to the lesser, from Worlds to Atoms; thus, finding by mathematical deductions, compared with astronomical observations, that all the globes of the solar System, Sun, Planets and Comets gravitate, or have a mutual tendency towards each other, and that this reciprocal attraction is proportional to the quantities of solid matter they respectively contain; he very justly concludes, that every single particle in any one of their solid masses both attracts, and is attracted by, every other particle of matter, contained in every globe throughout the solar System.^*

BUT as the Heavens exhibit no phaenomena from which we can, directly and with equal certainty, infer the existance and universal extent of the contrary principle of repulsion, as subsisting between the at­mospheres of the heavenly bodies; we are here obliged to use a contrary method, and to reason up­ward from the powers and properties which, by their effects, we discover to belong to those parcels of air upon which we can make experiments; to the effects, which the same powers and properties would naturally produce, in those vast collections of air, which con­stitute [Page 20]the atmospheres of the several globes; and if by tracing the necessary operations of these powers, step by step, we can at length arrive at any of the grand phaenomena of nature, we may with the highest reason conclude, that these phaenomena are the effects of those powers.

WE shall therefore endeavour to prove from authors of the best credit, or from experiments which any one may try at his leisure,

1. THAT there is a mutual repellency subsisting between the particles of air, whereby they continually endeavour to recede from each other; in consequence of which, that fluid is indefinitely dilatable from the centrifugal activity of its particles, as well as com­pressible by any foreign power.

THE honourable Robert Boyle, Esq found by expe­riments, that air might be so raresied as to occupy 13769 times the space it fills when in its natural state near the surface of the Earth^‡; other experi­ments prove, that it may be so condensed, as to be contained in [...] part of the same space^* therefore multiplying 13769 by 60, it appears, that the cubic space it is capable of filling, under different circum­stances, may be as 826140 to 1. But the cube root of 826140 is 94 nearly; therefore the central distances of the particles from each other, as discoverable by actual experiment, may be as one to ninety-four, and that from their centritugal activity.

[Page 21] MOREOVER, Sir I. Newton from experiments con­cludes this repulsive power to be so great, as that a cubic inch of air, condensed as it is with us, if removed one semidiameter of the Earth above its surface, where it would be free from the pressure of an incumbent at­mosphere, would so expand itself by virtue of this power, as to fill the whole sphere of Saturn's orbit; nay he adds, "and far beyond it"^†

2. THIS mutual repellency of the particles of air is greatly increased by heat. For if a bladder not more than half filled with air, be tied up tight, and laid before the fire, the additional expansive power, which the air contained therein acquires from the heat, will swell the bladder to its utmost extent, and will at length burst it with an explosion.

3. THE particles of air, although mutually repel­lent amongst themselves, are with regard to other mat­ter, in the common state of gravitation or attraction. This is evident from their being condensed in the form of atmospheres round the solid globes of the System, and attending them through all their revolutions.

4. THE mutual repellency of the particles of air is indefinitely greater, in proportion to the quantities of repelling matter, than the mutual attraction, subsist­ing betwixt the solid particles of attracting matter.

THE truth of this proposition will appear, at least highly probable, if we consider that the quantity of [Page 22]matter contained in our air near the surface of the Earth, and which gravitates in common with other matter towards its center, is so small, tho' condensed by the incumbent weight of the whole atmosphere, that one quart of it weighs no more than eight grains, as appears from experiments; yet so great is the re­pulsive power of its particles, that a small quantity of it, condensed by art in the barrel of an air-gun, of the size and bore of a common fowling-piece, is sufficient, when permitted suddenly to expand itself through the tube, to discharge a musket ball, in the same manner as fired gun-powder, the noise excepted, and with the like fatal effects.

WE shall now proceed to prove the following pro­position, viz. If two corpuscles of matter be in a state of mutual repulsion at any given distance, as A. B, Fig. 1. where they repel each other with a given force, say=1. and this repulsion decreases, either as the distances Ac, Ad, Ae, Af, &c. or as the squares, cubes, or any higher powers of those distances, increase, the extent of that repulsion is indefinite.

FOR suppose A and B to be two such particles re­pelling each other at the distance A B with a force=1. continue the line A B indefinitely through the equidistant points c, d, e, &c. let the particle A be fixed, B movable: Let us suppose this power to de­crease,

• 1st, simply, as the distances increase; then, if we suppose the particle B to move successively through c, d, e, &c. this power, at those several distances, will be [Page 23]as follows, viz. at c=½, at d=⅓, at e=¼, &c. in infinit.
• 2. If it decrease as the squares increase, it is at c=¼, at d=1/9, at e=1/16, &c. in inf.
• 3. If as the cubes, it is at — c=⅛, at d=1/27, at e=1/64, &c, in inf.
• 4. If as the biqua­drates, it is at c=1/16, at d=1/81, at c=1/256;, &c. in inf.

&c. onwards indefinitely.

NOW whatever the distance assigned between the two particles may be, and whatever may be the index of the power which expresses the ratio of the decrease of their repulsive force, this force will in every case be expressible by a fraction whose numerator is 1, and its denominator equal to the given distance, in­volved according to the index of the given power, as is evident from inspection of the foregoing fractions as they stand. It can never, therefore, become equal to nothing, until the denominator of the fraction be­comes infinite, which never can be at any assignable distance, however great; therefore, the extent of this mutual repulsion is indefinite. Q.E.D.

IN mathematical reasoning, the smallest quantities are not to be disregarded, unless supposed smaller than any assignable, as in fluxions, &c.; for, as all the huge masses of the solar System, are composed of par­ticles of matter inconceivably small, so their me­chanical effects upon each other are proportional to the numbers of such component particles they re­spectively contain; and though the mutual effects of [Page 24]two single particles, at any given distance, might be indefinitely small, yet we may easily conceive, that, when indefinite numbers of such particles are consoli­dated into one mass, and exert their influence from a common center, that that influence may be as exten­sive as the solar System, and perhaps as the material universe. Thus the Sun, by the united attractive force of his constituent particles, regulates the motions of the Comets, even at their aphelia, where their distances from him confound the human imagination; amounting in some of the to many thousands of millions of miles; yet, at those amazing distances from the Sun, they are by his influence retained in their proper orbits, without any deviation, till at length they are brought back to him again, many of them after excursions of some hundreds, and possibly some of them after thousands of years; in effecting which every particle of matter in the Sun, however small, bears a part.

ON the other hand, the atmospheres of the heavenly bodies consist of particles mutually repellent; not consolidated indeed, as that would be incompatible with the nature of the fluid composed of them, but each, in consequence of the mutual attraction between its constituent particles and the globe it surrounds, condensed into a fluid mass.

IN this case also, though the mutual repellency between two single particles might at a given distance be indefinitely small, yet the influence of two such masses upon each other, when thus condensed, may [Page 25]be as extensive as in the former case of attraction. The united repulsive force of the particles in each, would in like manner exert itself as from their com­mon center.

LET us, for illustration of the subject, suppose this repellent power between the particles A and B, Fig. 1, to decrease as the squares of the distances increase, then at a distance equal to 100000000 A B, or, in other words, suppose the particles A and B removed one hundred millions of times further asunder than represented in the figure, the repulsive force would then be equal to 1/10000000000000000, or to one ten-thousand-million-millionth part of what it is at the simple distance A B; which, though small indeed, is yet something, for the same distance remaining, viz. 100000000 A B, if ten thousands of millions of millions of such particles of equal bulk and repellency were condensed round the point A as a center, then the repulsive force between the mass at A and the particle B would be equal to that between the two single par­ticles A and B, at the distance A B; once more, suppose the same distance, viz. 100000000 A B to remain, and the particle B to have the same number of repellent particles condensed round it, as we have already supposed, to be condensed round A; then the repulsive power subsisting between the two masses would be ten thousands of millions of millions of times greater than between the two particles A and B, at the distance A B.

WE have seen that air consists of particles thus re­pellent; nor can we discover any bounds to that re­pellency [Page 26]by any experiments we can make; and if to other experiments we add Sir Isaac Newton's reasoning from some of his own, we must conclude them inde­finitely so, and consequently that their repulsive force decreases regularly according to some certain ratio of the increase of their distances, viz. either as the distances simply, or as their squares, cubes, or some other powers thereof.

IT seems most agreeable to mathematical reasoning, to suppose, that all powers, whether attractive or repel­lent, which act in right lines, to, or from the centers of the attracting or repelling bodies, are proportioned to the inverse ratio of the squares of the distances of their centers. For, if we consider these powers as represented by lines, or rays, converging from every point of the visi­ble concave of the Heavens to a given point as a center in case of attraction, and diverging equally from said center in case of repulsion; and suppose a corpuscle of matter to be placed in any part of that space, the num­ber of such rays, intercepted by, and falling upon said corpuscle, whether they were converging or diverging, would be in that ratio, viz. inversely as the squares of the distances of said corpuscle from such central point. The truth of this assertion is capable of the most sim­ple mathematical demonstration; which may be here omitted, as it has been already sufficiently demonstrated by others: Upon it indeed depends the truth of Sir I. Newton's position, viz. that ‘the density of the Sun's rays is reciprocally as the squares of the distances from the Sun’; from whence he deduces the propor­tions of light and heat, enjoyed by the several Planets of the System.

[Page 27] IF we conceive such supposed rays to move with a uniform velocity, and to be attended with any mo­memtum whatever, whereby, when converging, they impel the corpuscle to, or repel it, when diverging, from said central point; as the whole momentum would be proportional to the number of the intercepted rays, the tendency of the corpuscle to, or from the center, that is, its attraction or repulsion would also be in the aforesaid ratio. That the attraction of gravitation is regulated by this law, astronomical observations suffi­ciently demonstrate.

BUT Sir I. Newton concludes from experiments, to which all theoretic opinions must ever give way, that the mutual repellency between the particles of air is reciprocally as their distances only, or "nearly so."^*

IF this be the case, the physical effects of two par­ticles, and consequently of two fluid masses composed of such particles, upon each other, must be yet vastly greater than if it were reciprocally as the squares of the distances, as we before supposed, as will evidently appear upon inspection of the following scheme; in which the first series represents the distances, increasing in arithmetical progression; the second, the repellent force, decreasing as the distances increase only; the third, the same force considered as decreasing accord­ing to the increase of the squares of the distances; thus:

[Page 28] HERE it is evident that at the distance 3 A B, if the repellency decreased only as the distances increase, that that force would be equal to ⅓ but if it decreased as the squares of the distances increase, it would at the same distance be equal only to 1/9; at the distance 5 A B, in the former case, it would be ⅙, in the latter 1/25 if at the distance 8 A B, in the former it would be 1/8, in the latter 1/64, &c. But 3 is the square-root of 9 or 1/3 of 1/9, 1/5 of 1/25, and 1/8 of 1/64, &c.

IT therefore follows, that if this repellent force does actually decrease simply as the distances increase, the quantity of it at any given distance is greater, in the direct ratio of that distance, than if it decreased in the ratio of the squares of the distances; consequently, the sensible effects of two such fluid masses, condensed, as we have already supposed, round the particles A and B, would be proportionably more extensive.

NOW, what are the atmospheres of the Sun, Planets and Comets, but vast collections of these repellent particles of air, condensed round their respective globes, by the mutual gravitation subsisting be­tween them, in like manner as we have already sup­posed them to be condensed round the corpuscles A and B (Fig. 1.)? But if, according to Sir l. Newton, so small a quantity as a cubic inch of our air, if left to itself at the height only of one semidiameter of the Earth, might exert this force, so as to ful the whole sphere of Saturn's orbit; it demonstrably follows, that the atmospheres aforesaid, which consist of huge masses of the same fluid, may extend their influences as far [Page 29]at least; and though the condensation of each round its own globe, by virtue of their mutual gravitation, would prevent the scattering of its component particles, and confounding itself with the atmospheres of the neighbouring Planets (as would undoubtedly be the case, however great their distances might be, did the cause of that condensation cease) yet it is by no means impossible, nor yet improbable, that under some cir­cumstances, the physical effects of two such atmospheres upon each other may become very apparent: For if we suppose two equal Planets with their atmospheres (as A and B Fig. 2) to pass near to each other, these atmospheres being fluid, and that in the highest con­ceivable degree of fluidity, would give way to the least degree of external force, and in consequence of their mutual repulsion would recede from each other; but the attractions of their respective globes would prevent their leaving them wholly, while each would so far retire, as to be depressed in the parts next the other, and to swell out in the opposite parts, changing their spherical figures to two oblong spheroids as at C and D (Fig 2.) But this is said upon the supposition that both the globes and their atmospheres are equal, each to the other respectively. Whereas, if we sup­pose the body A to be an immense globe like the Sun, having an atmosphere proportionably large,^* and [Page 30]consequently containing many thousands, and perhaps some millions of times the quantity of this repellent fluid contained in the atmosphere of B; which we may now consider as a Comet, of equal magnitude with the Earth, but surrounded, like other Planets of the same species, with an atmosphere of great extent, when compared with the magnitude of the globe itself; and if we suppose also the visible effects of the mutual repellency of these atmospheres to be reciprocally as the quantities of the repellent fluid contained in them, the atmosphere of the Comet might be prodigiously lengthened when in the neighbourhood of the Sun, and repelled to great distances behind the nucleus, while at the same time, the natural spherical form of the Sun's atmosphere would not be sensibly disturbed by that of the Comet. This reasoning may be illus­trated by considering the effects of the contrary prin­ciple of mutual attraction or gravitation subsisting between the globes themselves, which effects are sub­jects, both of mathematical computation, and of astro­nomical observation: For it is very certain that Comets perform their revolutions round the Sun, in conse­quence of this mutual gravitation, in orbits very ex­centric, and nearly parabolical; whereas it is proba­ble, that the joint efforts of all the Comets which ever appeared, would scarcely disturb the repose of the Sun in the center of the System.

A Comet, in its descent through the planetary orbs, approaches the Sun with an accelerating velocity, un­til [Page 31]it arrives at its perihelion; entering deeper and deeper within the spheres, both of the repulsion of the Sun's atmosphere, and of the activity of his rays; in consequence whereof the cometic atmosphere is con­tinually rarefying during this descent, both from the expansion it undergoes, by means of that re­pulsion, and from the increasing heat, which it ac­quires, as it approaches the Sun, which heat, as already observed, contributes greatly to the repellency of the particles of the aerial fluid. By the con­currence of these causes, if admitted, a tail must ne­cessarily be formed, the length of which would depend, partly upon the quantity of air contained in the cometic atmosphere, and partly upon its proximity to that of the Sun, and consequently would increase until the Comet arrived at its perihelion, or rather, from the continuance of the causes, until a few days after, which is most agreeable to observation. When a Comet is at this stage of its revolution, if it pass near the Sun, the tail usually extends from its head through vast regions of space, exhibiting a curious spectacle to distant Worlds.

IF a cometic atmosphere consisted of an unelastic fluid, and were thus repelled by the atmosphere of the Sun. it would put on the form of a very oblong spheroid, both ends of which would be terminated by a regular curvilinear surface as at A and a (Fig. 3); but as it is an elastic fluid, whose constituent particles are, amongst themselves, mutually repellent, as soon as ever the at­traction of the nucleus, which before condensed them spherically round itself, is diminished in any part of that atmosphere, by this opposite repulsion, the particles [Page 32]in that part become more and more at liberty to exert their own inherent repellency, which they would at length do quaquaversum, were it not that the repellent power of the Sun's whole atmosphere, so far predomi­nates over their own mutual repellency, as constantly to keep them in a direction nearly opposite to the Sun's center; but as this is not sufficient totally to prevent a lateral dilatation, the tail grows broader and broader, as it extends from the nucleus, while the longitudinal expansion is rather increased than impeded thereby, until the whole train, in a fair view of it, wears nearly the form of a parabolic curve; the several parts of which are less and less distinct, as they are further distant from the head, until at length, spectators shall, at the same time, differ ten or twenty degrees in their esti­mation of its apparent length, according to the differ­ent acuteness of their sight.^* (See Fig. 4. B b.)

AFTER the perihelion, as the Comet recedes from the Sun, the mutual repulsion of the two atmospheres gradually decreases, while the mutual gravitation of the Comet and its own atmosphere proportionably gets the better of it, until at length the exiled atoms are drawn home, and that perhaps without the loss of a single one; unless the tail should happen to sweep the sphere of some Planet in its way; in which case, if the atmosphere of the Planet were large, the mutual repellency of the two would rather occasion a bifurca­tion of the cometic atmosphere than a union or con­fusion [Page 33]of them; but were that of the Planet very small, and did we suppose that in consequence thereof, that of the Comet might leave a small portion of its tail behind; still no detriment to us could reasonably be expected or feared, upon that account, considering the amazing rarity of the Comet's Tail. The cometic atmosphere being gradually re-condensed round its nucleus as before, would provide it with a suitable garment for winter quarters in the remote parts of its orbit; agreeable to the ingenious hypothesis of Doctor Williamson.

THE thickness of a Comet's tail, or the diameter of a section made perpendicularly thro' it, is amazingly great towards its extremity; the apparent breadth of the tail of the Comet of 1680, where its distance from the Earth was equal to the distance of the Earth from the Sun, was equal to above three apparent diameters of the Sun^*; consequently the real thickness of that part of the tail was about three millions of miles, or between three and four hundred diameters of the Earth, yet, says Sir I. Newton, ‘the smallest Stars were visible through it without any diminution of their lustre.’ How inconceivably rare then must it have been! Perhaps for some thousands, not to say millions of miles, reckoned from its extremity towards its head, it might not have contained more air than a well blown bladder does with us: And that this is not only possible, but even probable, appears from Sir I. New­ton's computation of the expansion which a cubic inch [Page 34]of our air is capable of, already repeatedly referred to.

SO much for the third general proposition, which the reader will receive or reject, according to the weight of the evidence offered in support of it. It is however hoped that candor will be exercised, and that the whole will not be exploded, merely, for want of accuracy in the method, or of perspicuity in handling a subject so difficult of access.

A short digression here may not be amiss, in order to prevent the uncomfortable impressions, which the appearance of a Comet is apt to make upon the minds of some, and which were greatly increased dur­ing the appearance of the Comet of 1769, by an in­judicious publication in one of the southern news­papers, wherein it was insinuated, that if that Comet should pass between the Earth and the Sun, the Earth would pass through the tail of the Comet; the con­sequence of which, the writer supposed, might be fatal even to the World itself. But from the foregoing observations it is evident, that such apprehensions were groundless. The ingenious and learned Mr. Whiston has indeed endeavoured to prove, in order to support his Theory, that a Comet passing near the Earth in its descent towards the Sun, occasioned the universal deluge, by leaving behind it a sufficient quantity of vapors from its atmosphere and tail, (in which he supposed the Earth to be inveloped,) when condensed into rain, to drown the World. He fur­ther supposed that a Comet, in its ascent from the Sun, might be sufficiently heated to cause a general Conflagration, should it find the Earth in the same [Page 35]situation. In consequence of which the fears of many are usually alarmed, at the appearance of a Comet, by the apprehension of some grand catastrophe. But if the tails of Comets are so extremely rare, as we find they must necessarily be, and as every one must be convinced they are, who ever saw the Stars shining through them; how can we conceive that the Earth in passing through one of them, could carry off oceans sufficient to submerge "all the high hills under the whole Heaven"? to effect which, many of our oceans would, doubtless, be necessary. On the contrary, it seems probable, from the foregoing considerations, that the tails of all the Comets which ever appeared, could not, jointly, furnish water sufficient for one ocean only.— If so, the near approach of a Comet must be found ut­terly insufficient to account for the superior waters of the deluge, or the forty days rain mentioned by Moses; whatever disorders might otherwise be occasioned in both globes, by their mutual gravitation, in such vi­cinity. And had the Earth passed directly through the midst of the tail of the Comet of 1769, or of any other, it is not in the least probable, unless the Comet had come near enough to injure us upon other ac­counts, that the Earth could have carried off with it, or that the Comet would have imparted to us, vapors sufficient, even for a common thunder-shower. But this is submitted to the reader; who, perhaps, upon the perusal of the second part of this essay, may be of opinion, that a Comet, in its ascent from the Sun, is no more calculated to set the World on fire, than to drown it in its descent. But to return to our subject.

[Page 36] HERE a material objection may naturally arise, viz, why should the Comets appear with tails, and those, generally, of enormous lengths, while the Planets, which have atmospheres as well as the Com­ets, are always seen without them, even when at equal distances from the Sun?

TO this it may be answered; that the atmospheres of the Planets are so small, in proportion to the globes they surround, compared with those of Comets, that whatever may be the cause of the tails of the latter; if the same cause act upon their several atmospheres, proportionably to the spaces they respectively occupy, the tails of Comets may appear of astonishing lengths, while those of the Planets shall be totally insensible. In other words; the Planets in similar situations shall have no tails at all.

THE Earth, as has been repeatedly observed, is a Planet, with which, as it is the place of our abode, we are well acquainted: The height of its atmosphere is generally computed at about fifty miles; beyond which, its density is not sufficient to reflect the rays of the Sun in the crepusculum or twilight: The diameter of the Earth is about 8000 miles, the dia­meter then of the Earth and atmosphere together is about 8100 miles; from whence it appears by com­putation, that the space occupied by the atmosphere alone is to the space occupied by the Earth alone, as 1 to 26 nearly.

THE diameter of the atmosphere of a Comet, as before observed, is, at a medium, equal to ten times [Page 37]the diameter of its nucleus; and, spheres being as the cubes of their diameters, the magnitude of a Comet and its atmosphere together is equal to one thousand times the magnitude of the Comet alone; consequently, the space occupied by the atmosphere, is to the space occupied by the nucleus as 999 to 1.

LET us suppose a Comet of equal bigness with the Earth, having an atmosphere as above described, to be so situated, as that the Sun, the Earth and the Comet, may be in the Angles of an equi­lateral triangle: Let us suppose also the visible effect of the repelling power of the Sun's atmosphere upon those of the Earth and the Comet, to be in pro­portion to the spaces they respectively occupy: The magnitudes of these atmospheres being one to the other as 999 to 1/26, that is as 25974 to 1; when a spectator upon the Earth might see the tail of the Comet extend through an arch in the Heavens of 60°; had the atmosphere of the Earth a tail, arising from the same cause, in proportion to the space it oc­cupied, to a spectator upon the Comet it would sub­tend an angle of no more than 0° 0′ 8″ 81‴ or 60° di­vided by 25974, an angle which the best instruments could never discover in an object so dubiously defined: In other words, when the Comet would have a tail 60° in length, a Planet at the same distance from the Sun would have none at all.

THE atmospheres of the inferior Planets are pro­bably less in proportion, and those of the superior, larger than that of the Earth. But this subject may [Page 38]more properly be introduced in the second part of this Essay.

The principle of repulsion by which we have en­deavoured to account for the tails of Comets, and their opposition to the Sun, may receive further confirma­tion by considering some of the phaenomena which the atmosphere of a Comet would exhibit in the neigh-bourhood of the Sun, were the Sun divested of his atmosphere, or were his atmosphere deprived of its repellent principle: We shall therefore endeavour to prove the following proposition, viz.

WERE the Sun without an atmosphere, or some other appendage in its nature repellent to the atmos­phere of Comets, and the aethereal spaces void of re­sistance; instead of one tail, and that always turned from the Sun, every Comet would have two; the direction of one being towards, the other from the Sun, of which the former would be the most consi­derable, and both would increase as the Comet ap­proached the Sun, from the increasing gravitation towards him.

FOR though a Comet's atmosphere in its natural state surrounds the nucleus in a spherical form, (a form that all fluids which tend to a common center of gravity must put on) yet if we suppose this atmos­phere to be affected by its gravitation towards the Sun, and to consist of an unelastic fluid, like water; those parts of this fluid which were nearest to, and most remote from the Sun, would have their gravita­tion towards the center of the Comet lessened by the [Page 39]Sun's attraction, and thereby, from a spherical form the whole would be changed to an oblong spheroid. This is in fact the case with our ocean, which is changed from a sphere to a spheroid by the gravita­tion of the Moon; the axis of which spheroid, revol­ving round [...] of the Earth in consequence of the diurnal motion, causes the flux and reflux of our sea. But the atmosphere of a Comet is indefinitely elastic, and thence capable of an unlimited expansion; therefore; in proportion as it is in any part elevated above the superficies of the sphere it occupies when in its natural quiescent state, such parts are proportionably at liberty to expand themselves, by virtue of the mu­tual repulsion of their component particles, which are now in great measure freed from that constraint which before condensed them;^* which would change both ends of the spheroidal figure, to forms similar to the tails which commonly attend the Comets, being broader as they become rarer towards their extremities, on one side and the other: But the tail next the Sun would be the most considerable, the effects of his attraction on that side being greatest; and, should the Comet approach near enough to the Sun, a continued aerial stream would be formed from one globe to the other, whereby the Sun, by his superior attractive power, would by de­grees rob the Comet of its atmosphere, condensing the same round his own globe, without leaving the Comet a sufficiency for the purposes of life and vegetation in its solitary retreat without the planetary spheres. But as the Sun has an atmosphere, repellent to the atmospheres of the other heavenly bodies, these in­conveniences [Page 40]are thereby prevented, and every globe of the system retains its own, unimpaired, through every stage of its revolution.

THAT the projection of the tail of a Comet from its nucleus, and its perpetual opposition to the Sun, do arise from the mutual repellency of the atmos­pheres of Sun and Comet, as we have endeavoured to prove in the foregoing pages; may be confirmed and il­lustrated, if not demonstrated by electrical experiments.

THE electric fluid is an element, as distinct from all others which we are acquainted with, as air is from water; and if there be any such thing existing in na­ture, as pure elementary fire, this claims that character, in preference to all others. For fire, in its common: form, is so far from being a pure element, of itself, that the presence of air is necessary to its very being. Whereas the electric fire, in many experiments, acts with greater freedom in vacuo, than in the open air. But, different as this wonderful substance is from all others, we find, that some of its properties greatly resemble those of air, already treated of: And, if the following propositions, which are introduced to prove that resemblance, can be cofirmed by experiments; and it be allowed, that similar causes, naturally, pro­duce similar effects; it is presumed, that the solutions of the cometic phaenomena, offered in this essay, when composed with those experiments, will be found not inadequate to the phaenomena.

Prop. 1. THERE is a mutual attraction, subsisting betwixt the electric fluid and common matter; in [Page 41]consequence of which, the former is capable of, and liable to a condensation round the latter; in the same manner as air, by common gravitation, is condensed round the heavenly bodies, in the form of atmospheres.

THE truth of this proposition has been abundantly proved by Doctor: Franklin, and other writers upon this subject: Indeed, the success of all our electrical experiments depends upon this property: We may add further, that all the phaenomena which have ever been observed, in which that element is concerned, from the attraction of small hairs, dust, &c. in consequence of the attrition of amber, to the most severe blaze and irresistable force of lightning; de­pend in a great measure, upon the same property.

Prop. 2. AIR, as we have seen, is a fluid, the par­ticles of which mutually repel each other: So, also, is the electric element.

THE truth of this proposition is demonstrated by the following

AS ancient geographers imagined the polar and equatorial regions, or the frigid and torrid zones of the earth, were uninhabitable, in consequence of the extremes of heat and cold, to which those climates are exposed: So, modern astronomers have passed a similar judgment upon the superior and inferior Planets, especially on Saturn and Mercury; concluding, that our water would always boil upon the latter, and be frozen upon the former; and that merely in consequence of their different distances from the Sun^*. Whence it has been naturally concluded, that the textures of their various fluids, and of their inhabitants, to whose uses these fluids are adapted, are very different from what they are found to be upon our Earth: And, considering the near ap­proaches of most Comets to, and the vast elongations of all their orbits from the Sun, it has been generally [Page 52]supposed, that no material race of beings could subsist under such amazing vicissitudes of heat and cold, as those bodies must, from their different situations, ne­cessarily be exposed to; consequently that they are uninhabited.

BUT the conclusiveness of this reasoning depends upon the truth of the following Proposition; ad­vanced indeed by Sir Isaac Newton; but not supported by experiments, which were, with him, the criterion veritatis; viz. that, ‘The heat of the Sun is as the density of his rays, that is reciprocally as the squares of the distances from the Sun. ^*’

HERE, we are again reduced to the disagreeable necessity, of dissenting from the opinion of the greatest GENIUS that ever dignified human reason; which, considering the justly celebrated fame of that illustri­ous author, may be stigmatized as ignorance or vanity: But it is hoped that the reader will wave that imputation, if he shall judge, upon the whole, that Sir Isaac himself would have altered his opinion, upon the evidence which we shall produce in support of the contrary position: We may, however, lay down this as a maxim, that, in the prosecution of any science, the progress of the mind must necessarily be retarded, in proportion to the implicit assent we give to the de­cisions of any man, however great. We shall there­fore, without further apology, endeavour to prove that the heat of the Sun, as perceived by us, and as dis­coverable by its effects upon other substances exposed to his rays, does not depend upon the density of those [Page 53]rays only, though they are necessary to the very ex­istence of that heat; but, equally upon the concurrent operation of another cause, which we shall presently consider; from whence it will follow, that these causes, wherever they co-exist, whether upon the Earth, or upon the heavenly bodies, will naturally produce similar effects.

IN the mean time, before we engage in the dis­cussion of planetary heat, as depending upon the seve­ral distances of the Planets from the Sun; it may throw some light upon this subject if we consider the portion of that heat which falls to our own share, and the distribution of it throughout the various climates of the Earth.

THE surface of the Earth has, by geographers, been divided into five zones, viz. one torrid, including all the regions between the tropics, upon every part of which the Sun shines perpendicularly twice every year: Two frigid, which are situated between the polar-circles and the poles, and endure the rigors of perpetual winter, as the former is always basking in a summer Sun: And two temperate, which experience the vicissi­tudes of winter and summer, and, in some parts of them, in their extremes; these are situated between the frigid and torrid zones, in both hemispheres. In the first of these, the seasons are much more uniform than in the others, the days and nights being nearly of equal lengths, the year round; and although the heat may, for a constancy, be greater therein than in any other climate, yet it is not liable to such great and sudden changes as are experienced in the temperate [Page 54]zones; for, during a whole annual revolution of the earth, the difference of the degrees of heat, experienced in the torrid zone, as determined by the thermometer, are not so great as those which, sometimes, happen in the temperate zones, within the compass of a few hours^*: Much more do they fall short of the extremes which are endured in the latter, in the opposite seasons of the year.^† But what is above asserted of the torrid zone is to be understood only of the low, inhabited and cultivated countries, the mountainous regions with which those climates abound being excepted, for rea­sons which will hereafter appear.

THE axes of the several Planets whose diurnal ro­tations have been discovered are inclined, more or less, to the planes of their respective orbits; conse­quently, their superficies are divisible into zones and [Page 55]climates, corresponding with those of the Earth: And it is, at least, highly probable, that the various cli­mates of each globe, during its periodical revolution round the Sun, experience as great vicissitudes of heat and cold as those of the Earth: Nor is it unlikely that, in the equatorial regions of the different Planets, there may be at the same time as great varieties in the degrees of heat they respectively enjoy, as there are in the temperate zones of the earth in the different sea­sons of the year; but supposing all this, the inequalities in the distribution of heat to the several Planets and their various climates would vanish, when compared with those extremes which they would necessarily be expo­sed to, at their several distances from the Sun, upon the supposition that the Sun's heat were as the density of his rays; for were that really the case, the heat of Summer upon Mercury would be about seven times as great as upon the Earth, and above twice as hot as boiling water with us. On the other hand our sum­mer heat would be above ninety times greater than that of Saturn, the difference being more than seven times as great as that between our summer heat and the heat of red hot iron^*: For it is undoubtedly cer­tain, that the density of the Sun's rays is reciprocally as the squares of the distances from the Sun; from whence the above conclusions must necessarily follow, if the heat be proportional to that density.

IT is certainly then a question well deserving a phi­losophical enquiry; whether there be not some me­dium [Page 56]provided in nature, which, being distributed in different proportions to the several Planets, may so attemper the heat of the Sun to their respective dis­tances from him, as that the inhabitants of all may be equally happy in the enjoyment of it; and that one globe may receive as much benefit, and be exposed to as little injury, from that heat, as any other through­out the system. This indeed seems to be an object so worthy of the attention and providence of the great PARENT of the universe, that a philosophic mind would naturally embrace such an hypothesis, had it, but, the most slender evidence to support it. This medium, we shall find, is actually provided in the element of air, which is, in various proportion, con­densed round, and constitutes the atmospheres of the Earth and the heavenly bodies.

AS air is an element, to which we, and probably the inhabitants of the other Planets, are more indebted than is generally imagined; a short dissertation upon some of the advantages which accrue to us, and pro­bably to them, from its presence, may be acceptable to the reader.

AIR is a grand medium in nature, through which an all-bountiful providence conveys to us many of the conveniences, comforts, and delights of life. Upon Air depends the ascent of vapors, and their condensa­tion into clouds, whence they descend in dews and grateful showers, to refresh and fructify the Earth.— Upon Air we depend for the twilight, which affords us an agreeable gradation of shades from day to night; without which we should instantaneously plunge from [Page 57]the light of the Sun to midnight darkness; and again emerge from total darkness to the full lustre of day; which would be unsufferable to our organs of sight, upon their present constructure. Air is also the ve­hicle of sounds, whether articulate or inarticulate; consequently without it, we should not only be de­prived of the artless melody of the woods, and of the raptures which accompany the masterly execution of musical composition; but, which is of infinitely greater importance to us; there could be no language, no communication of ideas, but by dumb signs; no liberal arts nor sciences in the world. Therefore if we could subsist without this element, all mankind would be like the unhappy few among us who are said to be born deaf and dumb.

IT has already been shewn from experiments, that Air is necessary both to the support of animal life, and to the subsistance of flame: And how far the very being of fire, in any shape, and even of heat itself, may depend upon it, the reader may judge from the following experiments and observations of Mr. Boyle, related in Shaw's Abridgement of his Works.

"COALS," says Mr. Boyle, ‘being put glowing into a receiver, in three minutes after beginning to pump, the fire totally disappeared.—Other coals being suspended, in the open air, at the same time, continued burning 'till a great part was reduced to ashes^*. Lighted match was found more diffi­cult to put out by exhausting the air than kindled charcoal, nevertheless in about seven minutes the [Page 58]fire was extinguished, beyond the possibility of re­covery by re-admitting fresh air.’ Here we see that air is necessary to the subsistance, not only of flame, but of fire that emits no flame at all. To these we may add an easy experiment, which any person may try at his leisure, without the assistance of a pneumatic engine, viz. A composition may be made of allum and flour, which being well mixed together, reduced to a cinder in a crucible, pulverized, and otherwise prepared by a second heat, in a phial secured from the free com­munication of the external air, acquires an igneous quality, which, if the phial be kept stopped, it will retain unimpaired, even for many years; nor will it shew any appearance of fire more than any other matter confined in the same manner; but if at any time a few grains of it be let out upon any com­bustible substance in the open air, it will by the fresh air be instantly changed into fire, and kindle the sub­stance upon which it falls. This powder is com­monly called the black phosphorus. From a small quantity of it the experiment may be repeated with success for years together, provided care be taken, whenever the phial is opened to let out any of the powder, to stop it again immediately, to prevent the too free access of the external Air. Here we have a substance which has all the qualities of fire inherent in it, and retains them for a long time, and yet can never exhibit them but upon the admission of fresh Air. But to return to Mr. Boyle: In page 603 he concludes from experiments, which he had been mak­ing in condensed Air, that ‘the consumption of mat­ter by fire is greater in proportion to the quantity of Air contained in the (same) receiver; or rather [Page 59]in a still greater proportion,’ as he found by some subsequent experiments. Therefore, as the consump­tion of matter by [...] without flame, must be pro­portional to the intensity of the heat which consumes it, we may conclude from this last observation, that the intensity of the heat in any enkindled substance, is nearly proportional to the density of the surrounding Air, and depends in a great measure upon it. In page 604 Mr. Boyle concludes from other experiments, that ‘fire is more easily kindled in Air much compressed, than in common Air.’ Now it is certain that, the more intense the heat, the quicker the same combusti­bles are kindled by it; thus bodies, exposed to the foci of different burning-glasses, will take fire sooner from some than from others, according as the powers of those glasses to condense the Sun's rays in their foci (caeteris paribus) are greater, by which condensation the heat is proportionably increased: But as in these ex­periments, made in condensed Air, Mr. Boyle kindled his fire with the rays of the Sun, thus collected in the focus of a burning-glass, and found, as above, that the same glass would more easily kindle substances, in compressed Air, than in common Air, it follows, that, the density of the Sun's rays remaining the same, the heat with which they were accompanied was increased by increasing the density of the Air; in like manner as it the density of the Air had remained the same, and the density of the rays had been increased, by using glasses of stronger powers: To which may be added that Mr. Boyle always found it extremely difficult, and sometimes impossible to kindle any substance whatever in an exhausted receiver, either by the rays of the Sun, or even by red-hot iron in contact with gun-powder itself. The conclusion is obvious to every capacity.

[Page 60] HAVING thus exhibited to the reader an imperfect sketch of the principal uses to which our Air is subservi­ent; wherein among other things, we have seen the ne­cessity of its presence and co-operation in the product­ion of heat by the rays of the Sun, in common expe­riments: We shall now in further prosecution of the subject, proceed to prove that the heat of the Sun, as enjoyed by the inhabitants of the Earth in general, depends, not only upon the density of the Sun's rays, but, equally, upon the density of the sur­rounding atmosphere. This we shall prove from the testimonies of travellers of the most undoubted repu­tations who crossed the seas, and undertook the most dangerous and fatiguing journies which, perhaps, have ever been performed by man, with no other view than to promote the cause of science; particularly Don George Juan and Don Antonio de Ulloa, and their attendants who were sent by the Courts of France and Spain to South-America, to measure a degree of the meridian under the Equator: In the execution of which commission, they were obliged to take their stations, and make their observations upon some of the highest mountains upon the Earth, viz. the Andes in the neighbourhood of Quito, under the Equinoctial.

IT is well known that the tops of high mountains are at all seasons very cold, and are, for the most part, covered with snow the year round. But those above-mentioned, exhibit a scene truly curious; for from their summits to the plains below, inclusively, may be found at the same time, all the varieties of heat and cold, which are to be met within every climate of the Earth, at all seasons of the year^*

[Page 61] ONE observation made by Don de Ulloa upon the spot, is very remarkable, and much to the present point: He says, ‘the region of continual congelation began upon the several mountains at the same height above the level of the Sea, as determined by equal heights of the mercury in the Barometer.’ But Sir Isaac Newton asserts, and deduces the certainty of it from "actual experiments," that ‘as to our own air, the density of it’ (i. e. at any height) ‘is as the weight of the whole incumbent air, that is, (says he) as the height of the mercury in the barometer.’ ^* It therefore follows, that the region of "continual con­gelation," or perpetual frost commenced upon all those mountains, where the air was of the same den­sity. Above that certain height, the density of the air lessen'd, and the cold increased accordingly in severity, till the tops of the mountains presented all the horrors of winter, which are to be found in the polar regions. Whereas below that height, as the density of the air increased, from the increase of the incumbent pressure, the heat of the Sun also increased; till the inhabitants of the plains below suffered all the inconveniencies of the torrid zone.

NOW, the density of the Sun's rays being the same in the several cases, and the tops of the mountains be­ing above the common region of the clouds, and conse­quently enjoying the presence of the Sun much more than the plains below; it follows, that although the rays of the Sun may be the sine qua non, without which the inhabitants of the Earth would enjoy no heat at all, yet, the degree or quantum of their heat depends upon the density and co-operation of the aerial medium through which those rays are transmitted to them.

[Page 62] AS the proportionality of the Sun's heat to the den­sity of his rays, is a point, upon which, as proved or disproved, many curious questions in natural philoso­phy may turn, every argument which tends to deter­mine that point will doubtless be acceptable to the reader; and if he should be already satisfied in his own mind, from the foregoing observations, that the heat of the Sun is not as the density of his rays simply, yet it is hoped that he will patiently attend to one argument more, which is drawn from Sir Isaac Newton's own principles, and naturally results from his computation of the amazing, inconceivable degree of heat, which must have been acquired by the Comet of 1680, at its perihelion, upon that supposition; which, though it has never been controverted, but generally allowed to be just, and quoted accordingly, must, if true, have occasioned the exhibition of some phaenomena, which could not have escaped the notice of the many curious astronomers of that day. According to this great au­thor's calculation, this Comet, by its near approach to the Sun at its perihelion, "acquired a degree of heat two thousand times greater than the heat of red-hot iron^*"; but from previous experiments he concludes, that the heat of red-hot iron is but twelve times greater than that which dry earth acquires when exposed to the summer's Sun: With what an amazing lustre then must the Comet have glowed, merely from the heat it acquired during its proximity to the Sun! Therefore at its first appearance after the perihelion, it must have shone, not with a borrowed or reflected light, as it did before it arrived at that stage, but by its own newly acquired lustre, far exceeding (perhaps) the brightest Star in the Heavens; for if iron heated but twelve times more [Page 63]than dry earth exposed to a summer's Sun, becomes red-hot, and from that heat emits a splendor, indepen­dent of the Sun's rays, we may defy the human ima­gination to conceive of that splendor, when yet in­creased two thousand fold, and exhibited by a globe of equal dimensions with the Earth. Furthermore, Sir Isaac computes that a globe of red-hot iron equal to the Earth, or the Comet, supposed of equal bigness, would scarcely cool in fifty thousand years; therefore the Comet, being two thousand times hotter, if it were a globe of iron, would require 50000 multiplied by 2000, or one hundred millions of years to cool in. But if we suppose the Comet to cool a hundred times faster than an iron globe of the same magnitude, equally heated; it could not lose all its heat under a million years^*; nay, at the end of five hundred thousand years it would still be a thousand times hotter, and consequently brighter than red-hot iron. But as the period of this Comet is supposed to be short of six hundred years, [several appearances of Comets in past ages being supposed to be different visits from the same Comet, after intervals of five hundred and seventy five years;] if at every perihelion it acquired a degree of heat which it could not lose under a million years, and had short of six hundred years to discharge itself of the heat acquired at each revolution, how astonishing upon these principles must be the accumulations of its heat during those several revolutions! so great! [Page 64]that one would imagine, that for ages after its peri­helion it would be visible merely from its own lustre; and that when, from its distance, its diameter would become insensible, it would still be seen as a lucid point, twinkling among the Stars: Yet so far was this from being the case, that in three months from its pe­rihelion, viz. from 8th December to 9th March it totally disappeared, though the Earth was in a situation to view it for a considerable time after: Sir Isaac Newton says ‘on the ninth and tenth of February to the naked eye the head appeared no more.’

THIS Comet, at least its tail, was discovered by Mr. Flamstead two days after its perihelion, viz. on the 10th December; from which time till its total disappear­ance, it was constantly observed by astronomers; but none of their observations take notice of any extraor­dinary brightness it exhibited, more than is usual in the appearance of other Comets. Sir Isaac indeed observes, that ‘in the month of December, just after it had been heated by the Sun, it did emit a much longer tail, and more splendid than in the month of November before, when it had not yet arrived at its perihelion.’ But this he asserts of the tail only, for soon after he adds, ‘the head of this Comet at equal distances from the Sun and from the Earth, appeared darker after its perihelion than it did be­fore:’ It is true he accounts for it by supposing the ‘nucleus to be environed by a denser and blacker smoak than before.’ But this is difficult to recon­cile with what he says a page or two back, when speaking of the same Comet, viz. ‘by so fierce a heat, vapors and exhalations, and every volatik [Page 65]matter must have been immediately consumed and dissipated^*.’ May we not add, and the whole solid mass calcined or vitrified? Therefore, if that dulness in its appearance, after the perihelion, was owing to clouds and vapours, it is evident, from that great author's own reasoning, that the Comet could not have been exposed to so great an intensity of heat, in that vicinity to the Sun; since all such heterogeneous exhalations must have been consumed and dissipated thereby, as fast as they arose from the head; if indeed any volatile or evaporable matter, or any degree of moisture whatever could have remained in the head after such an inconceivable ignition. Finally, had the Comet ever acquired so great an intensity of heat, it is probable the inhabitants of the Earth would never have lost sight of it to the end of time; much less would it have totally disappeared in three months after its perihelion.^†

[Page 66] SIR I. Newton, in making his computation of the heat sustained by this Comet, first took it for granted, "that the heat of the Sun is as the density of his rays," (as we have seen before): In the next place, he con­sidered the density of these rays with us, at the mean distance of the Earth from the Sun, as a fixed and certain standard, with which the density of the rays at every other distance might be compared: Then, after exposing dry earth to the summer Sun, he compared the heat contracted thereby, with that of boiling water and red-hot iron, and found by experiments the pro­portional degrees of heat in them to be nearly as 1, 3, and 12 respectively: In the last place he considered the heat acquired by the dry earth aforesaid, as the standard of our summer heat, and annexed it to the mean density of the Sun's rays with us: And upon this foundation he seems to have constructed his general scale of heat for the solar system. But it is presumed that the reader is by this time satisfied, that the main proposition upon which that great author's reasoning was founded must fail for want of support, however just his conclusions drawn from it may be: For we now find regions of eternal frost under the equinoctial itself, the rigors of which are scarcely exceeded in the polar regions; and this, at the height of but two or three perpendicular miles above the common surface of the Earth, and where but few clouds interpose to hide the beams of the Sun. If therefore we suppose ourselves carried up forty or fifty miles higher, or to the very top of the atmosphere, we may well shud­der at the idea of such a situation, even if exposed to the unclouded rays of a perpendicular Sun.

[Page 67] WE are now naturally led to consider some exten­sive purposes which the great Author of Nature pro­bably had in view, when he formed the atmospheres of, and annexed them to, the several Planets and Comets of our System. We have seen from indispu­table authority, that the density of the Sun's rays alone does not produce a competency of heat, for the com­fortable subsistance of the inhabitants of the Earth, even in its hottest climates, but, that a certain density of air is equally necessary, for these rays to operate upon, and to co-operate with them, in promoting the various purposes of life and vegetation. This air we are abundantly furnished with from the atmosphere which surrounds us, the density of which at every height, is proportional to the pressure of the incum­bent fluid. We may conclude from analogy that the atmospheres of the other Planets, and of the Comets, are designed for, and adapted to the same purposes for which the atmosphere of the Earth was originally pro­vided: And if we suppose the general stock of heat, which falls to the share of any Planet, to be in a ratio compounded of the density of the Sun's rays and the density of the air upon the surface of the Planet, it is easy to conceive, that these globes may be severally furnished with such atmospheres, as may render them comfortable habitations, whatever their distances from the Sun may be. That the heat of the Sun is actually thus dispensed to the Planets necessarily follows from the experiments and observations contained in the foregoing pages, upon the supposition that they are surrounded with aerial atmospheres, suited to their se­veral distances from the Sun; that they have such at­mospheres is already proved, and the Creator, has, [Page 68]doubtless wisely proportioned their respective quanti­ties and densities according to those distances.

THE tails of Comets are nothing more than expan­sions of their atmospheres, whose lengths depend upon their nearness to the Sun (as before observed) and de­crease as they recede from the Sun, becoming invisible to us (generally) before their heads disappear: There­fore we have the highest reason to conclude that when these globes are in the most remote parts of their or­bits, or at their aphelia, their tails wholly subside, and their atmospheres resume spherical forms, like those of the Earth and of the other Planets, surrounding their nuclei at equal altitudes in every part; the Sun's at­mosphere being too remote to have any sensible effect upon them by its repellency. The air must of con­sequence be prodigiously dense near the surfaces of their globes, being compressed by the weight of such a vast incumbent fluid; whereby the Sun's rays, though weaker, or less dense than with us in the ratio of the squares of the distances, may, upon our principles, be rendered as active with them, and as productive of such degrees of heat as are necessary for the purposes of animal and vegetable life, as with us, or any other Planet of the System.

BUT, (as was observed by Doctor Williamson,) should these atmospheres continue of the same density, through all parts of their orbits, the degrees of heat which their inhabitants must undergo at their peri­helia would be unsufferable: To prevent which, the great Author of Nature has made sufficient provision; for as they approach the Sun, they are by the repulsion of his atmosphere (or some other cause equivalent to such supposed repulsion) gradually eased of that incumbrance, [Page 69]the cometic atmosphere being gradually rarefied and driven behind its body through vast spaces of the Heavens; what remains from time to time being more and more rarefied by the increasing action of the Sun's rays upon it, and repelled as rarefied; till at length, if they come near enough to the Sun, the inhabitants may have little more than pure aether to breathe in. Thus the Comet of 1769, (than which but few have gone nearer to the Sun) before it ar­rived at its perihelion, although it projected a most astonishing tail, yet the remaining atmosphere was dense enough to hide the nucleus it surrounded; the Comet, when viewed through the best telescopes, pre­senting only a dubiously defined luminous appearance: But when it made its re-appearance in the evening about the latter end of October, its atmosphere had undergone so great a degree or rarefaction in passing its perihelion, that it was sufficiently pellucid to dis­cover the nucleus, which appeared plainly and dis­tinctly through it.

MAY we not then conclude, even with certainty, that as a Comet is perpetually varying its distance from the Sun, so the density of its atmosphere is con­tinually changing through the various stages of its revolution; and thence, that its inhabitants may at all times enjoy as much benefit, and receive as little injury, from the Sun's rays, as the inhabitants of any other Planet in the solar System?

AS the primary Planets revolve in orbits nearly cir­cular, they have no occasion for such vast atmospheres as are necessary for the Comets in the remote regions of the Heavens to which they retire; but are sur­rounded [Page 70]with such, as infinite wisdom saw best suited to their several distances from the Sun; such as might have no redundancies to be thrown off in tails at one time more than another; the nearly equal distances of each from the Sun, in the several parts of its orbit, requiring nearly an equal density of atmosphere at all times.

FROM the premises we may conclude, that the at­mospheres of the inferior Planets are smaller, and those of the superior ones larger in some proportion, than that of the Earth, in order that their densities near their respective superficies, may be so proportioned to their several distances from the Sun, as that they may equally share the benefit of his rays. For want of proper astronomical observations, to determine this point with regard to the other Planets, we can pro­nounce with certainty only concerning Mars: As that Planet is further distant from the Sun than the Earth, his atmosphere, for the reasons above assigned, ought to be larger than our own; accordingly it appears (from the observations referred to in Part I. Note* Page 12) that the height of his atmosphere above his surface is at least equal to two thirds of his diameter; which is much greater than that of the Earth, though it falls vastly short of those of the Comets. Observa­tions of future occultations of fixed Stars by Jupiter, Saturn and the other Planets, made with better instru­ments, may possibly determine this point with regard to them also.

SEVERAL objections may be raised against the prin­ciples advanced in this Essay, to all which we hope to give satisfactory answers. As

[Page 71] 1. IT may be objected, that, as the light of the Sun is confessedly proportional to the density of his rays, that is inversely as the squares of the distances from the Sun, the inconveniencies arising therefrom to the inhabitants of the Comets, at their perihelia and aphelia, might be nearly as great as those which would arise from the Sun's heat were it distributed in the same pro­portion; or at least, if it did not render the Comets uninhabitable, would make the conditions of their in­habitants, at times very uncomfortable. For example, were the light of the Sun adapted to their various pur­poses, at their mean distances, at their aphelia, they might not enjoy a sufficiency; and on the contrary at their perihelia, the splendor would be unsufferable.

THIS objection, it is presumed will vanish upon a careful attention to the structure of the eyes of terrestrial animals, whose pupils contract or dilate involuntarily, according as the density of the rays which pass thro' them and fall upon their retinas is greater or less, whereby more or fewer of those rays are admitted, as may be re­quisite for distinct, inoffensive vision: Thus most per­sons can see to read by candle light near as well as by day light, whereas the quantities of light reflected from objects in the two cases scarely bear any pro­portion one to the other. But the aperture of the pu­pil is much greater in the former than in the latter, and more rays in proportion are consequently admitted. Moreover, there are some animals with us which re­tire to their holes and caves at the approach of day, whose purposes are as well answered by the glimmer­ing [Page 72]light of the Stars, as those of others are by the presence of the Sun; there are yet others which can behold the Sun in his meridian splendor, without of­fence. Now if we only suppose that the inhabitants of Comets in general have, in the original formation of their optic organs, the power of contracting and dilating their pupils, according to the strength or weakness of the light which is transmitted through them, we may easily conceive, that the rays of the Sun might be no more offensive to them at one time than at another; for at their aphelia their pupils might be dilated to their utmost extent; on the other hand, at their perihelia, they might be contracted to physical points, if the splendor of the Sun so required, whereby a proportionally smaller quantity would be admitted.

THE light of the Sun which the Cometarians enjoy at their aphelia is indeed much greater than we should be apt to imagine; for let us consider the Comet of 1680, whose period is the longest and its aphelion distance the greatest of any one known, being (according to Dr. Halley) to the mean distance of the Earth from the Sun nearly as 138 to 1; but from the reasoning of Doctor Smith, in his optics it appears, that the pro­portion of our day light to moon light with a full moon, is nearly as 90000 to 1;^* and the light of the Sun upon the Comet at its last mentioned stage is to his light with us but as about 1 to 19000, therefore if we divide 90000 by 19000 we shall find that Moon­light with us is to Sun-light upon the Comet, nearly as 1 to 4¾, and consequently that the light of the Sun [Page 73]enjoyed by the inhabitants of that Comet at its aphe­lion is nearly five times as great as the light of our full Moon. But it is still much greater upon account of the largeness and density of the atmosphere; for it is certain, that our day light, which is equally diffu­sed upon all terrestrial objects, and renders them vi­sible; depends upon the reflection of the Sun's rays, from the atmosphere, together with the hete­rogeneous corpuscles floating in it; without which all such objects would be as obscure as at midnight, even with the Sun shining in full splendor above the hori­zon; excepting those upon which the direct rays of the Sun might fall, or such as might be faintly illumi­minated by the reflection of those rays from neigh­bouring objects: The Heavens would appear perfectly black, and the smallest Stars would appear, at noon­day, which is prevented, only, by the illumination of the atmosphere. The beautiful azure, which we ob­serve in the Sky, after the atmosphere is purged of its vapors, by a storm or thunder-gust, arises from the appearance of this black Sky, through the air, which is now become more transparent, than when charged with a heterogeneous collection of opaque corpuscles.

THE atmospheres of Comets, being much larger and denser than that of the Earth, reflect a much greater proportion of the Sun's rays; their hemispheres next the Sun must therefore be more illuminated, and their day light increased, in the same proportion; although the light arising from the direct rays of the Sun would be considerably weakened thereby.

[Page 74] THIS reasoning may be illustrated by calling to mind the effects of two great ecliples of the Sun, one of which happened on the 5th or 6th day of August, 1766; the other on the 19th day of January, 1768; which effects, most persons among us, whose attention was turned that way, may recollect; during the for­mer the air was very clear, and the sky cloathed in a fine blue, excepting, here and there, where a few sum­mer clouds were scattered: In the midst of this eclipse the air was darkened to such a degree, that, although the Sun shone unclouded, a sickly gloom seemed to spread over the face of nature. In the latter, (though a much greater eclipse) the air was full of vapors, the reflection of the Sun's rays from which was so copious, as to render it offensive to the eyes to look at the Heavens, before the eclipse began, and in the middle of it, the darkness occasioned thereby would scarcely have been noticed, had not the eclipse been known beforehand.

THE inhabitants of Comets enjoy another advan­tage from their great atmospheres, which is peculiar to them alone; for, in the hemisphere turned from the Sun, they can have no dark nights like those of the Planets; but, in consequence of the reflection of the Sun's rays from those atmospheres, must be fa­voured with perpetual twilight if not day light; for a cometic atmosphere is enlightened by the Sun by night as well as by day, excepting only a column, which is nearly cylindrical at the aphelion, whose base is a great circle of the Comet, and whose altitude is equal to the height of the atmosphere; which, (considering the [Page 75]great extent of the latter,) bears but a small propor­tion to the whole atmospheric hemisphere; this column includes that part of the atmosphere which is eclipsed by the shadow of the globe itself, and, if the diameter of the atmosphere is equal to ten diameters of the globe^*, does not contain [...]th part of the whole visi­sible hemisphere; and is still less upon account of the refraction of the Sun's rays, which shortens and con­tracts the cone of the shadow. Whence it is probable that the darkest nights of the Cometarians, at their aphelia, are much lighter than our brightest moon­light nights. But this is submitted to the judgment of the reader.

2. IT may further be objected; that if the atmos­pheres of Comets undergo such amazing, rarefactions and condensations, as they necessarily must, from the alternate projections and retractions of their tails, it is difficult to conceive that they can at all times be fit for the respiration of their supposed inhabitants.

THIS objection might, perhaps, have remained un­answerable, had it not been for the genius of that truly great philosopher Doctor Edmund Halley; who, if he was not the original inventor of the diving-bell, yet, by a sagacity peculiar to himself, improved it to a de­gree of perfection, which might, before, have rather been wished for, than expected.^† In this bell persons may be let down with safety to the bottom of the sea; but the included air differs in density at every depth below the surface of the water. At the depth of [Page 76]thirty-two or thirty-three feet, as the air may occa­sionally change, or less, in proportion as salt water is heavier than fresh, the density of the air within the bell is double the density of the external air; at double that depth the density is triple; at three times, four­fold, and so on. Now if the bell be let down without proper precaution, the too sudden condensation of the air within, would give the adventurers extreme pain, as they sometimes found by experience; and should the bell sink suddenly to the bottom of the sea, the consequence might be fatal to them; for the same reason that a square case-bottle, filled only with com­mon air above the surface of the water, and corked tight, if it were let down with the divers in the bell, would from the increasing pressure of the condensing air without it, be crushed inwards, and broken in pieces, in the same manner as if the air in the bottle had been exhausted by an Air-Pump above the sur­face of the water; which effect would be prevented by the smallest hole in the cork, provided the bell were let down leisurely, so that the air as it condensed without, might gradually insinuate itself through the hole into the bottle: On the contrary were the bottle corked at the bottom of the sea, and the bell drawn up from the depth of nine or ten fathoms; before the bell could arrive at the surface of the water, the bot­tle would burst outwards, from the expansive force of the condensed air within it; which might also be prevented by a similar precaution. And it is doubt­less from the latter cause that some persons who have ascended to the tops of high mountains, have been seized with reachings, vomitings and other inconve­niences [Page 77]related by travellers; the external air of the atmosphere at such heights being too rare to counter­act by its pressure, the expansive force of the denser air which is interspersed throughout the various vessels and organs of animal bodies.

BUT Doctor Halley testifies from his own experience, that ‘if the diving-bell be let down (or drawn up) gradually, about twelve feet at a time with an in­terval of but a few minutes between, no inconve­nience would follow’; as the several organs of the body would by degrees be inured to the density of the air, as it increased or decreased at the several depths. The Doctor tells us, that he himself was hours toge­ther at the bottom of the sea in nine or ten fathoms of water; and felt himself as well as if he had been all the time on board the ship: But the density of the air he then breathed, must have been more than three times as great as that of the air above the surface of the water: In other words, he then breathed in air, compressed by the weight of between three and four of our atmospheres instead of one, and of ten or a dozen such as Don de Ulloa breathed in, when upon the tops of the mountains of Quito, without any inconvenience. Therefore, if time sufficient be allowed for the air, in­cluded in the several vessels of the human body, gra­dually to contract, expand or otherwise accommodate itself, to the increasing or decreasing density of the exterior air; no bad consequences or even inconve­niencies are to be apprehended, although the difference of density be exceeding great.

BUT by the gradual and regular approach of any Comet to, or recess from its perihelion, (at and near which its [Page 78]velocity is greatest and the consequent changes in its atmosphere are most sudden of all) the increase or decrease of its atmosphere, is much more regular, uniform, and insensible to its inhabitants, than any increase or decrease of the density of the air can be in the diving bell by Doctor Halley's method; for by the latter, a degree of rarefaction, or condensation, is effected in a few minutes, which might not take place in the cometary atmospheres under some days.

PERHAPS enough has been already said to remove every material objection; but if any difficulties yet remain, in the mind of the reader, on account of the vast changes which the several climates in each, and the atmospheres of all Comets must necessarily under­go, in the various parts of their orbits: the following additional observations are submitted to his conside­ration; which may tend to lessen those difficulties, if they do not wholly remove them, viz.

AS a Comet approaches its perihelion, that hemis­phere of its atmosphere which is next to the Sun, be­ing more immediately exposed to his rays, will feel the effects of his neighbourhood sooner than the opposite hemisphere, and consequently will be warmed, rarefied, and thrown off behind the Comet by the repulsion of the Sun's atmosphere, sooner than the other; the colder and denser parts of the fluid will of course continually flow in from the other side of the Comet to supply its place, in order to preserve, as near as may be, an equilibrium; in consequence of which there will be a constant suc­cession of the cooler air from thence; whereby the inhabitants on the hemisphere next the Sun may be [Page 79]continually refreshed with gales of wind during that vicinity, which would increase till the Comet arrived at its perihelion, when their velocity would be greatest of all; but even then they would not (from this cause) blow in sudden violent gusts like our hurricanes, but steadily, unless disturbed by causes from within the Comet's atmosphere; besides, as the velocity of the current increased, the den­sity of the fluid would lessen from the increasing rarefaction, whereby its momentum might continue nearly the same; for this momentum would be in a ratio compounded of the velocity of the fluid and its density together; and as the violence of our high winds, and their consequent effects depend, not upon the velocity, merely, but upon the momentum of the current, this brisk circulation of the cometic air may, (however great we suppose its velocity) be rather grate­ful than injurious to the Cometarians: And how unfit soever the air in such a rarefied state might be for their use, if stagnant, yet, when thus put in motion, it may be rendered sufficiently active to answer all the purposes of respiration. This reasoning is confirmed by daily experience: For it is not an uncommon thing for people of tender frames to faint in a close hot and rarefied air; and as the fan is generally near at hand, it is as common for the by-standers to apply it to their faces, which, by giving a brisk motion to the air, without any alteration of its density, generally revives them, in a short time, even when no other remedy is at hand.— This brisk motion of the air would also remove or pre­vent the disagreeable sensations of heat which the [Page 80] cometary inhabitants might otherwise suffer from an exposure to the Sun's rays at their perihelia: For, if a person sit with his face uncovered before the scorching blaze of a common fire, the motion of the air excited by a common fan, even without hiding the blaze from the face, is sufficient, not only to make the situation comfortable, but to change the painful sensation to an agreeable coolness: As any one will find upon trial.

IF we suppose that every Comet has a diurnal ro­tation round an axis of its own, the inhabitants may enjoy grateful vicissitudes, from the alternate ab­sence and presence of the Sun; and if we further suppose this diurnal motion to be performed contrary to its apparent heliocentric motion; the returns of day and night would be quickened as it approached the Sun, from the increase of its angular velocity round that globe, whereby the presence of the Sun, in that neighbourhood, would be of shorter duration, upon any one part of the Comet, and his heat might be rendered still less irksome to the inhabitants, on that account. It is true, no discovery has been made of any such diurnal motion, but as all the primary Planets, so far as our observations can reach, are dis­covered to have such motions, we may well be allowed to suppose that the Comets are not without them; espe­cially now we are endeavouring to prove their habi­tability, to which this motion is perhaps as necessary as to the habitability of the primary Planets. These rotations have been already discovered and determined in Venus, the Earth, Mars and Jupiter. Saturn, though a vast globe in itself, is so remote, and Mercury is so near the Sun, and so very small, that this motion has [Page 81]never been discovered in either, by our best instru­ments, but is justly inferred by analogy; which me­thod of reasoning will equally extend to the Comets of the System. The diurnal motions of the Planets indeed are performed nearly in the same directions with their annual, both motions in all, as far as they have been discovered, being direct, or from West to East, whereas the diurnal motions of Comets, accord­ing to the foregoing supposition, are performed con­trary to this rule: But this is no objection against the hypothesis; for Planets and Comets differ as widely, in almost every other particular; the annual motions of the former (as now observed,) are all direct, and are apparently confined within the limits of the zodiac, the latter move indifferently in all directions through the Heavens; the periodical revolutions of the former are made in orbits nearly circular, those of the latter are prodigiously excentric, and nearly para­bolical; all which seem wisely to be ordered, that a multitude of Worlds may exist at the same time, and be enlightened, warmed, and rendered prolific, by the rays of the same Sun, without interfering in their mo­tions, or disturbing the harmony of the System.

TO illustrate the reasoning in pages 78, 79. Fig. 5 is added; in which let S represent the Sun; to which Comets in general, though perhaps equal in magni­tude to our Earth, are, without a figure, but as drops of the bucket. ^* Let C represent a Comet with its at­mosphere [Page 82]and tail, the dark curve line c k d g h on one side, and c i a e f on the other, may serve to give an idea of the motion of a parcel of the cometary air from its more condensed state behind the Comet at c, through its various stages of rarefaction and repulsion; as that part of the atmosphere next the Sun, viz. that in or near the line S b which connects the centers of the Sun and the Comet, is rarefied, the denser air from behind at c must necessarily flow in to pre­serve as near as possible an equilibrium, and continue so to do as long as the rarefaction continues to increase. The air next the Sun being thus rarefied, that at c would take a turn round the nucleus through k and i, but before these seperate parcels came to d or a the rarefaction would so increase, that they would begin to ascend, and as they ascended, at a and d they would repel each other; they would still keep rising by their increasing rarefaction, through S b as through a tun­nel, and increase in their mutual repellency as they re­ceded from the center of the Comet, till at length at l and m the repellency of the Sun's atmosphere would compel them to retire through g h or e f whence they would proceed to the extremity of the tail, the remain­ing parcels of air in the same cometic hemisphere would take a similar course (as represented by the faint strokes in the figure) whether their distances from the Comet's surface were greater or less, till at length the rarefaction of the Comet's atmosphere would become as great, as the repulsive power of the Sun's atmosphere could effect, or the Comet's vicinity to the Sun, require.—We shall offer one observation more, for the conside­ration of the reader before we close the subject, viz.

[Page 83] WHEN the air of our atmosphere appears by the ther­mometer to be extremely cold, it does not affect the senses so disagreeably, if the atmosphere be in a calm stagnant state, as at other times, when the mercury is ten or even twenty degrees higher, (and consequently the weather warmer) with a brisk wind, as has been frequently observed by those who attend to their thermometers. Now when a Comet is at its greatest distance from the Sun, its atmosphere, being uniformly condensed round its globe, might settle into a dead calm, for any distur­bances it could receive from without: For whatever influences baneful or salutary, the heavenly bodies may reciprocally communicate while in the neighbourhood of each other, the Comets in their aphelia, are removed to such inconceivable distances from all the other globes of the System, that their mutual effects, phy­sically considered must vanish. And in so calm an atmosphere, of so great a density, illuminated by the Sun's rays, the inhabitants of the Comets may, even when most remote from the Sun, be as warm, or at least as comfortable as the inhabitants of the Earth, or of any other Planet.

THIS subject cannot be better closed than in the words of Doctor Williamson, viz.^*

‘ONE of the primary ideas we form of the supreme Being is, that he is the source of life, intelli­gence and happiness, and delights to communicate them; the Earth we tread, the water we drink, and the very air in which we breathe, swarm with living [Page 84]creatures, all fitted to their several habitations.—Are we to suppose that this little globe is the only animated part of the creation, while the Comets, many of which are larger worlds, and run a nobler course, are an idle chaos, formed for the sole pur­pose of being frozen and burnt in turns?—We cannot admit the thought; the Comets are doubt­less inhabited.’

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