A discourse mathematical on the variation of the magneticall needle Together with its admirable diminution lately discovered. By Henry Gellibrand professor of astronomie in Gresham College.

A discourse mathematical on the variation of the magneticall needle Together with its admirable diminution lately discovered. By Henry Gellibrand professor of astronomie in Gresham College.

Table of contents

Title page

A DISCOVRSE MATHEMATICALL on the variation of the Magneticall Needle.

Errata Typographica.

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A DISCOVRSE MATHEMATICAL ON THE VARIATION OF THE MAGNETICALL Needle. Together with Its admirable Diminution lately discovered. By Henry Gellibrand Professor of Astronomie in Gresham College.

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Veniet tempus, quo ista quae nunc latent, in lucem dies extrahat, et longioris aevi diligentia.

Sen. Nat: Quaest. lib. 7. cap. 25.

LONDON, Printed by William Iones, dwelling in Red-crosse-street. 1635.

A DISCOVRSE MATHEMATICALL on the variation of the Magneticall Needle.

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TO write an Encomiastique of the Magnet is not my intention; the bold & confident attempts of Seamen, thorough those hidden paths of the vast Ocean, to all navigable parts of the earth, will spare me the labour; neither will these few lines permit me to speake of all Magneticall qualities, they that list may have recourse for their satisfaction herein, to that most learned worke (admired by all Foreiners) long agoe penned by Mr. Dr. Gilbert our countryman, sometimes Physitian to that Renowned Lady our late Queene. That which I here principally ayme at, is the deflection of the Needle from the Terrestriall Meridian, together with that abstruse and admirable variation of the Variation, lately discovered to the world.

The Variation then is the deflection of the Magneticall Meridian from the Terrestriall, the Horizon determining its Quantitie and Qualitie. These Meridians sometimes are coincident, and in such places there can be no Variation▪ Sometimes againe they are different, [Page] and then that Hemispere wherein either extremity of the Needle lyes, denominates its Quality, for if it be the Easterne Hemispere, it is called Easterly variation, if the westerne, westerly. And the Horizon supposed (as all great Circles are) to be divided into 360 parts, & each part subdivided into Centesmes or Millesmes, defines the Quantity as 10, 20, 30, &c. parts of variation.

For our more distinct proceeding (and to give some satisfaction to those that are not altogether Mathematicall) it will not be impertinent to our present purpose to give some probable Reason for this variation or swarving of the Needle from the meridians of most places.

Take a vigorous Magnet, convert it into a Sphericall Body, and having found its Poles, delineat the same with Meridians, the Aequinoctiall, and Parallells.

Then take a fine sowing Needle, and applying it to the intersection of a Meridian and the Aequinoctiall, it will be found precisely to point out the Poles of the stone; and if the Needle be moved towards either Pole, according to its owne direction, it will trace out a Circle over the said Magnetique Pole. But if this Sphaericall Magnet shall have a part of it excavated, as admitt about the forty fift Paralell, and forty fift Meridian or degree of longitude, the Needle moued forwards from the foresaid intersection, assoone as it shall draw neere to the limb of this excavation, will forthwith leave its former position, as lying in the plane of the prime meridian, and incline to the other solide side, not respecting at all the aforesaid Poles: and this deflection we call the variation of the Needle: And the greater will this variation be, the neerer it approacheth to the Poles. For [Page] whereas before the excavation each side being a like potent,Which may be some cause of those suddaine and exorbitant variations in the Nortwest and other Northerne & Southerne parts of the Earth. the Needle like an upright and indifferent Arbiter inclined to neither part; after the excavation it will convert it selfe to the more solide side, rejecting the imperfect and defective, and so much the more powerfully, by how much the other side shall be found to have lost more of its Magnetique vigorous substance.

The like will fall out if the Needle be so placed, as that it shall bisect the excavated part, for then will it acurately point out the true Poles of the stone formerly found, but suddainely swarue from them after any little dislocation, strongly applying it selfe to the more solide side.

As it is with this little, so likewise with the greater Magnet the Earth, whose solide magnetique parts are great Continents, the defective and excavated, the deepe and vast channell of the Sea. For if there were observations made in the midd way betweene the Easterly Coast of China and this westerne of Europe, it is very probable there would be found no variation at all, the Terrestriall and Magneticall Meridians being congruall; but if we shall incline to either side, as admit to the westward, the Needle will in like manner move it selfe to the Easterne Continent, as the more prevalent side, the channell of the Sea being a part deficient of the Sphericall Body of the Earth, and the Needle finding no incouragement to apply it selfe there to.

And so likewise for the Ocean, which palliates the imperfect parts of the Earth, it is evident, that if the [Page] Needle shall equally divide the Marine distance between two continents, there will be found no variation, as having both the Meridians united, the Needle equally disposing itselfe to either Continent. But if we shall swerue from this mediety, it will preponderate with the next continent.

For the evidence of the truth hereof, let a man examaine generally the variations of our most expert Seamen, (a Catalogue wherof we have annexed to that accurate worke of Mr. Wright in his detection and conection of errors in Navigation) (although by reason of the diversities of the setts of their Compasses and oftentimes unapt handling of their instruments, they very seldome concurre) obserued in the Atlantique Ocean, from the Equator unto the parts of Norway, all along the East Coast from the Meridian of the Azores, as farre North as hath beene discovered, and he shall finde the ordinary practise to testifie the truth hereof. As also after the same manner from the Equator southward, unto the streights of Magellan, and all along the backe parts of America in the South Sea, and on the East Coast unto the Cape of Bona Speranza, and he shall perceive the like agreement; But in fayling from the aforesaid Cape further Eastward, that sometime they doe finde it otherwise, the cause proceedeth from the different situation of the South Continent hitherto undiscovered. And whereas in the Atlantique Ocean about 135 leagues from the Ilands of Corues and Flores to the westward,Mr. Iohn Wells. there hath beene found by an able Artist no variation at all, it is a manifest argument, that the Place is equidistant from the Europaean or Affrican continents and the American: And as we approach to either [Page] Continent, so shall we find the Needle praepotent in its conversion; from whence the variation shall receive its denomination.

Therefore its absurd for Seamen to touch their Compasses with the allowance of the variation of any particular place, considering this variation is perpetually variable according to the shipps motion.

And moreover its absolutely necessary for that Seaman who sailes by his Compasse, continually to search the variatiō, that so by the Prosthaphaereticall application thereof, the true point of the compasse (which is his principall Mercurialist) may be rectified.

And this artifice may be performed by the assistance of the Planets or fixed Starres, and that either without or with their Altitudes. I shall not need to exemplify all varieties, as having elswhere copiously and demonstratively delivered the same. I thinke it only convenient to touch lightly at one or two, and such as best conduce to our present purpose.

And first without an Altitude. Finde two fixed Starres of equall right Ascention, and holding vp a thread and plummet, observe when the thread bisects them (for that is the exact Meridian of the place) and at that instant apply the Compasse to them; if the Needle precisely respect them, there is no variation; if not, then is the difference, the variation inquired; and that part of the Horizon to which it inclines, (alwayes accounting from the same intersection of the Horizon and Meridian) claimes it.

It may likewise be performed by the Amplitude Ortive or Occiduall of the Sunne, and as well by night as by day, by the rest of the Planets or fixed Starres, having [Page] onely their declinations and the Latitude of the places, either Geometrically by an Analemme, or Arithmetically by the helpe of the Canon of Triangles, for the ordinary termes of the proportion runne thus.

As the Sine complement of the Latitude, is to the Radius, So the Sine of the declination, to the Sine of the Amplitude. Turning therefore the Compasse to the Sunne or Starres, the difference of the Arcs is the variation inquired. Or by knowing the time of the Sunnes incidence into the Plane of the Prime verticall, after the like manner we may obtaine our desires.

But in these and some other wayes, we are confined to a certaine time: If therefore to the former requisits we shall add an Altitude, this Artifice may be performed at any convenient appearance of a Planet or Starre, as we shall by and by shew.

And for the more easie and certayne obtaining of the variation at Sea, I would advise the Sea-man to extend or prolong the capitall of his Needle, or fix some stile on it, so that it may cut the Flye at right Angles, for then the Compasse coverted to the Sun, this stile will readily shadow out the Azimuth Magneticall, and comparing it with that of the Sunne, he hath the variation neere enough without any further labour or preparation.

Thus hitherto (according to the Tenents of all our Magneticall Philosophers) we have supposed the variations of all particular places to continue one and the same: So that when a Seaman shall happly returne to a place where formerly he found the same variation, he may hence conclude, he is in the same former Longitude. [Page] For it is the Assertion of Mr. Dr. Gilberts. Variatio v [...]icuius (que) Loci constans est, that is to say, the same place doth alwayes retaine the same variation. Neither hath this Assertion (for ought I ever heard) been questioned by any man. But most diligent magneticall observations have plainely offred violence to the same, and proved the contrary, namely that the variation is accompanied with a variation. For whereas in the yeare 1580 Mr. Burrows (a man of unquestionable abilities in the Mathematiques) found the variation at Limhouse neere London to be 11 gr. 15 min. or neere one point of the Compasse; In the yeare 1622 Mr. Gunter sometimes professor of Astronomie in Gresham Colledge, found the variation in the same place to be but 6 gr. 13 min. And my selfe this present yeare 1634 with some friends had recourse to Dedpford (where Mr. Gunter had heretofore made the same observations with those at Limehouse) and found it not much to exceed 4 degrees.

For the clearer manifestation of this truth, (because all men have not beene furnished with or seene the fore said observations, I hold it necessary to transferre theirs hither, and subnect those which were lately made by my selfe with the assistance of some friends.

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Mr. Burrowes his observations made at Limehouse October. 16. 1580.

Before Noone. | After Noone. |
Elevation of the ☉ | variation of the shadow from the North of the Needle to the westwards. | Elevation of the ☉ | variat: of the shadow from the North of the Needle to the Eastward | variat. of the Needle from the Pole or Axis.
Gr. Min. | Gr. Min. | Gr. Min. | Gr. Min. | Gr, Min. Sec.
17: 0 | 52, 35 | 17: 0 | 30: 0 | 11: 17, 30
18: 0 | 50 8 | 18: 0 | 27. 45 | 11. 11. 30
19: 0 | 47 30 | 19: 0 | 24. 30 | 11. 30, 0
20: 0 | 45 0 | 20: 0 | 22: 15 | 11, 22. 30
21: 0 | 42 15 | 21: 0 | 19. 30 | 11. 22, 30
22: 0 | 38 0 | 22: 0 | 15: 30 | 11. 15. 0
23: 0 | 34 40 | 23: 0 | 12. 0 | 11. 20. 0
24: 0 | 29 35 | 24: 0 | 7: 0 | 11. 17, 0
25: 0 | 22 20 | 25: 0 | 0: 8 | 11. 14. 0
|  |  | From N to W |

The way of finding the variation after this manner is easy to be conceived. A time being chosen when the Heavens were like to be unmasked for the whole day, the altitudes of the Sunne were taken by him to each degree of a Quadrant both forenoone & Afternoone, from 17 to 25 gr. making nine severall observations. And at every Altitude turning his Needle to the Sunn, observed the degree and parts cut thereby. Then comparing the Magneticall Azimuthes made in the Forenoone, with those in the Afternoone answerable to the same Solar altitudes, the halfe of the difference is the variation. Thus at the altitude of 17 gr. in the [Page] Forenoone, the Needle cutting 52 gr. 35 min. and at the same altitude in the Afternoone 30 gr. 0 min: the difference is 22 gr. 35 min: the halfe whereof is 11 gr. 17 min: 30 Sec. the variation inquired.

Before we proceed to other Observations it will be requisite we take a review of these, and after correction of some of the apparent Altitudes, call the Solar Azimuthes to the Iudicature of a Triangle.

It is not unknowne that about this time of Mr. Burrowes his observations the accurate and corrected tables of the Sunnes motion were not published to the world, Neither the Angles of the Obliquity and Paralaxes of the Sunne so truely determined. And as for the refraction (which insinuates it selfe in every Altitude not exceeding 45 degrees) being then in its infancy and a stranger to the world it was not called into the account, no not by the Ancients themselues, nor limited before that great Astronomer and honorable Dane Tyche Brahe opened the same way. If therefore we shall examine these Altitudes acording to the late corrected grounds, we shall somewhat alter the variations of Mr. Burrowes, but so as that we shall not much differ from him, or suspect any great error in his worke.

These observations then of his, were made the 16 of October Anno 1580. And by Astronomicall Calculation should begin about 9 ½ in the Morning, and end about 2 ⅓ in the Afternoone. Let us now make choice of the Magneticall Azimuthes answerable to the Altitudes of 20 gr. being the fourth observation. The true place of the Sunne about this time correspondent to this Altitude according to the late Rud [...]lphins Tables, (with [Page] consideration to the difference of Meridians and Equation of Time) is thus found after the Centesimall way with the connexion of our British Logarithmes, but with some alteration of the Anomalies

Motion of the Apog. ☉ | Middle motion of the Sun
Sig. Gr. | Sig. Gr.
Epoch of Christ 2: 83455571829 | 9: 88667238532
yeares 1579 0: 270303979753 | 0: 111927414346
Month: Biss: 9. 0: 00 128419571 | 9: 00672704358
Dayes 14. 0: 000007030268 | 0: 137990576119
Her. 21. Min. 9 0: 00000413028 | 0: 08686013944
Sum of the mot. 3: 53895414449 | 7: 47943947299
Motion of the Apogaum substracted | 3: 53895414449
The middle Anomaly. | 3: 294048532850
Our Tabular middle Anomaly | 3: 29 00000
The difference of Anomalies | 0: 0 40485
The logarithme whereof is | 0. 6072976
The diff. of the Coaequated Anom. | 1. 01729167
The logarithme whereof is | 0: 0074454
The summe of the logarithmes | 0: 6147430
To which there answers | 04118538
which added to the tab: Coaeq: Anom. | 3. 271763527
Gives the inquired coaeq: Anomaly | 3. 275882065
To this the Apog: added | 3. 5 3895414
Gives the Sunnes place in ♏ | 2 9777479
The declination whereof is | 12548280
Whereunto adding a Quadrant | 90000000
The sum is the ☉ dist. from the Pole | 102548280

And because we haue made choice of the apparent Altitude of 20 gr. the Parallax (Kepler admitting little or none,) according to Tycho Brahe will be

found | Gr. 0047778
And the refraction | 0075000
The difference is | 0027222
Which substracted from the apparēt Alt. | 20000000
Giues the true Altitude of the ☉ | 19972778
The complement whereof is the distance |
of the ☉ from the Zenith | 70027222

That the insuing work may be the better cōceived, let FZB in the annexed scheme) be the Meridian of Limehouse; Z the zenith or Pole of the Horizon FOB. The elevation of the Pole PB. ZP the complement thereof. AE the Equator. The Southerne Parallell of the

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Sunne DSO. ZSH a Quadrant of a verticall Circle intersecting the Sunnes parallell, and passing through his Center in the point S. HS the Sunnes Altitude aboue the Horizon; and ZS the complement or his distance from the zenith. SI the declination. SP the distance of the Sunne from the Pole of the world. Also let the obscure periphery MZG be the Magneticall Meridian passing through the zenith Z, and deflecting or

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varying from the Terrestriall Meridian the Arc MB, the measure of the Angle MZB the variation inquired, which will be determined by comparing the magneticall Azimuth CZS with the Solar Azimuth PZS, always greater then the Magneticall before Noone, but lesse after Noone, where the deflection is Easterly as here with us in England and the adjacent parts. But the Magneticall azimuth is altogether found by instrumentall observation, the Solar by Calculation or the resolution of the obliquangular sphaericall Triangle PZS, whose sides are already knowne, and therefore the Angle PZS may be obtained by the fourth compend of the last chapter of the British Trigonometry after this manner.

| Logarithmes
PS. Dist of the ☉ frō the Pale 10254789 | 00269391447
ZS. Compl. of the ☉ altitude 7002722 | 02061682627
ZP. compl. of the Latitude 3846667 |
Sum of the sides 21104178 |
The halfe summe 10552089 | 99838665722
Diff. of the Base and ½ summ. 297299 | 87148768587
| 189318508383
The log. of the sine of 169996 | 94659254191
The complement 730004 |

The double whereof is 1460008 the Angle inquired PZS. But the Magnet. Azim. is 1350000 namely the Angle CZS. The differ. is 110008 MZB the variation inquired, which converted into sexagesmes is 11 gr. 0 min, 0 Sec. Mr, Burrowes by the helpe of the afternoons observation making it 11. gr. 22 min. 30 sec. the difference being 0 gr. 22 min. 30 sec.

The like may we doe with the same Altitude for the Afternoone, if we shall onely rectify the declination answerable to the Sunnes motion for the internall of time. The Sunnes true place was in 3179445 of ♏ the declination correspondent was 1261740. Then in the adiacent diagram, being the same with the former, save onely that the magneticall Azimuth CZS is (as it ought to be) greater then the Solar PZS, we have the same termes given, and therefore by resolving the

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Triangle PZS, the angle inquired PZS may be found either by the former or the third variety belonging to the 11 Probleme of the same booke, and agreeing as

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well with the Calculation of a Plane Triangle as of a Sphaericall, after this manner.

Gr. |
PS. dist. of the ☉ from the Pole 10261740 |
ZS. compl. of the ☉ Altitude 700272 [...] |
ZP. compl. of the Latitude 3846667 |
The sum of the sides 21 [...]129 | Logarithmes
The halfe summe 10555564 | 00162066869
Dif. of the half sum & sides PS 293824 | 12902261021
ZS 3552842 | 97642558829
ZP 6708897 | 99643117219
The summe of the logarithmicall sines | 210350003938
Logarithmical tang: of half the Angle | 105175001969
which found in the canō answereth to | 73104441 the
double whereof is the angle sought | 146208882 PZS
But the Magneticall Azim: is | 157750000 CZS
The difference is the variation | 11541118 CZP.

which converted into sexagesmes, is 11 gr. 32 min. 28: sec. And if with this we compare the morning variation, (which should have been the same with the afternoones) the difference will be 32 min: 28 sec: [Page] which may easily arise either from the Insufficiency or Fabrique or grosse handling of the Instruments. So that if we take the Arithmeticall meane, we may probably conclude the variation answerable to his time to be about 11 gr. 16 min: with which very neere concurreth Dr. Gilbert, finding it 11 gr. 20 min.

From these we proceed to Mr. Gunters Observations, the Table whereof we have here inserted, as we finde them in his mathematicall workes, where we see the variation not to exceede 6 gr. 13 min. His instruments were one Quadrant of three foote Radius for the Sunnes Altitudes: His Needles were two, the one of six, the other of 10 Inches in length. The Sunnes Azimuthes were found by calculation, and the Azimuthes Magneticall, taken off a Horizontall Plane by a line of Chordes, whose Semidiameter was twelue Inches.

Mr. Gunters Observations made at Limehouse Iun: 13 Anno 1622 after Noone.

Alt: ☉ | Azim: Mag | Azim: ☉ | Variation.
Gr. Min. | Gr. Min. | Gr. Min. | Gr. Min.
19 0 | 82 2 | 75 52 | 6 10
18 5 | 80 50 | 74 44 | 6 6
17 34 | 80 0 | 74 6 | 5 54
17 0 | 79 15 | 73 20 | 5 55
16 18 | 78 12 | 72 32 | 5 40
16 0 | 77 50 | 72 10 | 5 40
10 10 | 71 2 | 64 49 | 6 13
0 52 | 70 12 | 64 25 | 5 47

Thus haue we prooved that for the Interstice of 42 [Page] yeares, there hath beene an evident diminution of five, degrees variation. I deny not the Artifice to be very nice and subtle, and that an error may vnawares easily insinuate it selfe, which together with this great discrepance, moved some of us to be overhasty in casting an aspersion of error on Mr. Burrows observations,Mr. Iohn Marr. (though since upon noe just grounds) till an acquaintance of ours, lately applying Mr. Gunters owne Needle to the side of the Cubicall Stone of his Majesties Diall in White Hall garden, could not finde the variation so great as 6 gr. 15 min: formerly found; whereupon resolving with some friends to make an experiment hereof, we went to Diepford the last yeare 1633 the day of the Sunnes entrance into the summer Solstice, to the very same place where Mr. Gunter heretofore had made observation, and found it much lesse then fiue degrees; And afterwards calling into Question the Insufficiency of our Instruments, that all scruple might be quite taken away, we had recourse this present yeare 1634 Ioh. 12. stilo vet: to the same Garden of our learned and ingenuous friend Mr. Iohn Welles, with a Quadrant of six foote Radius for solar Altitudes, continually rectified with great care, and a Horizontall Quadrant of two foote Radius, for the determining of the Magneticall Azimuthes, exactly set to the Magneticall meridian, and in paralellisme to the Horizon; (otherwise great error might ensue) with two Needles of twelue Inches in length well touch't with good Magnetts; And for the better satisfaction, tooke with us also the very same Needle wherewith Mr. Gunter made the foresaid observations, in length ten Inches; all three must, accurately respecting [Page] the same Magneticall Meridian as we then proved. And least there might arise some diversity in the variation through the touches of severall Stones, I caused the one Needle first touch't by a very good Magnet, to be retouch't by another as good if no better, and the same wherwith most of our zea compastes are touch't, and yet found no difference betweene them. And so Iune the 14. 1634▪ we made these following observations, where we found the variation not to exceed 4 gr. 10 min. as appeareth by the Table annexed.

And because in the Resolutions of these obliquangular sphaericall Triangles for the Solar Azimuthes we have necessary use of the Latitude of the Place, the same we determine by the Sunnes meridian Altitude to be 51 gr. 30 min. after this manner.

| Gr.
The apparent Altitude of the Sunne | 62: 0: 0
The Parallax of the Sunne added | 0: 1: 22
Therefore the Sunnes true altitude | 62: 1: 22
Sunnes declination according to Tycho | 23: 31▪ 22
Hence the Altitude of the Equinoctiall | 38: 30. 0
The compl. wherof is the hight of the Pole | 51. 30. 0

Observations made at Diepford An.

Testibus ac secus is D. Welles. Harrison. Marr. Butler. Ho [...]ton. Ho [...]knell.

1634 Iunij 12 before Noone

Alt: ☉ vera | Azim. Mag | Azim. ☉ | variatiō
Gr. Min. | Gr. M. | Gr. M, | Gr. M.
44, 45. | 106, 0 | 110 6 | 4. 6
46, 30, | 109, 0 | 113 10 | 4 10
48. 31, | 113, 0 | 117 1 | 4 1
50, 54, | 118▪ 0 | 122, 8 | 4. 2
54, 24, | 127. 0 | [...]30 55 | 3 5

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After Noone the same day.

Alt. ☉ vera | Azi. Mag | Azim. ☉ | Variation
Gr. Min. | Gr: M. | G. Mn. | Gr, Min
44 37 | 114: 0 | 109. 53. | 4: 7
40 48 | 108: 0 | 103, 50 | 4: 10
38 46 | 105. 0 | 100, 48 | 4. 12
36 43 | 102, 0 | 97. 56 | 4. 4
34 32 | 99, 0 | 95, 0 | 4: 0
32 10 | 96: 0 | 91. 55 | 4: 5

These Concordant Observations can not produce a variation greater then 4 gr. 12 min. nor lesse then 3 gr. 55 min. the Arithmeticall meane limiting it to 4 gr. and about 4 minutes.

And for a further confirmation herof (though moved upon no just reason to suspect these observations, being every way as cautelous in them as possible we could) as soone as the intermission from my publique Exercise would permit, I caused the same instruments to be transported to Paules Cray in Kent, distant from London SE and by S about 12 miles, where the fourth of Iuly following (Fide Astronimica) I made the subsequent animadversions, hardly amounting to 4 degrees.

The Latitude of the Place I finde by the same large Quadrant to be 51 gr. 25 min. which thus I prove.

| o | i | ii
Iuly 21 the ☉ apparent meridian altit. was | 56: | 53: | 0
The Sunnes parallex to be added | 0, | 1. | 36
The sum is the Sunnes true Altitude | 56, | 54, | 36
The declination of the Sunne subtracted | 18, | 19, | 36
The Relique is the altitude of the equinoc. | 38. | 35. | 0
The compl: wherof is the hight of the Pole | 51. | 25, | 0

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Observations made at Pauls Cray in Kent Iuly: 4: 1634: After Noone.

vera alt. ☉ | Azim: mag | Azim: ☉ | variatio
Gr, Min: | Gr, Min: | Gr. Min: | Gr: Min
40: 55. | 111 30 | 107 30 | 4 0
40: 1 | 110 0 | 106 5 | 3 55
39: 41. | 109 30 | 105 34 | 3 56
38: 42. | 108 0 | 104 5 | 3 55
35: 32. | 103 30 | 99 32 | 3 58
34. 49. | 102 30 | 98 32 | 3 58
33, 41, | 101 0 | 97 0 | 4 0
32. 57: | 100 0 | 96 2 | 3 58
32: 9, | 99 0 | 94 58 | 4 2
31, 25, | 98 0 | 94 0 | 4 0
30: 39: | 97 0 | 93 1 | 3 59
29: 29: | 95 30 | 91 31 | 3 59
27, 51: | 93 30 | 89 28 | 4 2

It were needlesse having so many sufficient testimonyes, to produce any more; one onely observation made with circumspection being as good as one hundreth. So that its plaine, the observations made in this place do all make the variation to fall neere upon 4 degrees.

Hence therefore we may conclude that for the space of 54 yeares (the difference of time betweene Mr. Burrowes and these last observations of ours) there hath beene a sensible diminution of 7 degrees and better.

If any affected with magneticall Philosophy shall yet desire to see an experiment made for their owne particular satisfaction, where I may prevaile, I would advise them to pitch a faire stone parallell to the Horizon [Page] there to rest immoveable, and having a Needle of a convenient length strongly touch't by a vigorous Magnet to draw a Magneticall Meridian thereby, and yearely to examine by the application of the same (well preserved from the ayre and rust, its greatest enemies) whether time will produce the like alteration.

I will not here enter into a dispute concerning the cause of this sensible diminution, whether it may be imputed to the Magnet, or the Earth, or both. It is not unknowne to the world, how the Greatest Masters of Astronomie, which this age hath afforded, for the more easy salving the apparent anomalar motions of the fixed and erratique caelestiall lights, and avoyding that supervacaneous furniture of the Ancients, do with all alacrity embrace that admirable Copernicean Hypothesis of the diurnal, Annual, & Secular motions of the earth, in so much as conferring with that Great Astronomer D. Phil. Lansberg. in Zealand about Astronomicall matters, did most seriously affirme unto me, he should never be disswaded from that Truth. This which he was pleased to stile a truth, I should readily receive as an Hypothesis, and so be easily led on to the consideration of the imbecillity of Mans apprehension, as not able rightly to conceive of this admirable opifice of God or frame of the world, without falling foule on so great an absurdity. Yet sure I am, it is a probable inducement to shake a wavering understanding. And which adds somewhat more, I understand by Galilaeus a Florentine, and a most acute and learned Mathematician, that an Italian Gentleman of the Family of the Marsilii hath lately found the mutation of the Terrestriall Meridians, and penned a treatise thereof not yet published, which the said Galilaeus hath seene and read constupore, not without amazement. His words are these.

[Page] Surge di presente una quinta novita, dalle quale si possa arguir mobilita nel globo terrestre, mediante quello, che sottilissimamente va scoprendo l' illustrissimo S. Caesare della nobilissima famiglia de i Marsilij di Bologna, pur' Academico Linceo, il quale in una dottissima scrittura va esponendo, come ha osservato una continua mutazione, benche tardissima, nella linea Meridiana; della quale scrittura, da me ultimamente con stupore veduta, spero che douera fare coppia a tutti gli studio si delle maraviglie della natura.

Which in English runnes thus.

Now there ariseth a fift noveltie, from which we may argue the mobility of the tererestriall Globe, by the meanes of that which most subtllie the most mustruous Gentleman Sig: Caesar of the most noble familie of the Marsilij of Bologna a Lyncean Academicke also, hath discovered, who in a most learned tract plainely sheweth how he hath observed a continuall mutation, although very slow, in the Meridian line; which discourse, which with wonder I lately saw, I hope he will impart to all those who are Studious of the wonders of nature.

Surely if it be so, the due consideration of the premisses doth (as I conceive) lay open a faire way for the [Page] salving of that Irregular Motion imputed to the Axis of the Earth. I meane the alterable Obliquity of the Ecliptique, which to deny (as some yet doe, not without great violence offred to the Ancients) I shall hardly (without very strong argument) be perswaded. I pretermit likewise a suppofition which might be made of the correspondent motion of the Pole of the Magnet with those of some of the Planets, and divers other conceits, which must be all left to future times to discover, this Invention being but newly presented to the world in its infancy.

Gresham College 5 Cal. Aug. 1634.

Errata Typographica.

PAg. 7 lin. 1. for Gilberts read Gilbert. Pag. 7 lin, 16 for D [...]ford read Diepford. p. 9 lin 19 for the same way, read the way. p. 9 lin. 27 for 9 ½ read 9 ⅓. p, 13, lin, 12, for internall reade intervall: pag, 16 lin last for must reade most:

FINIS.

Information about this book

Title statement

A discourse mathematical on the variation of the magneticall needle Together with its admirable diminution lately discovered. By Henry Gellibrand professor of astronomie in Gresham College.

Gellibrand, Henry, 1597-1636.

Edition statement

1635

Publication

Publisher

Text Creation Partnership,

Place of publication

Ann Arbor, MI ; Oxford (UK) :

Date

2008-09 (EEBO-TCP Phase 1).

ID [DLPS]

A01586

ID [STC]

STC 11712

ID [STC]

ESTC S117951

ID [EEBO-CITATION]

99853160

ID [PROQUEST]

99853160

ID [VID]

18529

Availability

This keyboarded and encoded edition of the work described above is co-owned by the institutions providing financial support to the Early English Books Online Text Creation Partnership. This Phase I text is available for reuse, according to the terms of Creative Commons 0 1.0 Universal. The text can be copied, modified, distributed and performed, even for commercial purposes, all without asking permission.

Series

Early English books online.

Notes

(EEBO-TCP ; phase 1, no. A01586)

Transcribed from: (Early English Books Online ; image set 18529)

Images scanned from microfilm: (Early English books, 1475-1640 ; 1237:05)

Source

— A discourse mathematical on the variation of the magneticall needle Together with its admirable diminution lately discovered. By Henry Gellibrand professor of astronomie in Gresham College., Gellibrand, Henry, 1597-1636..

Extent

[2], 22 p.

Printed by William Iones, dwelling in Red-crosse-street,. London :: 1635.. (Reproduction of the original in the Bodleian Library.)

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