Page:Cyclopaedia, Chambers - Supplement, Volume 1.djvu/634

 EAR

Dempfter fays, it was cuftomary, in his time, for the king, and nobility of England, to go a hunting in the fpring, drefled in green, that they might be of a fimilar colour with the feafon. Hofm. Lex. in voc. The word comes from the Greek E«p, the fpring.

EARING, in a ihip, that part of the bolt-rope, which, at the four corners of the fail, is left open, in the fhape of a ring. iThe two uppermoft parts are put over the ends of the yard- arms, and fo the fail is made faft to the yard ; and into the lowermoft Earings the fheets and tackles are feized, or bent at the clew.

EARTH [tycly— Figure of the Earth— That the figure of the Earth is fpheroidical, is agreed upon by all : But whe- ther it be an oblong, or oblate fpheroid, i. e. whether the axis be longer or fhortcr than a diameter at the equator, has been for fome time a matter of doubt.

Three feveral methods have been propofed to determine this controverfy; i. By experiments, as by the different lengths of pendulums vibrating feconds, in different latitudes. 2. The figure of the earth's fhadow in lunar eclipfes ; And 3. By. the actual meafurement of the lengths of a degree on the meri- dian, in different latitudes.

It is certain, if the lengths of the degrees of latitude decreafe, as we go from the equator toward the poles, then the axis is greater, and the figure an oblong fpheroid ; but, on the con- trary, if thefe lengths increafe, as you remove towards the poles, the axis is lefs than a diameter at the equator, and con- sequently the figure an oblate fpheroid.

Mr. Caffini, and others, judge the Earth to be of an oblong fpheroidical figure ; and the cbfervations made in France, if entirely to be depended upon, prove this hypothefis to be a matter of fact.

Sir Ifaac Newton, Mr. Huygens, and others, make the earth to be an oblate fpheroid, higher at the equator than at the poles ; and this figure of the Earth is undoubtedly the true one, if the obfervations lately made near the arctic circle be admitted as certain and exact.

Mr. Celfius takes notice that the inftruments, aftronomical obfervations, and trigonometrical operations, performed in France, render the obfervations there made uncertain. The obfervations made at Tornea were undertaken at the charge of the king of France, by five fkilful gentlemen, three of them members of the royal academy at Paris, who were joined by Mr. Celfius and the Abbe Outhier. The trigo- nometrical part of the work was performed near the river of Tornea, the direction of which is the fame with the meridian of Tornea; the coafts of the gulph of Bothnia being found very inconvenient for that purpofe. By the favourable fitua- tion of five mountains, they formed eight triangles, which took in fpace enough for their defign. All the five gentle- men obferved, one after another, each angle of thefe trian- gles, fetting them down in writing feparately. JThey afterwards determined the diftance between Tornea and mount Kittis, under the fame meridian, by a bafis, mea- sured on the river, when frozen over, the length of which was 7406 toifes 5 feet, by the firft meafurement ; and, when meafured again, was barely four inches over. This diftance between the two places they found to be 55023^ toifes. The firft part of their work being thus finifhed, the next was to find the difference of latitude of thefe two places ; this they did by the help of a telefcope, fixed to a fector of nine feet, made at London, by the care and direction of Mr. George Graham, to whom the lovers of aftronomy are in- debted for the curious and well-contrived inftruments with which he has fuppHed them. The ftar they obferved at Tor- nea was a, Draconis. They repeated their obfervations three times, and the greateft difference between them was but two feconds. Removing to mount Kittis, they took the fame number of obfervations, of the fame ftar, without finding more than one fecond of difference. The refult was, that the amplitude of the arch, in the heavens, between Tornea and mount Kittis (allowing for the preceffion of the equinox, and the time elapfed between the two obfervations, according to Mr. Bradley's Theory) was 57 minutes 26 feconds. Hence the magnitude of a degree, on the earth, inter feeling the polar circle, was found to be 57437-r^ toifes, which is greater than a mean degree of France, or 57060 toifes, by 377-fjs toifes J an d differs 900 toifes from what it fliould have been, according to Mr. Caffmi's hypothefis : And if the correction, according to Mr. Bradley's theory, were omitted, the difference would have amounted to above a thoufand toifes ; the confequence of which is, that the Earth is not only flatted towards the poles, but that it is much more fo than Sir Ifaac Newton or Mr. Huygens thought it. This unexpected difference being fo very great, made them refolve upon a careful, as well as new kind of verification cf the whole. In the firft place, they repeated their aftronomi- cal obfervations three feveral times at Tornea and Kittis, with the fame inftrument, but on another ftar, viz. & Dra- conis : -The difference of latitude between the two places was found to be the fame) within three feconds and an half, with the firft : They then not only examined the truth of their meridian line, the exactnefs of the fector, in the different di- viftons upon the limb, chiefly in the two degrees employed

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in observing « and £ draconis, but fuppofed that in their tri ' gonometrical operations they had erred in each triangle b~- twenty feconds in each of the two angles, and forty f/ conds in the third ; and that all thefe errors tended to dim " nifh the length of the arch ; the calculation, upon this IIid porition, gives but 54I tcifes for the greateft error that could be "committed. See Phil. Tranf. N°. 457. Sect. 1. conv pared with Mr. Maupcrtuis's rig. of the Earth determined, p. 1.62 of the Traiuliit. by Mi. Murdoch. We have faid that. Mr. Celfius doubted uf the. accuracv.of the French obfervations, and he gives good reafons for hi/ doubts 1 heir angles were not accurately determined, and when thefe are properly corrected, thole very, meafures of a degree; in the fouth and north, of France, brought to overturn gi r Ifrac Newton's theory, have, like the meafures taken j n Lapland, confirmed.it. See Phil. Tranf. N.°. 457. Sect. 1. Mr. Celfius's diuertation, entitled, Jpe objervationibus p Q fignra tclluris deter mi panda, in GaJIta hahitis difqulfuio^ Upfal. 1738. 4 . may be farther confultcd. By the obfervations in the north of- France, extending from Paris to Dunkirk, a mean degree was, found to confift of 56960 toifes. And by the observations made in' the fouth of France, from Paris to Collioure, near the Pyrenees, a degree was, found to be 57097 toifes. Frpm. whence it would fol- low, that the Earth is an oblong fpheroid. But. Mr. Celfius obferves, that the diftance between the royal obfervatory at Paris, and the perpendicular to the meri- dian of Collioure, properly corrected*, will amount to no more than 358980 toifes. This, divided by the mean dif- ference of their latitudes 6°. 19'. 11". will give 56803 toifes for the length of a degree, one with another, between Paris and Collioure, which is pretty near the truth, and lefs than the length of a mean degree, found by Mr. Picard, and rated at 57060 toifes : So that the degrees decreafe as you go to- wards the equator; and confequently the Earth is higher there than at the poles, as Sir Jfaac Kcwtgn and Mr. Huy- gens determined. Phil. Tranf. loc. cit. The obfervations of fome gentlemen of the French academy, who have been at the pains to re-examine the meafures for- merly taken in France, confirm the Newtonian theory. And the obfervations made in Peru are an additional con- firmation of the fame. See Mr. Bouguer's book lately pub- liflied.

The meafure of a degree of the meridian in the latitude of 66°. 20'. was found to be 574.37-5% French toifes, but in the latitude of 49. 21'. only 57183 toifes. Suppcfmg thofe de- grees accurately meafured, the axis, or diameter, that paftes through the poles, will be to the diameter of the equator as 177 to 178. Hence, the Earth will be 22 miles higher at the equator, than at the poles », If the earth was of an uni- form dcnfity from the furface to the center, then, according to the theory of gravity, the meridian would be an exact ellipfis, and the axis would be to the diameter of the equator, as 230 to 231, and the difference of the femi-diameter of the equator, and the femi-axis, would be about 17 miles b. — [ a Treat. Pract. Geom. p. 44, 45. *. Ibid. p. 45.] See the article Degree.

According to Mr. Maupertuis's meafures, if the femi-diameter of the equator be = 1, and the femi-axis = a ; Then will 1 — a a =0.022. See Mr. Murdoch's Mercat. Sailing.

p -7*.

Or if the femi-diameter of the equator be to the diftance of

the focus of the generating ellipfis from the center, as m to r, then will mm : 1 r ; 1000 : 22. For, by the nature of the ellipfe, 1:^/1 — a a 1 ; tn 1 1, or 1:1 — a a : : m m : 1 ; confequently mm : r : : 1 : 0. 022, or mm.', I :.: -looo : 22. This is the analogy made ufe of to compute the meridional parts in a fpheroid. See Meridional Parts. Mr. Eifeniehmidt having compared the meafures of a de- gree, taken in different latitudes, by Riccioli, Snellius, csV. found that the lengths of degrees grew lefs towards the pole, and hence, admitting thefe meafures to be exact, he jufty inferred, that the figure of the Earth was oblong, and not oblate. This was the hypothefis Mr. Caffini endeavoured afterwards to confirm. Dr. Burnet, in his theory of the Earth, has mentioned Mr. Eifenfchmidt and his conclufion ; for which, both the Doctor, and the author he quotes, were roughly cenfurcd bv Mr. Jo. Keill, who draws the contrary conclufion from Eifenfchmidt's facts ; but Keill himfelf was here greatly miftaken : Eifenfchmidt and Burnet were mifled by inaccurate meafures, but did not draw falfe confequences from thofe fuppofed facts. Jo. Cafp. Eifenfchmidii diatribe de figura telluris elliptico-fpharcidc, &c. Argentorati, 1691. 4.

Changes of the Earth — Mr. Boyle fufpects there are great, though flow internal changes, in the mafs of the Earth. He argues from the varieties obferved in the change of the mag- netic needle, and from the obferved changes in the tempera- ture of climates. But as to the latter, we doubt whether he could have diaries of the weather fufheiently exact to di- rect his judgment. Boyle's Works abr. vol. I. p. 292, 293. See Sinking of the Earth.

Magnetifm of the Earth— The notion of the magnetifm of the Earth was ftarted by Gilbert ; and Mr. Boyle fuppofes

- " magnetic