Page:Encyclopædia Britannica, first edition - Volume I, A-B.pdf/569

 473 ASTRO N O M Y. on 0 will perceive H rifing higher toward Z. pole whilft (he defcribes the northern half of the eclip- InfpeCtator like manner, 0 being more ftrongly attracted than D, tic or from her third quarter to her firft; and it will farther towards M than D does; confebelow the horizon during her progrefs through the fouth- quently move the diftance between 0 and D will be increafed, ern half on 0, not perceiving his own motion, at the full. But in winter, when the earth is at E, and will D receding farther from him towards n; all efits north pole declines from the fun, the new moon at D fects fee appearances being the fame, whether D recedes is at her greatell depreflion below the horizon NIVS, and from and 0, or 0 from D. the full moon at B at her greateft height above it, rifing there is a number of bodies, as at her firft quarter and keeping above the horizon till Suppofe nowplaced round 0, fo as to form a ilexible {he comes to her third quarter C. At a mean ftate fhe orABCDEFGH, then, as the whole is attracted towards is 23-r degrees above the horizon at B and b, and as M,fluidthe ring; parts at H and D will have their diftance much below it at D and d, equal to the inclination of from 0 increafed; whilft the parts at B and F, being the earth’s axis F. SSB and Srs are, as it were, a ray ‘nearly at the fame diftance from M as 0 is, thefe pans of light proceeding from the fun to the earth; and will not recede from one another, but rather, by the obfhews, that when the earth is at e, the fun is above the lique of M, they will approach nearer to 0. horizon, vertical to the tropic of Cancer; and when the Henceattraction the fluid ring will form itfelf into an ellipfa earth is at E, he is below the horizon, vertical to the ZJBLnKFNZ, whofe longer axis nOZ produced will tropic of Capricorn. pafs through M, and its ftiorter axis BOF will terminate in B and F. Let the ring be filled with bodies, fo as to a fluid fphere round G; then, as the whole moves Chaf. XV. Of the Ebbing and Flowing form toward M, the fluid fphere being lengthened at Z and of the Sea. n, will aflume an oblong or oval form. If M is the moon, O the earth’s centre, ABCDEFGHxht fea coThe caufe of the tides was difcovered by Kepler, vering the earth’s furface, it is evident, by the above who, in his Introdufiion to the Phjftcs of the Heavens, reafoning, that whilft the earth by its gravity falls .tothus explains it: “ The orb of the attracting power, ward the moon, the water direCHy below her at B will “ which is in the moon, is extended as far as the earth, fwell and rife gradually towards her; alfo the water at ■“ and draws the waters under the torrid zone, adting D will recede from the centre, (ftriCtly fpeaking the cen“ upon places where it is vertical, infenfibly oh confined tre recedes from D), and rife on the oppofite fide.of tire “ feas and bays, but fenfibly on the ocean, whofe beds earth, whilft the water at B and F is deprefled, and falls “ are large, and the waters have the liberty of recipro- below the former level. . Hence,, as the earth turns round “ cation; that is, of rifing and falling.” And in the its axis from the moon to the moon again in 24^ hours, 70th page of his Lunar AJlronomy:—“ But the cauje there will be two tides of flood and two of ebb in that " of the tides of the fea appears to be the bodies of the time, as we find by experience. “ fun and moon drawing the waters of the fea.” This As this explanation ©f the ebbing and flowing of the hint -being given, Sir Ifaac Newton improved it, and fea is deduced from the earth’s conftantly falling toward wrote fo amply on the fubjeft, as to make the theory of the moon by the power of gravity, fome may find a difthe tides in a manner quite his own; by difcovering the ficulty in conceiving how this is poflible, when the moon caufe of their rifing on the fide of the earth oppofite to is full, or in oppofition to the fun, fince the earth rethe moon. For Kepler believed, that the prefence of volves about the fun, and muft continually fall towards the moon occafioned an impulle which caufed another in it, and therefore cannot fall contrary ways at the fame her abfence. time; or if the earth is conftantly falling towards the The power of gravity diminifhes as the fquare of the moon, they muft come together at laft. To remove this diftance increafes; and therefore the waters (Plate difficulty, let it be confidered, that it is not the centre XLIV. fig. 4.) at Z on the fide of the earth ABCDEFGH of the earth that defcribes the annual orbit round the next the moon M are more attracted than the central fun, but the common centre of gravity of the earth and parts of the earth 0 by the moon, and the central parts moon together; and that whilft the earth is moving are more attracted by her than the waters on the oppo- round the fun,, it alfo defcribes a circle round that centre fite fide of the earth at n; and therefore the diftance be- of gravity, going as many times round it.in one revolutween the earth’s centre and me waters on its furface tion about the fun as there are lunations or courfes of under and oppofite to the moon will be increafed. For, the moon round the earth in a year; and therefore the let there be three bodies at H, G, and D, if they are earth is conftantly falling towards the moon from a tanall equally attracted by the body M, they will all move gent to the circle it defcribes round the faid common equally faft toward it, their mutual diftances from each centre of gravity. In Plate XLV, fig. 1. let Af be the other continuing the fame. If the attraction of M. is moon, 77T.part of the tnpon’s orbit, and C the centre unequal, then that body which is molt ftrongly attracted of gravity of the earth and moon ; whilft the moon goes will move fafteft, and this will increafe its diftance from round her orbit, the centre of the earth defcribes the the other body. Therefore, by the law of gravitation, circle ged round C, to which circle gak is a tangent; M will attract H more ftrongly than it does 0, by which and therefore when the moon has gone from 71/ to a litthe diftance between H and 0 wilj.be increaled, and-a tle paft IV, the earth has moved from g to j and in that Vol. I. Numb, 20. 3 6D time
 * higheft at the change, moll deprefled and a fpeCtator