| XXX | (474) | XXX |
474 ASTRO N O M Y. time has fallen towards the moon, from the tangent at a little under and oppofite to the fun at 0 and //, and fall as much under and oppofite to the moon at Z and N, to f, and fo round the whole circle. The'fun’s influence in raifing the tides is but fmall in making what we call the neap-tides, becaufe the fun and comparifon of the moon’s: For though the earth’s dia- moon then act crofs-wife to each other. But, ftrictly meter bears a confiderable proportion to its diftance from fpeaking, thefe tides happen not till fome time after; bethe moon, it is next to nothing when compared with the caufe in this, as in other cafes, the aftions do not prodiftance of the fun. And therefore, the difference of duce the greateft effect when they are at the ftrongeft, the fun’s attraftion on the fides of the earth under and but fome time afterward. ' oppofite to him, is much lefs than the difference of the The fun being nearer the earth in winter than in fummoon’s attra&ion on the fides of the earth under and mer, is of courfe nearer to it in February and October oppofite to her; and therefore the moon muft raife the than in March and September; and therefore the greateft tides much higher than they can be raifed by the fun. tides happen not till fome time after the autumnal equiOn this theory, fo far as we have explained it, the nox, and return a little before the vernal. tides ought to be higheft diredily under and oppofite to the The fea being thus put in motion, would continue to moon; that is, when the moon is due north and fouth. ebb and flow for feveral times, even though the fun and But we find, that in open feas, where the water flows free- moon wpre annihilated, or their influence ftpould ceafe: ly, the moon M (Plate XLIV. fig. 4.) is generally part the as if a bafon of water were agitated, the water would north and fouth meridian, as at p, when it is high water continue to move for fome time after the bafon was left at Z and at n. The reafon isiobvious; for though the to ftand ftill. Or like a pendulum, which having been moon’s attraction was to ceafe altogether when Ihe was put in motion by the hand, continues to make feveral vipaft the meridian, yet the motion of afcent communi- brations without any new impulfe. cated to the water before that time would make it con- When the moon is in the equator, the tides are etinue to rife for fome time after; much more muft it do qually high in both parts of the lunar day, or time of the fo when the attraction is only diminilhed: as a little im- moon’s revolving from the meridian to the meridian apulfe given to a moving ball will caufe it ftill tp move gain, which is 24 hours 48 minutes. But as the moon farther than otherwife it could have done. And as ex- declines from the equator towards either pole, the tides perience (hews, that the day is hotter about three in the are alternately higher and lower at places having north or afternoon, than when the fun is on the meridian, becaufie fouth latitude. For one of the higheft elevations, which is that under the moon, follows her towards the pole to of the increment made to the heat already imparted. The tides anfwer not always to the fame diftance of which {he is neareft, and the other declines towards the the moon from the meridian at the fame places, but are oppofite pole ; each elevation defcribing parallels as far varioufly affefted by the a&ion of the fun, which brings diftant from the equator, on oppofite fides, as the moon them on fooner when the moon is in her firft and third declines from it to either fide ; and confequently, the quarters, and keeps them back later when fhe is in her parallels defcribed by thefe elevations of the water are fecond and fourth; becaufe in the former cafe the tide twice as many degrees from one another, as the moon raifed by the fun alone would be earlier than the tide is from the equator ; increafing their diftance as the moon increafes her declination, till it be at the greateft, raifed by the moon, and in the latter cafe later. The moon goes round the earth in an elliptic orbit, when the faid parallels are, at a mean ftate, 47 degrees and therefore (he approaches nearer to the earth than her from one another : and on that day, the tides are moft mean diftance, and recedes farther from it, in every lu- unequal in their heights. As the moon returns toward nar month. When (he is neareft, (he attra&s ftrongeft, the equator, the parallels defcribed by the oppofite eleand fa raifes the tides moft; the contrary happens when vations approach towards each cither, until the moon fhe is fartheft, becaufe of her weaker attraction. When comes to the equator, and then they coincide. As the both luminaries are in the equator, and the moon in Pe- moon declines toward the oppofite pole, at equal dirigeo, or at her leaft diftance from the earth, (he raifes ftances,„each elevation defcribes the fame parallel in the the tides higheft of all, efpecially at her conjunction and other part of the lunar day, which its oppofite elevation oppofition ; both becaufe the equatoreal parts have the defcribed before. Whilft the moon has north declinagreateft centrifugal force from their defcribing the largeft tion, the greateft tides in the northern hemifphere are circle, and from the concurring actions of the fun and when flie is above the horizon ; and the reverfe whilft moon. At the change, the attractive forces of the fun her declination is fouth. In Plate XLV. let NE5P>_ and moon being united, they diminiftt the gravity of the be the earth, NCS its axis, £^'the equator, T25 waters under the moon, and their gravity on the op- the tropic of Cancer, trs the tropic of Capricorn, pofite fide is diminiflred by means of a greater centrifugal ab the ardtic circle, cd the antardHc, N the north pole, force. At the full, whilft the moon raifes the tide un- S the fouth pole, M the moon, F and G the two emider and opppfite to her, the fun acting in the fame line, nences of water, whofe loweft parts are at a and d, raifes the tide under and oppofite to him; whence their (fig. 3.}, at Wand 5, (fig. 4.), and at foind c,. (fig. 5.), conjoint effect is the fame as at the change; and in both always 90 degrees from the higheft. Now when the cafes, occafion what we call the fpring-tides. But at moon is in her greateft north declination at M, the the quarters, the fun’s action on the waters at 0 and H higheft elevation G under her, is on the tropic of Can(Plate XLV. fig. 2.) diminifties the effect of the moon’s cer, TOn, and the oppofite elevation F on the tropic of action on the waters at Z and N fo that they rife a Capricorn trs ; and thefe two elevations defcribe the tropics
