| XXX | (476) | XXX |
476 ASTRO N O M Y. at her third quarter. In fpring, the phenomema of the and calls a lhadow towards that point of the heavens sirfl quarter anfwer to thofe of the third quarter in au- which is oppofite to the fun. This lhadow is nothing tumn ; and vice vsrfa. The nearer any time is to ei- but a privation of light in the fpace hid from the fun by ther of thefe feafons, the more the tides partake of the the opaque body that intercepts his rays. phenomena of thefe feafons ; and in the middle between When the fun’s light is fo intercepted by the moon, any two of them the tides are at a mean ftate between that to any place of the earth the fun appears partly or thofe of both. wholly covered, he is faid to undergo an eclipfe •, though, In open Teas, the tides rife but to very fmall heights properly fpeaking, it is only an eclipfe of that part of the in proportion to what they do in wide-mouthed rivers, earth where the moon’s lhadow or penumbra falls. opening in the direiftion of the ftream of tide. For, in When the earth comes between the fun and moon, channels growing narrower gradually, the water is accu- the moon falls into the earth’s lhadow; and, having no mulated by the opposition of the contra<5Hng bank ; like light of her own, Ihe fuffers a real eclipfe from the ina gentle wind, little felt on an open plain, but ftrong terception of the fun’s rays. When the fun is eclipfed and brilk in a ftreet; efpecially if the wider end of the to us, the moon’s inhabitants, on the fide next the earth, ftreet be next the plain, and in the way of the wind. fee her lhadow like a dark fpot travelling over the earth, The tides are fo retarded in their paffage through dif- about twice as fall as its equatoreal parts move, and the ferent flioals and channels, and otherwife fo varioufly af- fame way as they move. When the moon is in an e fected by linking again!! capes and headlands, that to clipfe, the fun appears eclipfed to her, total to all thofe different places they happen at all diftances of the, moon parts on which the earth’s lhadow falls, and of as long from the meridian ; confequently at all hours of the lu- continuance as they are in the lhadow. nar day. The tide propagated by the moon in the Ger- That the earth is fpherical (for the hills take off no man ocean, when the is three hours pal! the meridian, more from the roundnefs of the earth, than grains of takes 12 hours to come from thence to London-^Bridge * dull do from the roundnefs of a common globe) is eviwhere it arrives by the time that a new tide is raifed in dent from the figure of its lhadow on the moon; which the ocean. And therefore when the moon has north de- is always bounded by a circular line, although the earth clination, and we Ihould expeCl the tide at London to be is inceflantly turning its different fides to the moon, and greatel! when the moon is above the horizon, we find very feldom Ihews the fame fide to her in different eclipit is leal!; and the contrary when fhe has fouth decli- fes, becaufe they feldom happen at the fame hours. nation. At feveral places it is high water three hours Were the earth fhaped like a round flat plate, its lhadow before the moon comes to the meridian ; but that tide would only be circular when either of its fides direftly which the moon pulhes as it were before her, is only the faced the moon; and more or lefs elliptical as the earth tide oppofite to that which was raifed by her when Ihe happened to be turned more or lefs obliquely towards tbs moon when Ihe is eclipfed. The moon’s different phafes was nine hours pall the oppofite meridian. There are no tides in lakes, becaufe they are generally prove her to be round; for, as Ihe keeps Hill the fame fo fmall, that when the moon is vertical Ihe attraCls e- fide towards the earth, if that fide were flat, as it appears every part of them alike, and therefore, by rendering to be, Ihe would never be vifible from the third quarter all the water equally light, no part of it can be raifed to the firfl; and from the firli quarter to the third, Ihe higher than another. The Mediterranean and Baltic would appear as round as when we fay Jhe is full; befeas fuffer very fmall elevations, becaufe the inlets by caufe, at the end of her firll quarter, the fun’s light which they communicate with the ocean are fo narrow, would come as fuddenly on all her fide next the earth, that they cannot, in fo Ihort a time, receive or difcharge as it does on a flat wall, and go off as abruptly at the end of her third quarter. enough to raife or fink their furfaces fenfibly. Air being lighter than water, and the furface of the If the earth and fun were equally large, the earth’s atmofphere being nearer to the moon than the furface lhadow would be infinitely extended/ and all of the fame of the fea, it cannot be doubted that the moon raifes bulk ; and the planet Mars, in either of its nodes and much higher tides in the air than in the fea. And there- oppofite to the fun, would be eclipfed in the earth’s lhadow. fore many have wondered why the mercury does not fink Were the earth larger than the fun, its Ihadow would inin the barometer when the moon’s adtion on the particles creafe in bulk thefartherit extended, and would eclipfe the of air makes them lighter as Ihe palfes over the meridian. great planets Jupiter and Saturn, with all their moons, But we mull confider, that as thefe particles are ren- when they were oppofite to the fun. But as Mars, in dered lighter, a greater number of them is accumulated, oppofition, never falls into the earth’s lhadow, although until the deficiency of gravity be made up by the height he is not then above 42 millions of miles from the earth, of the column ; and then there is an equilibrium, and it is plain that the earth is much lefs than the fun ; for confequently an equal prefiure upon the mercury as be- otherwife its lhadow could not end in a-point at fo fmall fore ; fo that it cannot be affedted by the aerial tides. a diftance. If the fun and moon were equally large, the moon’s Ihadow would go on to the earth with an equal and cover a portion of the earth’s furface more Chap. XVI. Of Eclipfes : Their Number breadth, than miles broad, even if it fell direftly again!! the and Periods, A large Catalogue of ancient earth’s2000 centre, as feen from the moon ; and much more if and modern Eclipfes. it fell obliquely on the earth : But the moon’s lhadow is falls Every planet and fatellite is illuminated by the fun ; feldom 150 miles broad at the earth, »unlefs when it very
