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wise to the Moon. In winter, the phenomena are the same at full Moon as in summer at new. In autumn, the Earth's axis inclines sidewise to the Moon when new and full; therefore the tides ought to be equally high, and unequal in their returns at these times. At the first quarter, the tides of flood should be least when the Moon is above the horizon, greatest when she is below it; and the reverse at her third quarter. In spring, the phenomena of the first quarter answer to those of the third quarter in autumn; and vice versa. The nearer any time is to either of these seasons, the more the tides partake of the phenomena of these seasons; and in the middle between any two of them, the tides are at a mean state between those of both.

rise

308. In open seas, the tides rise but to very small Why the heights in proportion to what they do in wide-mouth-higher in ed rivers, opening in the direction of the stream of rivers than tide. For, in channels growing narrower gradually, in the sea, the water is accumulated by the opposition of the contracting bank. Like a gentle wind, little felt on an open plane, but strong and brisk in a street; especially if the wider end of the street be next the plane, and in the way of the wind.

happen at

from the

309. The tides are so retarded in their passage The tides through different shoals and channels, and otherwise all distan so variously affected by striking against capes and ces of the headlands, that to different places they happen at all Moon distances of the Moon from the meridian; conse- meridian quently at all hours of the lunar day. The tide at differproent places, pagated by the Moon in the German ocean when and why. she is three hours past the meridian, takes 12 hours to come from thence to London-bridge; where it arrives by the time that a new tide is raised in the ocean. And therefore when the Moon has north declination, and we should expect the tide at London to be greatest when the Moon is above the horizon, we find it is least; and the contrary when she has

The water never rises in lakes.

The Moon raises

tides in the

air.

south declination. At several places it is high-water three hours before the Moon comes to the meridian; but that tide which the Moon pushes as it were before her, is only the tide opposite to that which was raised by her when she was nine hours past the opposite meridian.

310. There are no tides in lakes, because they are generally so small, that when the Moon is vertical she attracts every part of them alike, and therefore by rendering all the water equally light, no part of it can be raised higher than another. The Mediterranean and Baltic seas have very small elevations, because the inlets by which they communicate with the ocean are so narrow, that they cannot in so short a time receive or discharge enough to raise or sink their surfaces sensibly.

311. Air being lighter than water, and the surface of the atmosphere being nearer to the Moon than the surface of the sea, it cannot be doubted that the Moon raises much higher tides in the air than in the sea. And therefore many have wondered why the mercury does not sink in the barometer when the Moon's action on the particles of air makes them lighter as she passes over the meridian. But we must consider, that as these particles are rendermercury ed lighter, a greater number of them is accumulated, in the bar- until the deficiency of gravity be made up by the not affect height of the column; and then there is an equili ed by the brium, and consequently an equal pressure upon the mercury as before; so that it cannot be affected by the aerial tides.

Why the

ometer is

aerial

tides.

CHAP. XVIII.

Of Eclipses: Their Number and Periods. A large
Catalogue of Ancient and Modern Eclipses.

312.

E

what.

VERY planet and satellite is illuminated A shadow by the Sun, and casts a shadow toward that point of the heavens which is opposite to the Sun. This shadow is nothing but a privation of light in the space hid from the Sun by the opaque body that intercepts his rays.

of the Sun

313. When the Sun's light is so intercepted by Eclipses the Moon, that to any place of the Earth the Sun and Moon, appears partly or wholly covered, he is said to un- what. dergo an eclipse; though, properly speaking, it is only an eclipse of that part of the Earth where the Moon's shadow or * penumbra falls. When the Earth comes between the Sun and Moon, the Moon falls into the Earth's shadow; and having no light of her own, she suffers a real eclipse from the interception of the Sun's rays. When the Sun is eclipsed to us, the Moon's inhabitants on the side next the Earth (if any such inhabitants there be) see her shadow like a dark spot travelling over the Earth, about twice as fast as its equatorial parts move, and the same way as they move. When the Moon is in an eclipse, the Sun appears eclipsed to her, total to all those parts on which the Earth's shadow falls, and of as long continue as they are in the shadow.

314. That the Earth is spherical (for the hills take A proof off no more from the roundness of the Earth, than that the grains of dust do from the roundness of a common Moon are

Earth and

globular

*The penumbra is a faint kind of shadow all round the perfect bodies. shadow of the planet or satellite, and will be more fully explained by and by.

and that

globe) is evident from the figure of its shadow on the Moon; which is always bounded by a circular line, although the Earth is incessantly turning its different sides to the Moon, and very seldom shews the same side to her in different eclipses, because they seldom happen at the same hours. Were the Earth shaped like a round flat plate, its shadow would only be circular when either of its sides directly faced the Moon; and more or less elliptical as the Earth happened to be turned more or less obliquely toward the Moon when she is eclipsed. The Moon's different phases prove her to be round, § 254; for as she keeps still the same side toward the Earth, if that side were flat, as it appears to be, she would never be visible from the third quarter to the first; and from the first quarter to the third, she would appear as round as when we say she is full: because at the end of her first quarter the Sun's light would come as suddenly on all her side next the Earth, as it does on a flat wall, and go off as abruptly at the end of her third quarter.

315. If the Earth and Sun were of equal magnithe Sun is tudes, the Earth's shadow would be infinitely extendmuch bigger than ed, and every where of the same diameter; and the the Earth, planet Mars, in either of its nodes, and opposite to the Moon Sun, would be eclipsed in the Earth's shadow. Were much less. the Earth bigger than the Sun, its shadow would in

and the

crease in bulk the farther it extended, and would eclipse the great planets Jupiter and Saturn, with all their moons, when they were opposite to the Sun. But as Mars in opposition never falls into the Earth's shadow, although he is not then above 42 millions of miles from the Earth, it is plain that the Earth is much less than the Sun; for otherwise its shadow could not end in a point at so small a distance. If the Sun and Moon were of equal magnitude, the Moon's shadow would go on to the Earth with an equal breadth, and cover a portion of the Earth's sur

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face more than 2000 miles broad, even if it fell directly against the Earth's centre, as seen from the Moon; and much more if it fell obliquely on the Earth but the Moon's shadow is seldom 150 miles broad at the Earth, unless when it falls very obliquely on it in total eclipses of the Sun. In annular eclipses, the Moon's real shadow ends in a point at some distance from the Earth. The Moon's small distance from the Earth, and the shortness of her shadow, prove her to be less than the Sun. And as the Earth's shadow is large enough to cover the Moon, if her diameter were three times as large as it is (which is evident from her long continuance in the shadow when she goes through its centre) it is plain that the Earth is much larger than the Moon.

ets never

ther.

316. Though all opaque bodies on which the Sun The prishines have their shadows, yet such is the bulk of mary plan the Sun, and the distances of the planets, that the eclipse primary planets can never eclipse one another. A one anoprimary can eclipse only its secondaries or be eclipsed by them; and never but when in opposition to, or conjunction with, the Sun. The Sun and Moon are so every month: whence one may imagine that these two luminaries should be eclipsed every month. But there are few eclipses in respect to the number of new and full Moons; the reason of which we shall now explain.

317. If the Moon's orbit were coincident with Why' there are the plane of the ecliptic, in which the Earth always so few moves, and the Sun appears to move, the Moon's eclipses. shadow would fall upon the Earth at every change, and eclipse the Sun to some parts of the Earth. In like manner, the Moon would go through the middle of the Earth's shadow, and be eclipsed at every full; but with this difference, that she would be totally darkened for above an hour and an half; whereas the Sun never was above four minutes totally eclipsed by the interposition of the Moon. But one The half of the Moon's orbit is elevated 5 degrees above Moon's

LI

nodes.

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