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is towards the Sun, and is thrown off, by Gravitation, the contrary Way. For as in our Air, the Smoke of a heated Body afcends from the Earth, towards which the Body gravitates; fo in the Heavens, where all Bodies gravitate towards the Sun, Smoke or Vapour muft afcend from the Sun; therefore the Vapour which is raised from that Side of a Comet which is towards the Sun, is turned back again, and thrown the contrary Way. Confequently when the Comet is oppofite to the Sun, and the heated Side is towards us, the Vapour, in returning back, is feen round the Edge of the Disk of the Comet, and helps to form what is vulgarly called the Hair.

N. B. The Tail or Beard of a Comet appears longer or fhorter, not only as it is really projected to a further or less Distance from the Head, but as it appears to us making a greater or lefs Angle, with a Line drawn from our Eye to the Comet. When we fee it at right Angles, or any great Angle, it appears long; but if at a small Angle it appears short.

Sir Ifaac Newton has computed that the great Comet, which appeared 1680 and 1681, was heated, by its near Approach to the Sun, to the Degree of near 2000 Times the Heat of red hot Iron.

Bodies that can endure fo intense a Heat, without being entirely diffipated and deH 3 stroyed,

ftroyed, muft needs be very hard and folid; fuch therefore, no doubt, the Comets are.

Helvetius, by measuring the Comet that. appeared 1665, found its Diameter to be three times as long as that of the Earth ; confequently the Body of that Comet muft be twenty-seven times greater than the Earth *.

The Comet which appeared 1759, is supposed to be the fame that appeared 1682, and before in 1607 and 1531; whose Period muft therefore be 75 or 76 Years; and whose Return may be expected 1835 or 1836. Its greatest Distance from the Sun, to its least, is as 60 to 1: and its greatest Light and Heat, to its least, as about 3600 to 1.

The Comet which appeared 1661, is supposed to be the fame that appeared 1532, whofe Period is therefore about 126 Years. It may be expected again 1789. Its greatest Distance to its leaft, is computed to be more than 100 to 1, and its greatest Light and Heat to its leaft, more than 10,000

to I.

The great Comet which appeared 1680 and 1681, is fuppofed to be the fame that appeared in the 44th Year before the Chriftian Era, and again A. D. 531 or 532, again 1106, and laftly 1680; therefore

*Spheres are to one another as the Cubes of their Di

ameters.

the

the Time of its Revolution must be 576 Years, and its next Appearance may be expected in the Year 2256. Its greatest Diftance to its leaft is above 20,000 to 1, and its greatest Light and Heat to its leaft above 400,000,000 to 1.

We know but little, or rather nothing certainly, of the Ufe of Comets. They feem, in their prefent State, to be very unfit for the Habitation of Animals; because of their intenfe Heat, when they are in one Part of their. Orbit, viz. nearest the Sun, and their extreme Cold in the oppofite Part. But yet God could, no doubt, make Creatures who fhould be capable of enduring both these Extremes. To imagine that the appearing of a Comet is 'ominous, and that it forebodes fome approaching Calamity to any Part of the Earth, is a fuperftitious Conceit, without any Foundation in Reafon; and it feems to be condemned in Scripture. Jerem. x. 2. Thus faith the Lord, learn not the Way of the Heathen, and be not difmayed at the Signs of the Heaven; for the Heathen are dif mayed at them.

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С НА Р. IX.

Of the Equation of TIME.

A

S the Moon wants two Days, over and above an entire Revolution in her Orbit, to complete a Synodical Month, or Lunation; (vid. Ch, iv. p. 68.) fo, for the fame Reafon, does the Earth want about four Minutes, more or lefs, over and above an entire Revolution on its Axis, to complete a natural Day from Noon to Noon. Or thus: the Earth completes its Revolution in 23 Hours, 56 Minutes; but the Meridian on which it was Noon fo long ago, is not yet come to Noon again; becaufe, fince then, the Earth has advanced forward, in its annual Orbit, about one Degree; and therefore we must wait fome time longer, before the Sun will come again upon the fame Meridian, and make Noon. But now that Meridian will at fometimes gain Noon again fooner, after a complete Revolution, than at other Times; fometimes in lefs than 4 Minutes, fometimes in more; because the Angle from the Sun to the two Points in the Earth's Orbit, the one where it was Yefter

day

day at Noon, the other where it is Today, when it has completed its Revolution, is lefs in Summer, when the Sun is farther off from the Earth, than in Winter, when he is nearer to it: and confequently, that Meridian will have the Sun upon it again fooner, after the diurnal Revolution is completed, when the Angle is lefs, than when it is greater; therefore the natural Days from Noon to Noon are not of equal Lengths, which is one Reason of the Difference betwixt equal Time, which is measured by a Clock that is fuppofed to go without any Variation, and to measure exactly 24 Hours from Noon to Noon; and apparent Time, which is measured by the apparent Motion of the Sun in the Heavens, or by a good Sun-Dial.

Fig. VIII.

Let A a Cc F be the Earth's Orbit, S the Place of the Sun, A a Cc the Arch which the Earth runs over, while it makes a complete Revolution on its Axis *. A B, a Semidiameter of the Earth from the Center to any Meri

* The Arch A a is in reality greater than Cc; for the Earth, as well as every other Planet, defcribes in its Motion round the Sun, equal Spaces, or Areas, in equal Times. That is, if the Earth moves from A to a, in the same Time that it moves from C to c; the triangular Space A Sa is equal to the Space C S c. But as the Triangle C Scis longer than AS a, it is manifeft that the Arch A a must be greater than Cc. However it is fufficient, in the prefent Cafe, to fuppofe them equal.

dian

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