heat of the comet which appeared in the year 1680, Plate I. when nearest the Sun, to be 2000 times hotter than red-hot iron; and that being thus heated, it must retain its heat until it comes round again; although its period should be more than twenty thousand years; though it is computed to be only 575. The method of computing the heat of bodies, keeping at any known distance from the Sun, so far as their heat depends on the force of the Sun's rays, is very easy; and shall be explained in the eighth chapter.

the orbits

not solid.

ods only of

93. Part of the paths of three comets is delineat. Fig. I. ed in the scheme of the solar system, and the years marked in which they made their appearance.There are, at least, 21 comets belonging to our sys- They tem, moving in all sorts of directions; and all those prove that which have been observed, have moved through the of the plaethereal regions and the orbits of the planets, with- nets are out suffering the least sensible resistance in their motions; which plainly proves that the planets do not move in solid orbits. Of all the comets, the periods The peri of the above mentioned three only are known with. three are any degree of certainty. The first of these comets known. appeared in the years 1531, 1607, and 1682; and is expected to appear again in the year 1758, and every 75th year afterward. The second of them appeared in 1532, and 1661, and may be expected to return in 1789, and every 129th year afterward. The third, having last appeared in 1680, and its period being no less than 575 years, cannot return until the year 2225. This comet, at its greatest distance, is about eleven thousand two hundred millions of miles from the Sun; and at its least distance from the Sun's centre, which is 49,000 miles, is within less than a third part of the Sun's semi-diameter from his surface. In that part of its orbit which is nearest the Sun, it flies with the amazing swiftness of 880,000 miles in an hour; and the Sun, as seen from it, appears a hundred degrees in breadth; consequently 40 thousand times as large as he ap

prove the

tances.

They pears to us. The astonishing length that this comet stars to be runs out into empty space, suggests to our minds at im- an idea of the vast distance between the Sun and mense dis- the nearest fixed stars; of whose attractions all the comets must keep clear, to return periodically, and go round the Sun; and it shews us also, that the nearest stars, which are probably those that seem the largest, are as big as our Sun, and of the same nature with him; otherwise, they could not appear so large and bright to us as they do at such an immense distance.

Inferenc

above phenomena.

94. The extreme heat, the dense atmosphere, the es drawn gross vapours, the chaotic state of the comets, seem from the at first sight to indicate them altogether unfit for the purposes of animal life, and a most miserable habitation for rational beings; and therefore some* are of opinion that they are so many hells for tormenting the damned with perpetual vicissitudes of heat and cold. But when we consider, on the other hand, the infinite power and goodness of the Deity; the latter inclining, the former enabling him to make creatures suited to all states and circumstances; that matter exits only for the sake of intelligent beings; and that wherever we find it, we always find it pregnant with life, or necessarily subservient thereto; the numberless species, the astonishing diversity of animals in earth, air, water, and even on other animals; every blade of grass, every tender leaf, every natural fluid, swarming with life; and every one of these enjoying such gratifications as the nature and state of each requires: when we reflect, moreover, that some centuries ago, till experience undeceived us, a great part of the Earth was adjudged uninhabitable; the torrid zone, by reason of excessive heat, and the two frigid zones because of their intolerable cold; it seems highly probable, that such numerous and

* Mr. WHISTON, in his Astronomical Principles of Religion.

large masses of durable matter as the comets are, however unlike they be to our Earth, are not destitute of beings capable of contemplating with wonder, and acknowledging with gratitude, the wisdom, symmetry, and beauty of the creation; which is more plainly to be observed in their extensive tour through the heavens, than in our more confined circuit. If farther conjecture be permitted, may we not suppose them instrumental in recruiting the expended fuel of the Sun; and supplying the exhausted moisture of the planets? However difficult it may be, circumstanced as we are, to find out their particular destination, this is an undoubted truth, that wherever the Deity exerts his power, there he also manifests his wisdom and goodness.

tem very

ble.

95. THE SOLAR SYSTEM, here described, This sys. is not a late invention; for it was known and taught ancient by the wise Samian philosopher PYTHAGORAS, and deand others among the ancients: but in latter times monstra was lost, till the 15th century, when it was again restored by the famous Polish philosopher, NICHOLAUS COPERNICUS, born at Thorn in the year 1473. In this he was followed by the greatest mathematicians and philosophers that have since lived; as KEPLER, GALILEO, DESCARTES, GASSENDUS, and Sir ISAAC NEWTON; the last of whom has established this system on such an everlasting foundation of mathematical and physical demonstration, as can never be shaken; and none who understand him can hesitate about it.

tem ab.

96. In the Ptolemean system, the Earth was sup- The Ptole posed to be fixed in the centre of the universe; mean sysand the Moon, Mercury, Venus, the Sun, Mars, surd. Jupiter, and Saturn, to move round the Earth. Above the planets, this hypothesis placed the firmament of stars, and then the two crystalline spheres all which were included in and received motion from the primum mobile, which constantly

K

The Ty.

chonic system partly true, and partly false.

revolved about the Earth in 24 hours from east to west. But as this rude scheme was found incapable of standing the test of art and observation, it was soon rejected by all true philosophers; notwithstanding the opposition and violence of blind and zealous bigots.

97. The Tychonic system succeeded the Ptolomean, but was never so generally received. In this the Earth was supposed to stand still in the centre of the universe or firmament of stars, and the Sun to revolve about it every 24 hours; the planets, Mercury, Venus, Mars, Jupiter, and Saturn, going round the Sun in the times already mentioned. But some of TYCHO's disciples supposed the Earth to have a diurnal motion round its axis, and the Sun with all the above planets to go round the Earth in a year; the planets moving round the Sun in the aforesaid times. This hypothesis being partly true and partly false, was embraced by few; and soon gave way to the only true and rational system, restored by COPERNICUs, and demonstrated by Sir ISAAC NEWTON.

98. To bring the foregoing particulars into one point of view, with several others which follow, concerning the periods, distances, magnitudes, &c. of the planets, the following table is inserted.

Place of Propor. P. opor-Prop.01 tion of tion of gravity tion of

SUN and tion of quanti- motion motion Square miles in

Planets. light & ty of

in its of its e

heat. matter. orbit. quator.

surface.

Cubic miles in solidity. destroyed,

1,828,911,000,00 232,577,115,137,000,000 days hours

82,000,000

13,000 2° 10' 5° 18'

Projectile force being

would fall to the Sun in No.

The Moon belonging to the Earth goes round its orbit in 27 days, 7 hours, 43 minutes.

45,969,335,840 121 0