Mercury—Venus—Earth—Mars—Jupiter—Saturn—Georgium Sidus is 4 will be 7 10 15 52 95 190
or, if a body projected from the Sun should continue to fly at the rate of 480 miles per hour, (which is about the swiftness of a cannon-ball), it could reach the orbit of Mercury in 9 years, Venus in 16 years, the Earth in 23 years, Mars in 34 years, Jupiter in 118 years, Saturn in 216 years, and the Georgium Sidus in 432 years.
These we consider as the regular bodies of our system; so regular, indeed, that every phænomenon respecting them is calculated for years before hand, and it is almost considered as a criminal error to be a minute of time wrong in the calculation. But we are sometimes visited by COMETS, which may also be recognized as a part of our system: of these our knowledge is very imperfect. By supposing that the same Comet has appeared at equal intervals of time; by observing that, like the planets, they describe equal areas in equal times; and by having three points in an ellipsis given to make out its eccentricity; from these data it was natural for mathematicians to suppose they could calculate the return of all Comets that had been scientifically observed: but the actual return even of that conspicuous one expected by Dr. Halley, has been thought by some not to be sufficiently ascertained: yet, on examining the balance of probability, as stated by Maupertius, Lalande, Messire and Martin, for its re-appearance, probably this doubt will be greatly lessened, if not removed. As new Comets are almost perpetually appearing, on which calculation hitherto has been silent, there is reason to expect, in a proper period of time, an adequate number of observations to decide the question, whether in general they revolve at stated times, or traverse our system without probability of return. Perhaps Comets of each description time and observation may confirm to us. We know that Comets accompanied with tails come very near the Sun, and from all quarters of the Heavens! that the tails keep opposite to the Sun; consequently they are only visible to us when seen obliquely to the Sun. Thus the Comet of August, 1797, was observed to have little or no train during any part of its appearance; but a faint hazy light diffused round it; these trains, like electrical and borealean light, do not refract the light of the fixed stars, &c. The appearance of the Comet of 1682, is copied in the _Eidouranion_. It descends from the top of the Machine; its train increasing in length and lustre till it arrives at the Sun, diminishing as it ascends. Its orbit is so eccentric that the small part of it visible is not sensibly to be distinguished from the parabolic curve; and in this representation it finally disappears in the roof of the Theatre; it being impossible, if its return were ascertained, to represent the extent of such an orbit, and its motion in it, with any degree of suitable proportion. The velocity of such of these as approach nearest to the Sun, particularly of the Comet of 1680, (whose appearance was tremendous) exceeds any swiftness that falls within observation; except that of the rays of light; it being nearly 2000 times greater than the swiftness of a cannon-ball, at the instant of its discharge; yet scarcely a thousandth part of the velocity of light passing from the Sun[4]. This Comet approached to within 40,000 miles of the Sun’s surface, and was calculated to be heated 2000 times hotter than red hot iron; a globe of iron the size of the Earth in this heat, would be 50,000 years in cooling. These amazing visiters, whom philosophy contemplates with awe very different from that terror with which superstition had long viewed them, moving in such amplitude of space, so numerous as they are, and so great as some of them, must have functions assigned to them proportionally important: either occasionally of terrific revolution; or more generally of recruiting the atmosphere of the planets in their successive appulse to any of them, and supplying the diminution of the solar fires. Perhaps too they are useful in preventing the central tendency of the planets to the Sun, from increasing more than in a certain degree; so that the apparent disturbances, thus produced, will be part of the necessary order and harmony of the system.