PUPIL. Yes, Sir; and I shall be glad to know the reason of it. TUTOR. If the attractive power of the sun were uniformly the same in every part of their orbits they would be true circles, and the planets would pass over _equal_ portions of their orbits in _equal_ times; that is, they would move from B to C, (Plate II. fig. 5.) in the same time as from A to B, &c. PUPIL. That is clear, but as their orbits are elliptical, when the planets are farthest from the sun, the velocity with which they move must be lessened as the attraction is decreased. TUTOR. And they must consequently pass over _unequal_ parts of their orbits in _equal_ portions of time. And, as _a double velocity will balance a quadruple or fourfold power of gravity or attraction_, it follows, that as the centripetal force is four times as great at A as at B (Plate II. fig. 4.) the centrifugal force will be twice as great, and would carry a planet from A to _a_ in the same time it would from B to _b_, and in its orbit from A to _c_ as soon as from B to _d_, and thereby describe the area, or space contained between the letters A S _c_, in the same time as the area or space B S _d_. For according to the laws of the planetary motions, in their periodical revolutions, _they always describe equal areas in equal times_. PUPIL. The orbits of the comets being very elliptical, the irregularity of their motions must be exceedingly great. TUTOR. Great, indeed!—One of them passed so near the sun as to acquire a heat which Sir Isaac Newton computed to be two thousand times hotter than red hot iron.[12] [Footnote 12: Dr. Herschel is of opinion, that bodies near the sun do not acquire so great a degree of heat as has been generally imagined.] PUPIL. Astonishing! If they pass so near the sun, the centripetal force must act powerfully on the body of the comet. TUTOR. And that force, you know, must be equalled by the projectile force; so you find they move when near the sun with amazing celerity.—But when arrived at their aphelion, where the influence of the sun is weak, what a transition! PUPIL. Wonderful, indeed!—Their motion is excessively slow, and the sun must appear little more than a fixed star. Surely they cannot be inhabited, can they? TUTOR. We cannot speak positively; but, as they differ so much from the planets, which we have reason to suppose are so, it is imagined they are designed for some purpose unknown to us. PUPIL. When is the earth in its perihelion? TUTOR. In December; and our summer half year is longer than the winter half, by about eight days. PUPIL. I suppose this is occasioned by the inequality of the earth’s annual motion. TUTOR. It is; and this inequality is the cause of the difference of time between the sun and a well regulated clock; the latter keeps equal time, whilst the former is constantly varying.