20 OBJECTS, ADVANTAGES, AND 



between us and him. But the comets give light always of themselves, 

 being apparently vast bodies heated red hot by coming in their course 

 far nearer the sun than the nearest of the planets ever do. Their motion 

 is much more rapid than that of the planets: they both approach the 

 sun much nearer, retreat from him to much greater distances, and take 

 much longer time in going round him than any of the planets do. Yet 

 even these comets are subject to the same great law of gravitation, 

 which regulates the motions of the planets. Their year, the time they 

 take to revolve, is in some cases 75, in others 135, in others 300 of our 

 years ; their distance is a hundred times our distance when furthest off, 

 and not a hundred and sixtieth of our distance when nearest the sun ; 

 their swiftest motion is above twelve times swifter than ours, although 

 ours is a hundred and forty times swifter than a cannon ball's ; yet their 

 path is a curve of the same kind with ours, though longer and flatter, 

 differing in its formation only as one oval differs from another by the 

 string you draw it with having the ends fixed at two points more distant 

 from each other ; consequently the sun, being in one of those points, is 

 much nearer the end of the path the comet moves in, than he is near 

 the end of our path. The motion, too, follows the same rule, being 

 swifter the nearer the sun ; the attraction of the sun varies according to 

 the squares of the distances, being four times less at twice the distance, 

 nine times less at thrice, and so on ; and the proportion between the 

 times of revolving and the distances is exactly the same, in the case of 

 those remote bodies, as in that of the moon and the earth. One law 

 prevails over all, and regulates their motions as well as our own : it is 

 the gravity of the comets towards the sun, and they, like our own earth 

 and moon, wheel round him in boundless space, drawn by the same 

 force, acting by the same rule, which makes a stone fall when dropped 

 from the hand. 



The more full and accurate our observations are upon these heavenly 

 bodies, the better we find all their motions agreeing with this great 

 doctrine ; although, no doubt, many things are to be taken into the 

 account beside the force that draws them to their different centres : 

 thus, while the moon is drawn by the earth, and the earth by the sun, 

 the moon is also drawn directly by the sun ; and while Jupiter is 

 drawn by the sun, so are his moons ; and both Jupiter and his moons 

 are drawn by Saturn : nay, as this power of gravitation is quite uni- 

 versal, and as no body can attract or draw another without being 

 itself drawn by that other, the earth is drawn by the moon, while 

 the moon is drawn by the earth ; and the sun is attracted by the 

 planets which he draws towards himself. These mutual attractions 

 give rise to many deviations from the simple line of the ellipse, and 

 produce many irregularities in the simple calculation of the times and 

 motions of the bodies that compose the system of the universe. But 

 the extraordinary powers of investigation applied to the subject by the 

 modern improvements in Mathematics, have enabled us at length to 

 reduce even the greatest of the irregularities to order and system ; and 

 to unfold one of the most wonderful truths in all science, namely, that 

 by certain necessary consequences of the simple fact upon which the 

 whole fabric rests, the proportion of the attractive force to the distances 

 at which it operates, all the irregularities which at first seemed to 



