PLEASURES OF SCIENCE. ID 



the proportion of the squares, lessening as the distance increases : at 

 two miles from the earth, it is four times less than at oue mile ; at 

 three miles, nine times less; and so forth. It goes on lessening, but 

 never is destroyed, even at the greatest distances to which we can reach, 

 and there can be no doubt of its extending indefinitely beyond. But, 

 by astronomical observations made upon the motion of the heavenly 

 bodies, upon that of the moon for instance, it is proved that her movement 

 is slower and quicker at different parts of her course, in the same manner 

 as a body's motion on the earth would be slower and quicker, accord- 

 ing to its distance from the point it was drawn towards, provided it 

 was drawn by a force acting in the proportion to the squares of the 

 distance, which we have frequently mentioned ; and the proportion of 

 the time to the distance is also observed to agree with the rule we have 

 referred to. Therefore, she is shown to be attracted towards the earth 

 by a force that varies according to the same proportion in which gravity 

 varies ; and she must consequently move in an ellipse round the earth, 

 which is placed in a point nearer the one end than the other of that 

 curve. In like manner, it is shown that the earth moves round the sun 

 in the same curve line, and is drawn towards the sun by the same 

 force ; and that all the other planets in their courses, at various dis- 

 tances, follow the same rule, moving in ellipses, and drawn towards the 

 sun by the same kind of power. Three of them have moons like the 

 earth, only more numerous, for Jupiter has four, Saturn seven, and 

 Herschel six, so very distant that we cannot see them without the help 

 of glasses; but all those moons move round their principal planets, as 

 ours does round the earth, in ovals or ellipses ; while the planets, with 

 their moons, move in their ovals round the sun, like our own earth 

 with its moon. But this power, which draws them all towards the sun, 

 and regulates their path and their motion round him, and which draws 

 the moons towards the principal planets, and regulates their motion 

 and path round those planets, is the same with the gravity by which 

 bodies fall towards the earth, being attracted by it. Therefore, the 

 whole of the heavenly bodies are kept in their places, and wheel round 

 the sun, by the same influence or power that makes a stone fall to the 

 ground. 



It is usual to call the sun, and the planets which with their moons 

 move round him, (twelve in number, including the four lately discovered, 

 and the one discovered by Herschel,) the Solar System, because they 

 are a class of the heavenly bodies far apart from the innumerable fixed 

 stars, and so near each other as to exert a perceptible influence on one 

 another, and thus to be connected together. The Comets belong to the 

 same system, according to this manner of viewing the subject. They 

 are bodies which move in elliptical paths, but far longer and narrowei 

 than the curves in which the earth, and the other planets and their 

 moons roll. Our curves are not much less round than circles ; the 

 paths of the comets are long and narrow, so as, in many places, to be 

 more nearly straight lines than circles. They differ from the planets 

 and their moons in another respect ; they do not depend on the sun for 

 the light they give, as our moon plainly does, being dark when the earth 

 comes between her and the sun ; and as the other planets do, those of 

 them that are nearer the sun than we are, being dark when they come 



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