96 THE POPULAR SCIENCE MONTHLY. 



varies directly as the product of the masses, and inversely as the 

 square of the distance between the two bodies. When more careful 

 observations are introduced, it is seen that the planets exhibit some 

 slight deviations from the movements which they would have were 

 each planet only acted upon by the attraction of the sun. These 

 deviations do not invalidate the principle of attraction. They have 

 been shown to arise from the mutual attractions of the planets them- 

 selves. Each of the planets is thus seen to attract each of the other 

 planets. The intensity of this attraction between any pair of the 

 planets is proportional to the masses of these planets, and varies in- 

 versely as the square of the distance between them. "We may use 

 similar language with regard to the satellites by which so many of 

 the planets are attended. Each satellite revolves around its primary. 

 The movements of each satellite are mainly due to the preponderating 

 attraction of the primary." Irregularities in the movements of the 

 satellites are well known to astronomers, but these irregularities can 

 be accounted for by the attraction of other bodies of the system. The 

 law of attraction thus seems to prevail among the small bodies of the 

 system as well as among the large bodies. It is true that there are 

 still a few outstanding discrepancies which can not yet be said to 

 have been completely accounted for by the principle of gravitation. 

 This is probably due to the difficulties of the subject. The calcula- 

 tions which are involved are among the most difficult on which the 

 mind of man has ever been engaged. We may practically assume 

 that the law of gravitation is universal between the sun, the planets, 

 and the satellites ; and we may suppose that the few difficulties still 

 outstanding will be finally cleared away, as has been the case with so 

 many other seeming discrepancies. But even when these admissions 

 have been made, are we in a position to assert that the law of gravi- 

 tation is universal throughout the solar system ? We are here con- 

 fronted with a very celebrated difficulty. Do those erratic objects 

 known as comets acknowledge the law of gravitation ? There can be 

 no doubt that in one sense the comets do obey the law of gravitation 

 in a most signal and emphatic manner. A comet usually moves in an 

 orbit of very great eccentricity ; and it is one of the most remarkable 

 triumphs of Newton's discovery, that we were by its means able to 

 render account of how the movements of a comet could be produced 

 by the attraction of the sun. As a whole, the comet is very amenable 

 to gravitation, but what are we to say as to the tails of comets, which 

 certainly do not appear to follow the law of universal attraction? 

 The tails of comets, so far from being attracted toward the sun, seem 

 actually to be repelled from the sun. Nor is even this an adequate 

 statement of the case. The repulsive force by which the tails of the 

 comets are driven from the sun is sometimes a very much more intense 

 force than the attraction of gravitation. 



I have no intention to discuss here the vexed question as to the 



