GRAVITATIOX. 



evident that this motion will be produced by the united 

 influence of the force with which the weight was thrown 

 off by the hand, and the tendency down towards the cen- 

 tre. This last represents gravitation. And although 

 the motion of the weight in this experiment is in many 

 respects different from that of a planet, yet the illustra- 

 tion is good in this respect ; viz. it shows how the con- 

 stant operation of a tendency towards the centre, may 

 cause a body to revolve about that centre in a circular or 

 eliptical orbit. 



It will, however, be justly said, that if the tendency of 

 one body to another is universal, it will be apparent not 

 only between a planet and the sun, but between one planet 

 and another. This is the fact. In the adjoining dia- 

 gram, let S rep- 

 resent the sun, M 

 Mars, and J Jupi- 

 ter. Whenever 

 the two planets, 

 in their regular 

 revolutions, pass 

 near each other, 

 each is drawn out 

 of its regular path. 

 Instead ofpassing 

 in the curves of 

 their orbits, they 

 approach each 

 other and move 

 in the paths rep- 

 resented by the dotted lines in the figure. When Mars, 

 which revolves more rapidly than Jupiter, passes beyond 

 the influence of the latter, both return to their original 

 paths. 



There are many other disturbances in the planetary 

 motions, which no ingenuity could explain before the 

 simple principle of gravitation was discovered. They 

 are so numerous, and correspond so exactly with the the- 

 ory, as to leave not the slightest room to doubt, that 

 among all the heavenly bodies, sufficiently near us to 

 have their motions observed, there is this uniform ten- 

 dency to move towards each other. 



