LAW OF GRAVITATION. 217 



statement of what would happen : if the force 

 were to be simply in the direct ratio of the dis- 

 tance, any number of planets might revolve in 

 the most regular and orderly manner. Their 

 mutual effects, which we may call perturbations 

 if we please, would be considerable ; but these 

 perturbations would be so combined with the 

 unperturbed motion, as to produce a new motion 

 not less regular than the other. This curious 

 result would follow, that every body in the system 

 would describe, or seem to describe, about every 

 other, an exact elliptical orbit ; and that the times 

 of the revolution of every body in its orbit would 

 be all equal. This is proved by Newton, in the 

 64th proposition of the Principia. There would 

 be nothing to prevent all the planets, on this 

 supposition, from moving round the sun in orbits 

 exactly circular, or nearly circular, according to 

 the mode in which they were set in motion . 



But though the perturbations of the system 

 would not make this law inadmissible, there are 

 other circumstances which would do so. Under 

 this law, the gravity of bodies at the earth's sur- 

 face would cease to exist. Nothing would fall or 

 weigh downwards. The greater action of the 

 distant sun and planets would exactly neutralize 

 the gravity of the earth : a ball thrown from the 

 hand, however gently, would immediately become 

 a satellite of the earth, and would for the future 

 accompany it in its course, revolving about it in 

 the space of one year. All terrestrial things 



