1 76 IN STARRY REALMS. 



instant, and you will find that they reach the ground at 

 the same moment. It is true that with very light bodies 

 there is some difference, but this is caused by the resist- 

 ance of the air. There is no resistance to the motion of 

 satellites in the way we are now considering. It is there- 

 fore plain that the masses of the satellites can in no 

 degree account for the huge disparity between the periods 

 of their revolutions. Our search has now become greatly 

 narrowed, for the only remaining point of difference be- 

 tween the two systems is found in the masses of the 

 primaries. To this, therefore, we must look for an 

 originating cause of the differences of the periods of revo- 

 lution of the two satellites. If the two central masses 

 had been equal, there can be no doubt that the two 

 periodic times would also have been the same. The value 

 of this reasoning consists in the deduction which it enables 

 us to make. "We know the periodic times, and from that 

 knowledge we are enabled to calculate the masses of the 

 two central bodies, by which the satellites that were re- 

 volving in those periodic times have been controlled. 



This point is of such fundamental importance in astro- 

 nomy, that I feel I must pursue it a little further, so as to 

 enable us actually to solve the majestic problem of weigh- 

 ing our earth against the sun. 



In the first place, it can be shown that the greater the 

 mass of the sun, the greater must be the rapidity with 

 which the planet revolves, in order to sustain its position 

 in a circular orbit at a specified distance. The planet 

 would, if unacted upon by any force, pursue a rectilinear 

 path and move on continuously with an unaltered velocity. 

 The presence of a central force constantly compels the 



