4o6 POPULAR LECTURES AND ADDRESSES. 



than if they be given moving in any random 

 directions and with any velocities considerable in 

 comparison with the velocities which they would 

 acquire in falling from rest into collision. In 

 this connection it is most interesting to know from 

 stellar astronomy, aided so splendidly as it has 

 recently been by the spectroscope, that the relative 

 motions of the visible stars and our sun are 

 generally very small in comparison with the 

 velocity (612 kilometres per second) which a body 

 would acquire in falling into the sun, and are com- 

 parable with the moderate little velocity (29*5 kilo- 

 metres per second) of the earth in her orbit round 

 the sun. 



To fix the ideas, think of two cool solid globes, 

 each of the same mean density as the earth and 

 of half the sun's diameter, given at rest, or nearly 

 at rest, at a distance asunder equal to twice the 

 earth's distance from the sun. They will fall to- 

 gether and collide in exactly half a year. The 

 collision will last for about half an hour, in the 

 course of which they will be transformed into a 

 violently agitated incandescent fluid mass flying 



