168 ON THE OKIGIN OF THE PLANETAEY SYSTEM. 



ing heated. You all know that friction heats the 

 bodies rubbed. Every match that we ignite, every 

 badly greased coach-wheel, every auger which we work 

 in hard wood, teaches this. The air, like solid bodies, 

 not only becomes heated by friction, but also by the 

 work consumed in its compression. One of the most 

 important results of modern physics, the actual proof 

 of which is mainly due to the Englishman Joule, is 

 that, in such a case, the heat developed is exactly pro- 

 portional to the work expended. If, like the mechani- 

 cians, we measure the work done by the weight which 

 would be necessary to produce it, multiplied by the 

 height from which it must fall, Joule has shown that 

 the work, produced by a given weight of water falling 

 through a height of 425 metres, would be just suffi- 

 cient to raise the same weight of water through one 

 degree Centigrade. The equivalent in work of a 

 velocity of eighteen to twenty-four miles in a second 

 may be easily calculated from known mechanical laws ; 

 and this, transformed into heat, would be sufficient to 

 raise the temperature of a piece of meteoric iron to 

 900,000 to 2,500,000 degrees Centigrade, provided that 

 all the heat were retained by the iron, and did not, as 

 it undoubtedly does, mainly pass into the air. This 

 calculation shows, at any rate, that the velocity of the 

 shooting-stars is perfectly adequate to raise them to 

 the most violent incandescence. The temperatures 



