Dr. J. R. Mayer on Celestial Dynamics. 393 



of this globe ; if we again substitute for the earth the sun, with 

 a mass 350,000 greater, and for a height of a few metres celes- 

 tial distances, we obtain a generation of heat exceeding all ter- 

 restrial measures. And since we have sufficient reason to assume 

 the actual existence of such mechanical processes in the heavens, 

 we find therein the only tenable explanation of the origin of the 

 heat of the sun. 



The fact that the development of heat by mechanical means 

 on the surface of our globe is, as a rule, not so great, and can- 

 not be so great as the generation of the same agent by chemical 

 means, as by combustion, follows from the laws already discussed ; 

 and this fact cannot be used as an argument against the assump- 

 tion of a greater development of heat by a greater expenditure 

 of mechanical work. It has been shown that the heat generated 

 by a weight falling from a height of 367 metres is only g^th 

 part of the heat produced by the combustion of the same weight 

 of coal ; just as small is the amount of heat developed by a weight 

 moving with the not inconsiderable velocity of 85 metres in one 

 second. But, according to the laws of mechanics, the effect is 

 proportional to the square of the velocity ; if therefore the weight 

 move 100 times faster, or with a velocity of 8500 metres in one 

 second, it will produce a greater effect than the combustion of an 

 equal quantity of coal. 



It is true that so great a velocity cannot be obtained by human 

 means ; everyday experience, however, shows the development of 

 high degrees of temperature by mechanical processes. 



In the common flint and steel, the particles of steel which are 

 struck off are sufficiently heated to burn in air. A few blows 

 directed by a skilful blacksmith with a sledge-hammer against a 

 piece of cold metal may raise the temperature of the metal at the 

 points of collision to redness. 



The new crank of a steamer, whilst being polished by friction, 

 becomes red-hot, several buckets of water being required to cool 

 it down to its ordinary temperature. 



When a railway train passes with even less than its ordinary 

 velocity along a very sharp curve of the line, sparks are ob- 

 served in consequence of the friction against the rails. 



One of the grandest constructions for the production of motion 

 by human art is the channel in which the wood was allowed to 

 glide down from the steep and lofty sides of Mount Pilatus 

 into the plain below. This wooden channel, which was built 

 about thirty years ago by the engineer Rupp, was 9 English miles 

 in length ; the largest trees were shot down it from the top to the 

 bottom of the mountain in about two minutes and a half. The 

 momentum possessed by the trees on their escaping at their 

 journey's end from the channel was sufficiently great to bury 



