ORIGIN OF TNE SUN'S HEAT. 277 



The fact that the development of heat by mechanical 

 means on the surface of our globe is, as a rule, not so great, 

 and cannot 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 argu- 

 ment against the assumption 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 ^th part of the heat pro- 

 duced 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 sec- 

 ond. 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 de- 

 velopment 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 re- 

 quired to cool it down to its ordinary temperature. 



When a railway train passes with even less than its ordi- 

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

 observed 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 



