SOURCES OF HEAT. 827 



The friction of two bodies one against the other is 

 an important source of heat. Although this source is 

 not ordinarily used for producing large quantities of 

 heat, such as are obtained by combustion, still ad- 

 vantage is taken of it for producing heat on a small 

 scale on exceedingly numerous occasions for lighting 

 fires. The most ancient method of lighting a fire was 

 by pressing the end of an elongated piece of wood be- 

 tween two other pieces and making it rapidly revolve 

 by means of a bow and string of catgut, in the manner 

 in which the drill-bow is used. In the case of flint and 

 steel, the friction of the flint against the steel raises the 

 temperature of the metallic particles, which fly off 

 heated to such an extent that they ignite the easily in- 

 flammable tinder or fusee upon which they fall. In 

 the more modern matches friction is used for producing 

 sufficient heat to make the very inflammable matter 

 with which the match is tipped catch fire. 



Pressure and percussion produce heat. A piece of 

 lead, a few centimetres long and broad, and 1 or 2 cm 

 thick, becomes sensibly heated if it is placed upon a 

 hard support, such as a flat stone or an anvil, and is 

 steadily and vigorously hammered for some time. 



This is only a special case of the more general and 

 highly important principle, that heat appears whenever 

 mechanical work is done without producing an equal 

 amount of mechanical work in some other form. 

 Experiments prove that for every 424 kilogramme- 

 metres of work which disappear there appears one 

 kilogramme -degree of heat. Conversely, when me- 

 chanical work is produced by heat, as in a steam 



