390 THE CONNECTION AND EQUIVALENCE OF FORCES. 



by rapid motion tiie iron tires of carriage wheels become fre- 

 quently so hot that they cannot be touched. In grinding 

 needle-points the steel is heated to redness, and the detached 

 particles burn with sparks. The wooden breaks of railway 

 carriages become frequently so hot by friction that their sur- 

 face emits an ernpyreumatic odour. 'By the friction of an 

 iron grater, particles of white sugar can be melted and heated 

 so far as to acquire the taste of burnt sugar (Caramel). The 

 heat evolved by the friction of two pieces of ice is sufficient to 

 melt them. 



In the English steel-foundries a bar of steel 10 or 12 

 inches long, by being heated at one end in a forge, is welded 

 by hammering to another slender bar, 10 or 12 feet long, 

 without the necessity of further direct application of heat a 

 point of great importance for the preservation of the good 

 quality of the steel. Every spot on which the powerful blows 

 of the hammer rapidly descend, becomes red hot, and to the 

 spectators the red glow of heat appears to run up and down 

 the bar. This glow is produced by the blows of the hammer, 

 and corresponds to an amount of heat sufficient to raise many 

 pounds of water to the boiling point ; whilst the end of the 

 bar heated in the fire would scarcely by itself raise to the 

 same temperature as many ounces of water. 



According to the preceding views a precise connection 

 exists between the blows of the hammer (the cause) and the 

 heat produced (the effect) ; and natural philosophers have 

 devised the most ingenious experiments to show this relation. 

 The working power is in this case converted into heat. If 

 the view of Mayer be correct, then should we by this amount 

 of heat thus obtained, be able to reproduce the same amount 

 of work, viz., the same number of blows of the hammer. But 

 a closer view of the point shows that we require to elevate 

 the hammer, and that therefore its working power was not 

 inherent, but only lent to it. The hammer was elevated by a 

 water-wheel set in motion by a certain weight of water falling 



