280 APPLICATION OF HEAT TO THE ARTS. SECT. XXVII. 



and gas lamps restore to our use the light and heat of the sun of 

 the early geological epochs which have rested as dormant powers 

 under the seas and mountains for unnumbered ages. The sun 

 is therefore the source of the mechanical energy of all the heat 

 and motion of inanimate things, of all the motions of the heat 

 and light of fires and artificial flames, and of the heat of all 

 living creatures. For animal heat, and weights raised or re- 

 sistance overcome, are mechanical effects of the chemical com- 

 bination of food with oxygen ; and food is either directly or 

 indirectly vegetable, consequently dependent upon the sun. 



Professor Helmholtz of Bonn has put in a strong point of view 

 the enormous store of force possessed by our system by compar- 

 ing it with its equivalent of heat. The force with which the 

 earth moves hi its orbit is such, that if brought to rest by a 

 sudden shock, a quantity of heat would be generated by the blow 

 equal to that produced by the combustion of fourteen such earths 

 of solid coal ; and supposing the capacity of the earth for heat as 

 low as that of water, the globe would be heated to 11,200 Cent. 

 It would be quite fused and for the most part reduced to vapour. 

 If it should fall to the sun, which it would certainly do, the 

 quantity of heat developed by the shock would be four hundred 

 times as great. 



The application of heat to the various branches of the mechan- 

 ical and chemical arts has within the present century effected a 

 greater change in the condition of man than had been accom- 

 plished in any equal period of his existence. Armed by the ex- 

 pansion and condensation of fluids with a power equal to that of the 

 lightning itself, conquering time and space, he flies over plains, 

 and travels on paths cut by human industry even through moun- 

 tains with a velocity and smoothness more like planetary than 

 terrestrial motion ; he crosses the deep in opposition to wind 

 and tide ; by releasing the strain on the cable, he rides at 

 anchor fearless of the storm ; he makes the lightning his mes- 

 senger ; and like a magician he raises from the gloomy abyss of 

 the mine the sunbeam of former ages to dispel the midnight 

 darkness. 



The principal phenomena of heat may be illustrated by a 

 comparison with those of sound. Their excitation is not only 

 similar but identical, as in friction and percussion ; they are both 

 communicated by contact and radiation ; and Dr. Young observes 

 that the effect of radiant heat in raising the temperature of a 



