S*CT. XXV. APPLICATION OF HEAT. 229 



ing and that which is already in the atmosphere. In 

 calm weather, vapor accumulates in the stratum of air 

 immediately above the evaporating surface, and retards 

 the formation of more ; whereas a strong wind accele- 

 rates the process, by carrying off the vapor as soon as 

 it rises, and making way for a succeeding portion of 

 dry air. 



The latent heat of ah* and all elastic fluids may be 

 forced out by sudden compression, like squeezing water 

 out of sponge. The quantity of heat brought into action 

 in this way is very well illustrated hi the experiment of 

 igniting a piece of timber by the sudden compression of 

 air by a piston thrust into a cylinder closed at one end : 

 the development of heat on a stupendous scale is exhib- 

 ited in lightning, probably produced in part by the violent 

 compression of the atmosphere during the passage of 

 the electric fluid. Prodigious quantities of heat are 

 constantly becoming latent, or are disengaged by the 

 changes of condition to which substances are liable in 

 passing from the solid to the liquid, and from the liquid 

 to the gaseous form, or the contrary, occasioning endless 

 vicissitudes of temperature over the globe. 



There are many other sources of heat, such as com- 

 bustion, friction, and percussion, all of which are only 

 means of calling a power into evidence which already 

 exists. 



The application of heat to the various branches of the 

 mechanical and chemical arts has, within a few years, 

 effected a greater change in the condition of man than 

 had been accomplished in any equal period of his exist- 

 ence. Armed by the expansion and condensation of 

 fluids with a power equal to that of the lightning itself, 

 conquering time and space, he flies over plains, and trav- 

 els 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 hi opposi- 

 tion to wind and tide ; by releasing the strain on the 

 cable, he rides at anchor fearless of the storm ; he makes 

 the elements of air and water the carriers of warmth, 

 not only to banish winter from his home, but to adorn it 

 even during the snow-storm with the blossoms of spring; 

 and, like a magician, he raises, from the gloomy and 

 U 



