-1859] WRITINGS OP JOSEPH HENRY. 133 



the surface, beyond the depth of the influence of external 

 chmate, the temperature uniformly increases at the rate of 

 about 1° F. for every 50 feet. Our globe then consists of a 

 mass of matter which has been gradually cooled from a state 

 of intense heat, and at its surface has arrived at a con- 

 dition of equilibrium, the heat which its surface gives off 

 into space being just compensated by that received from the 

 sun. The permanency of our temperature therefore depends 

 upon that of the great central luminary of our system itself. 

 But whether 



" The sun himself shall fade, and ancient night 

 Again involve a desolate abyss," 



must be left for future consideration. 



The ideas which are here given had their origin in the 

 attempts which were made to produce self-moving machines. 

 The possibility of such contrivances appeared to be sanctioned 

 by the apparently spontaneous motion of men and lower 

 animals. The idea that these motions were the results of 

 the chemical action of food had not yet entered the mind; 

 and it was only after many fruitless attempts, and the ex- 

 penditure of much thought, time, and labor that the con- 

 clusion was at length arrived at that a machine is a mere 

 instrument for the application and modification of power or 

 energy, and that in no case can it do more work or produce 

 more changes in matter, or in other words, it can break 

 apart no more atoms than are equivalent to the pov/er which 

 has been applied to it. The same amount of power which 

 we apply at one extremity of a machine, properly estimated, 

 is equal to the sum of the resistances at the other, and the 

 two precisel}^ balance each other. From considerations of 

 this kind we arrived at the conception of the correlation of 

 the physical forces and the re-conversion of the equivalent 

 of one into that of the other. 



We may do the same work by heat properly applied, or 

 by a fall of water, or by muscular energy. For example, a 

 disc of iron may be made to revolve rapidly with a mill 

 driven by a fall of water, and if this is allowed to rub with 

 some pressure against another iron plate a great amount of 



