252 PRINCIPLES OF THE MECHANICAL THEORY OF HEAT. 



tlie vessel is perhaps just as great as that which is developed in the vessel through 

 the compression of the air. In this experiment the escaping air has to overcome 

 the resistance of the atmosphere, and thus to perform a mechanical work. 



In another series of experiments, to the vessel A, in which air had been com- 

 pressed to 22 atmospheres, was screwed, by means of a short metallic pipe, an 

 equally large vessel B, exhausted of air, and after both vessels, A and B, had been 

 placed in the same reservoir, holding 16^ pounds of water, a suitably adapted 

 cock was opened, so that half of the air compressed in A could flow over into B. 

 Through this process no observable change of temperature was produced in the 

 water surrounding the vessels A and B, whence Joule draws the conclusion that 

 no change of temperature occurs when air expands in such a ivay as to create no 

 mechanical power. 



When the two receivers A and B were placed in separate vessels of water, a 

 lowering of temperature of 2°. 36 F. was observed in the vessel which con- 

 tained the receiver A, out of which flowed the compressed air, wbile the water 

 which suiTounded the receiver B, into which the air flowed, acquired a nearly 

 equal elevation of temperature. 



Hirn, also, {Theorie mechaniqiie cle la chaleur, Paris, 1865,) has made a series 

 of experiments for the determination of the mechanical equivalent of heat, among 

 which we adduce that on the development of heat through the compression of 

 lead as being distinguished at once for its simplicity and conclusiveness. 



A cylinder A, of wrought iron, 350 kilograms in weight, which we will call 

 the hammer, is suspended by two pairs of strings about three metres in length, 

 as is shown by Fig. 6. Opposite to this cylinder is suspended in like manner 



Figr. 6. 



a prismatic block of sandstone, of the weight of 941 kilograms, which we will 

 term the anvil, and which is furnished on the side opposed to the hammer with an 

 iron plate C. Between the hammer and anvil is placed a cylindrical piece of lead 

 P, having a weight of 2.948 kilograms, and supported by a light wooden holder, 

 {Holzgahel.) This piece of lead is in part hollowed out in the direction of its axis. 

 Its temperature before the experiment Avas ascertahied by means of a thermometer 

 temporarily introduced into the cavity to be 7°. 87. 



The haiiimer was now drawn back 'by a pulley until it reached the position A', 

 and then again released. In recovering its position of equilibrium, it delivered 

 a strong blow upon the lead, which compressed and heated. Yet Avas not the 

 entire living force of the falling hammer spent in the compression of the lead; 

 for, after impact, the stone block and iron cylinder Avere again driven somcAvhat 

 apart. According to an experiment of this kind, the height of fall of the 



