realizing the Advantages of the Air-Engine. 3 



This quantity is known by the name of Joule's equivalent, 

 and may be otherwise termed the dynamical specific heat of 

 liquid water at ordinary atmospheric temperatures. 



3. Illustrations of this law. — The dynamical specific 

 heats of other substances may be determined either by direct 

 experiment, or by ascertaining their ratios to that of water. 

 For example, to heat one pound of atmospheric air, maintained 

 at a constant volume, by one degree of Fahrenheit, requires 

 the expenditure of 



130*5 foot-pounds of mechanical power.* 



This is the real dynamical specific heat of air. The appa- 

 rent dynamical specific heat of one pound of air, under con- 

 stant pressure, is (for a degree of Fahrenheit) 



183-7 foot-pounds;* 

 the difference, or 532 foot-pounds, being the mechanical 

 power exerted by the air in expanding, so as to preserve the 

 same pressure, notwithstanding the increase of its temperature 

 by one degree. The apparent specific heat of air at constant 

 pressure exceeds the real specific heat in the ratio of 1-41 : 1. 

 All quantities of heat may be thus expressed by equivalent 

 quantities of mechanical power. The heat required to raise 

 one pound of liquid water from the freezing to the boiling 

 point, and to evaporate it at the latter temperature, is 



1147°' 5 x 772 = 885,870 foot-pounds, 

 of which 180°-0 x 772 = 138,960 foot-pounds is what is termed 

 sensible heat, or the heat employed in raising the tempera- 

 ture of the water, while the remainder 



967°-5 x 772 = 746,910 foot-pounds 



is the latent heat of evaporation of one pound of water at 

 212° Fahr., being the heat which disappears in overcoming 

 the mutual attraction of the particles of water, and the exter- 

 nal pressure under which it evaporates. 



The mechanical equivalent of the available heat produced 



* It is worthy of remark, that the values of the specific heats of air were 

 predicted, to a close approximation, by means of the Mechanical Theory of Heat, 

 three years before they were ascertained by M. Regnault's experiments. (Trans. 

 Roy. Soc. Edin., vol. xx. ; Comptes Rendus, 1853.) 



A 2 



