MOTIVE POWER OF HEAT. 167 



it is probable, from various experiments which 

 have been made, that the density of vapor follows 

 very closely the simple laws which are so accurately 

 verified by the ordinary gases;* and thus it may 

 be calculated from Regnault's table giving the 

 pressure at any temperature within those limits. 

 Nothing as yet is known with accuracy as to the 

 density of saturated steam between 100 and 230, 

 and we must be contented at present to estimate it 

 by calculation from Regnault's table of pressures; 

 although, when accurate experimental researches 

 on the subject shall have been made, considerable 

 deviations from the laws of Boyle and Dalton, on 

 which this calculation is founded, may be dis- 

 covered. 



34. Such are the experimental data on which 

 the mean values of // for the successive degrees of 

 the air- thermometer, from to 230, at present 

 laid before the Royal Society, is founded. The 

 unit of length adopted is the English foot; the 

 unit of weight, the pound ; the unit of work, a 



count upon at present, we might neglect it altogether, and 

 take dp/Mi simply, as the expression for /*, without com- 

 mitting any error of important magnitude. 



* This is well established, within the ordinary atmos- 

 pheric limits, in Regnault's Etudes Meteorologiques, in the 

 Annales de Chimie, vol. xv. , 1846, 



