Mr. W, J. M. Rankine on the Mechanical Action of Heat. 181 



the effect of one cubic foot of water as calculated above to that 

 which con'esponds with his definition, we must deduct ^, which 

 leaves 



5,865,781 lbs. raised one foot. 



M. de Pambour's own calculation gives 

 6,277,560, 

 being too large by about one-fifteenth. 



Explanation of Tables to be used in calculating the Pressttre, 

 Volume, and Mechanical Action of Steam, treated as a Perfect 

 Gas. 



(32.) The object of the first of the annexed tables is to facili- 

 tate the calculation of the volume of steam of saturation at a 

 given pressure, of the pressure of steam of saturation at a given 

 volume, and of its mechanical action at full pressui'e. 



The pressures are expressed in pounds avoirdupois per square 

 foot, and the volumes by the number of cubic feet occupied by 

 one pound avoirdupois of steam, when considered as a perfect 

 gas ; those denominations being the most convenient for mecha- 

 nical calculations in this country. 



The columns to be used in determining the pressure from the 

 volume, and vice versa, are the third, fourth, sixth and seventh. 



The third column contains the common logarithms of the 

 pressures of steam of saturation for every fifth degree of the 

 Centigrade thermometer from —30° to +260°; that is to say, 

 for every ninth degree of Fahrenheit's thermometer from —22° 

 to +500°. 



The fourth column gives the differences of the successive terms 

 of the third column. 



The sixth column contains the common logarithms of the 

 volume of one pound of steam of saturation corresponding to the 

 same temperatures. 



The seventh column contains the differences of the successive 

 terms of the sixth column, which are negative ; for the volumes 

 diminish as the pressures increase. 



By the ordinary method of taking proportional parts of the 

 differences, the logarithms of the volumes corresponding to in- 

 termediate pressures, or the logarithms of the pressures corre- 

 sponding to intermediate volumes, can be calculated with great 

 precision. Thus, let X + A be the logarithm of a pressure not 

 louiid in the table, X being the next less logarithm which is 

 found in the table ; let Y be the logarithm of the volume cor- 

 responding to X, and Y — k the logarithm of the volume corre- 

 sponding t(j X + /< ; let II be the difference between X and the 

 next greater logarithm in the table, as given in the fourth column, 



