228 SIXTH RKI'ORT 1836. 



rometric pressure of 0»-76 (= 29-922 inches) to be 10462. From 

 which it appears that if G be the measure of gravity at the place 

 where this experiment was made (the Observatory of Paris), the 



value of ~ is 0°i'76 x 10462 G, oi- 7951-12 x G. For any 



other place this quantity must be multiplied by the ratio of the 

 force of gravity at that place to the force represented by G. The 

 numerical value of G is 9'^-80896, equivalent to 32-1824 English 

 feet. The above coefficient of G is obtained on the supposition 

 that the air is perfectly void of moisture. It has been ascertained, 

 by a process wliich will be touched upon at a subsequent part of 

 the Report, that the ratio of the density of air completely satu- 

 rated witli vapour, to air perfectly dry under the same pressure, 

 is 0-99749. On multiplying 795*1 ""-1 2 by this factor the result 

 is 797l™"09, which applies to air containing its maximum quan- 

 tity of humidity. The mean of the two values, 7961°*- 10, may 

 be supposed to apply to the usual state of the atmosphere. Its 

 equivalent in English fathoms is 4353-26. A correction should 

 also be given to the factor 0-00375, on account of the effect of 

 vapour. When the temperature increases the quantity of vapour 

 in the atmosphere augments at the same time, and as the density 

 of vapour under the same pressure is greater than that of air, a 

 given quantity of humid air will dilate more than an equal quan- 

 tity of dry air : it has been usual in consequence to change the 

 above factor to 0-004. As the temperature hitherto spoken of 

 is always that indicated by the air thermometer when a mercu- 

 rial thermometer is employed, a correction may be thought ne- 

 cessary, on account of the different rates of expansion of mercury 

 and air. The experiments, however, of MM. Petit and Dulong 

 show that this correction is insensible between 0° and 100° cen- 

 tigrade, and only begins to be of considerable magnitude at a 

 temperature of 300° centigrade*. Within the same limits the 

 increase of elastic force was found to be proportional to the in- 

 crease of temperature, the volume being constant f. 



By the means above indicated, one relation between the press- 

 ure, density, and temperature of an aeriform body has been ex- 

 perimentally assigned, and the two constants which the equation 

 expressive of this relation involves have been determined with 

 great exactness. But in addition to this equation the mathe- 

 matician requires another for the solution of any question in 

 which the effect of variation of temperature is to be taken into 

 account. For instance, if it were proposed to determine the 



♦ Memoir on the Dilatation of Gases. Jotirnal de I'Ecole Poll/technique, 

 call. 18. p. 213. t p. 200 of the same Memoir. 



