88 Daniell on the Correction 



.00243 as the mean. Now these estimates derived from baro- 

 metrical experiments, made at different temperatures and com- 

 pared with known heights, must have included also the ex- 

 pansion due to the mean quantity of vapour ; and upon refer- 

 ring to the table we shall find that the medium of the two 

 combined effects is exactly .00243 for every degree of tempera- 

 ture : for 0.85895 deducted from 100000, leaves 0.14105, which 

 divided by 58, the total number of degrees from 32° to 90 gives 

 .00243. 



General Roy, again, has fixed the point of temperature at 

 which the specific gravity of mercury to the atmosphere is 10.435 

 at 32° the average of his experiments, making it, however, a 

 little lower. Sir George Shuckburgh places it at 3I|. It is 

 worthy of remark that the point which approaches the nearest 

 by the table to exact coincidence is 31°, for at 32° the effects 

 of temperature being null, a fall of one degree is necessary lo 

 neutralize the expansion of the vapour. 



But it may be asked, Does vapour, thus acting upon the 

 atmosphere, and changing the degree of its absolute elasticity, 

 act in such a manner as to change it equally at all heights, " so 

 that though different at different times, it shall always at any one 

 time be the same at all different heights ?" 



This requires a little consideration. Vapour, uninfluenced by 

 circumstances of external temperature, would, I conceive, be 

 subject to the same law as the permanent gases of the atmo- 

 sphere ; it would decrease, namely, in density, in a geometrical 

 progression for equal ascents. By the application of logarithms, 

 we may therefore ascertain what the decrease of density would 

 be for any given height, and consequently its constituent tem- 

 perature. 



Let it be required to know the decrease of density which would 

 take place in an atmosphere of vapour of the elasticity of 0.200 

 inches, and whose constituent temperature is therefore 32° in 

 an ascent of 10,000 feet ; we must first find the height of an 

 homogeneous atmosphere of such vapour equivalent to 0.200 

 inches of mercury. 



