THEORY DEDUCED FROM PHYSICAL LAWS. 33 



the cloud, and the expansion of the air under the cloud, 

 but the exact amount of this descent for a given cloud, can- 

 not, at present, be calculated, because it is not known ex- 

 actly what the temperature of the atmosphere is at very 

 great heights, and, therefore, the comparative specific gravity 

 of the cloud and of the surrounding air, cannot be precisely 

 estimated. It will, however, be near four hundred yards 

 for a fall of the barometer one inch, and eight hundred 

 yards for a fall of two inches under the cloud. 



I would not wish to be understood here as saying by im- 

 plication, that the numbers used in this paper are strictly 

 correct. These numbers are introduced chiefly for the pur- 

 pose of illustrating the theory. Yet as they are all within 

 the range of nature and generally near approximations to 

 the truth, they may be assumed as true, until future inves- 

 tigations shall furnish strictly accurate results. 



JUNE 10, 1841. 



I have, at various times since the experiments recorded 

 above were made, performed a much more extended series 

 of similar experiments, with a similar urn or vessel of glass, 

 in which the cloud could be seen when moist air was 

 used. 



To enable me to use higher pressures and greater relative 

 rarefications on opening the stop cock, I had a condensing 

 pump attached to the vessel, by means of which I could 

 force in air to any desirable amount. After this was done, 

 I let the instrument stand till the air within was the same 

 in temperature as the air without. I then measured how- 

 much higher the mercury stood on the outer leg of the gage 

 than on the inner. This gave me the degree of condensa- 

 tion. I then opened the stop cock, and let the air fly out, 

 and at the moment of equilibrium of pressure within and 

 without, I closed the cock, holding fast the air within, at 

 the moment of its greatest cold produced by the expansion. 



