of Barometrical Mensuration. 89 



But its specific gravity, compared to dry air, is as 2.317 to 

 557.800, therefore 2.317 : 557.800:: 10435 : 2,512,146; then 

 2,512,146x0.2 ins. = 502429 ins. = 41.869 feet. 



Height of homogeneous Feet. Modulus of Lo- Diff. of Logs, of 



Vapour. garithms. Densities. 



41869 : 10,000 :: .4342945 : .1037198 



Density of va- 

 pour at 32* 



Log. of .200 = .3010300 

 — .1037198 



= .1973102 Log. of .157 



which is the elasticity of vapour at 25°. 



To select another example from the other extremity of the 

 scale, let us take vapour at 212°, and 30 ins. elasticity. The 

 height of the homogeneous atmosphere we shall find 56,567 feet. 

 56,567 : 10,000 feet :: .4342945 : .0767752 



Density of va- 

 pour at 212" 



Log. of 30.00 = .4771212 

 — .0767752 



= .4003460 = 25.14 



the elastic force of vapour at 203|. 



In this manner I have found, by various trials and for dif- 

 ferent altitudes, that the elastic force of vapour would decrease 

 in such a proportion as to lose 1° of constituent temperature 

 for every 1250 feet; or, in other words we may say, more cor- 

 rectly, that it is a necessary condition of an atmosphere of pure 

 vapour, that its constituent temperature increase from above 

 downwards 1° for every 1250 feet. But this can never be ex- 

 actly the case with vapour in the state of mixture as it exists 

 in our atmosphere. The nearest approximation to it is when 

 its constituent temperature is very considerably below the 

 general temperature ; as, for example, when the point of pre- 

 cipitation is at 40°, and the heat of the air 70°. The heat of 

 the air, decreasing 1° for about every 290 feet of elevation, 

 would not attain the precipitating point at a less height than 

 8700 feet. Within this range, then, the vapour would be left 

 to its own law of density, but still modified by the excess of heat. 

 In the case of complete saturation, however, it is evident tliat 



