412 



RECORD OF SCIENCE FOR 1887 AND 1888. 



Table XXII. — Initial depreHston or compJement of tlie dew-point necessary in order that 

 cloud or haze may form in a mass of rislny air at the ijiven altitudes above the starting 

 point. 



Conversely, when the air is drawn or forced down from the upper 

 regions it comes under greater pressure and is compressed and warmed 

 at the same rate as above, namely, 1° Fahr. for 180.5 feet or O.^SS 

 Fahr. for 100 feet. But this latter warming (like the preceding cool- 

 ing) is so largely afi'ected by radiation of heat, absorption of solar heat, 

 the mixing with air that has a different temperature, and by the absorp- 

 tion of heat in the process that is the reverse of condensation, i. e., that 

 of evaporating fog and other forms of water, that the actual average 

 rate of cooling, as we ascend in the air, is barely one-half of the above, 

 i. e., 0.033 Fahr. per 100 feet or OPQ C. per 100 meters. 



The fourth column in Table xxii shows the elevation at which haze 

 or cloud begins to be formed by the actual observed rate of cooling in 

 an ascending mass, the initial depression of whose dew-point is known. 

 Conversely, if we actually observe the height to which a mass of air 

 has ascended, when it begins to become hazy or cloudy, we can from this 

 last column deduce, at least approximately, the corresponding depres- 

 sion of the dew-point that obtained at its place of starting. 



The cooling by ascension is therefore not a truly adiabatic process. 

 Espy's rule of an ascent of 100 yards for each degree of depression of 

 the dew point is thus seen to be a close approximation. 



As soon as the dew-point is reached in this process of ascension and 

 cooling the condensation of vapor is accompanied by evolution of the 

 latent heat of steam, that is to say, of the enormous amount of heat 

 originally required to convert the water into invisible vapor, and which 

 has been carried from the surface of the earth up into the cloud region, 

 where it is now ready to be set free. 



