THtS FORCES THAT PROBtJCE STOBMS. 



61 



the extent of one half, as just explained, and the result is 

 that the actual cooling of the mixed mass is only 1° instead 

 of 2°. This double process being continued to greater heights 

 in the atmosphere, the absolute cooling of the ascending mass 

 is only half a degree for every hundred yards of height. 



The facts here stated may be proved by pumping out air that 

 has been deprived of vapour from the receiver of an air pump, 

 until expansion of the air within takes place to an extent equal 

 to that which occurs, on air being removed from the surface 

 of the earth to the height of one hundred yards in the atmos- 

 phere, when, as we have seen, the temperature sinks 1°. But 

 if the air in the receiver be saturated with vapour, some of 

 the vapour will be condensed by the cold of expansion, and 

 then the temperature of the mixture will be found to be 

 reduced only about half a degree. It follows from this expe- 

 riment that, in an atmospheric column that is ascending to 

 higher regions, and in which condensation of vapour is taking 

 place, the heat liberated reduces the cooling to about one half 

 of what it would otherwise be ; and two adjoining masses or 

 atmospheric columns of the height of, say four thousand eight 

 hundred yards, the one undisturbed by condensation, and the 

 other having condensation going on within it, would have the 

 temperatures as put down in the following table at the 

 heights named, the dew-point and temperature at the surface 

 of the earth being supposed to be both at 80^ : — 



Now it is evident that in the part of the atmosphere which is, 

 say one thousand yards high, the clear air of the temperature 



