188 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1920. 



soon as the sun has set, these clouds and the air masses they fill cool 

 more rapidly than does the clear atmosphere. They, therefore, fall 

 to lower levels, warm up to higher temperatures than they originally 

 had, and evaporate. 



It will be interesting, in this connection, to note the logical effect of 

 a certain ingenious, often proposed, and at least once experimentally 

 tried, method of artifically inducing rainfall, namely, the liberal 

 sprinkling of a cloud mass with liquid air. The result is, of course, 

 an initial cooling of the cloud, followed, as above explained, by a 

 much greater warming. Instead of rain being induced by this proc- 

 ess, as its many inventors would confidently expect, the chilled cloud 

 is certain to grow warmer and diminsh in size, and, if considerably 

 chilled may grow so much warmer as to disappear entirely. Indeed, 

 this particular liquid air scheme is not a rain-making process at all, 

 but, on the contrary, a rain deterrent. 



NOT AIR THAT IS HEATED, BUT AIR THAT IS NOT HEATED, IS THEREBY 



WARMED. 



This particular paradox may suggest the superiority of " absent 

 treatment" ; nevertheless, it is perfectly sound. Heated air, as we 

 know, is driven up by the surrounding denser air, and dynamically 

 cooled, but the air that drives it does so by dropping to a lower level, 

 where it is more or less compressed and correspondingly warmed. In 

 other words, while the particular air that was heated rises and gets 

 colder than it was initially, other air that was not heated at all falls 

 lower and thus gets warmer. It is not the air that is heated but air 

 that is not heated that gets warmer. 



NOT AIR THAT IS CHTLLED, BUT AIR THAT IS NOT CHILLED IS THEREBY 



COOLED. 



The explanation of this paradox is very similar to that of the one 

 just given, and is equally simple. As the chilled air descends, certain 

 other air is thereby raised and dynamically cooled. That is, while 

 the particular air that was cooled descends and thus gets warmer than 

 it was originally, other air that was not chilled at all is forced up, 

 expands, and gets colder. It is not the air that is chilled (unless it 

 haj^pens to be on or near the surface where it can not fall to a lower 

 level) but air that is not chilled that gets colder. 



MIXING BRINGS THE AIR TO A NON-UNIFORM TEMPERATURE. 



To the laboratorian familiar with beakers and calorimeters ; to the 

 housewife skilled in the art of the cups and kettles ; and to all the 

 rest of us, nothing is more certain — nothing more in accord with daily 



