RISING AND COOLING OF WARM AIK. 1 19 



and iii consequence of it the air rising up from the 

 ground expands more and more, and in so doing 

 musl use up a part of its heat, which thus becoino 

 combined or latent. For it is well known that the 

 air, by sudden and powerful rarefaction, may be 

 cooled even to below the freezing-point of water, 

 as well as that, on the other hand, by sudden and 

 powerful compression air may be rendered so hot 

 as to ignite tinder. 



The temperature of the higher strata of the 

 atmosphere, so far as it depends on the influence 

 of the heated soil, is, of course, subject to daily 

 and yearly oscillations. The daily highest tem- 

 perature does not occur* till after the maximum 

 on the earth's surface. While, for instance, in 

 the summer months, this is attained on the plains 

 about 3 p.m., Kaemtz found it on the Rigi, at a 

 height of live thousand feet, at 5 p.m. ; Saussure 

 found it on the Col du Geant (in the Mont Blanc 

 region) at a height of ten thousand feet at 6 p.m. • 

 that is, at an hour when the temperature of the 

 earth was already diminishing, and when, there- 

 fore, the upward current was also slackening. The 

 more the surface of the earth is cooled at night, 

 the colder does the air become that is in contact 

 with it ; but this effect can extend to but a small 

 height, because the air that is thus cooled be- 

 comes at the same time heavier, and therefore 

 ceases to rise. 



