SECT. XV. DENSITY OF THE ATMOSPHERE. 119 



for every degree of Fahrenheit's thermometer. Experience shows 

 that the heat of the air decreases as the height above the surface 

 of the earth increases. It appears that the mean temperature of 

 space is 226 below the zero point of Fahrenheit by the theories 

 of Fourier and Pouillet, but Sir John Herschel has computed it 

 to be - 239 Fahr. from observations made during the ascent in 

 balloons. Such would probably be the temperature of the sur- 

 face of the earth also, were it not for the non-conducting power of 

 the air, whence it is enabled to retain the heat of the sun's rays, 

 which the earth imbibes and radiates in all directions. The 

 decrease in heat is very irregular ; each authority gives a 

 different estimate, because it varies with latitude and local 

 circumstances, but from the mean of five different statements it 

 seems to be about one degree for every 334 feet ; the mean of 

 observations made in balloons is 400 feet, which is probably 

 nearer the truth. This is the cause of the severe cold and perpetual 

 snow on the summits of the alpine chains. In the year 1852 four 

 ascents in a balloon took place from the meteorological observatory 

 at Kew, in which the greatest height attained was 22,370 feet. 

 The observations then made by Mr. Welsh furnished Sir John 

 Herschel with data for" computing that the temperature of space 

 is minus 239, that is 239 below the zero point of Fahrenheit, 

 that the limiting temperature of the atmosphere is probably 77 

 degrees below that point at the equator, and 119 below it at the 

 poles, with a range of temperature from the surface of 161J in 

 the former case, and 119J in the latter. During these ascents it 

 was found that the temperature of the air decreases uniformly 

 up to a certain point, where it is arrested and remains constant, 

 or increases through a depth of 2000 or 3000 feet, after which it 

 decreases again according to the same law as before. Throughout 

 this zone of constant temperature it either rains, or there is a 

 great fall in the dew point ; in short, it is the region of clouds, 

 and the increase of temperature is owing to the latent or absorbed 

 heat set free by the condensation of the aqueous vapour. In the 

 latitude of Kew the cloud region begins at altitudes varying 

 between 2000 and 6500 feet, according to the state of the 

 weather. 



Were it not for the effects of temperature on the density of the 

 air, the heights of mountains might be determined by the 

 barometer alone ; but as the thermometer must also be con- 



