CLOUDS SURMOUNTING POBESTS. 41 



the deepest mine ; on the ocean's surface and on the dry land a 

 thousand miles away. 



But, as has been mentioned, while moisture may be found in the 

 atmosphere everywhere, it is not always present in the same quantity 

 in the same space. In almost any circumstances, but more especially 

 where there is an unlimited supply of moisture, the quantity varies 

 with the temperature, and varies with the temperature to a very 

 great degree. According to Sir John Leslie, it doubles with every 

 rise of twenty-seven degrees of temperature; and, according to 

 subsequent observers, it varies to a still greater extent, the doubling 

 taking place at different intervals, which increase slowly with the 

 temperature — the mean being 23° 4' — from the freezing point to 100° 

 Fahrenheit. 



According to Leslie's calculation, the air at a temperature of 59° or 

 60° will hold twice as much as at 32°, or the freezing point ; and at 

 86° it will hold twice as much again. And, according to later 

 observers, at 121° it will hold twice as much again, or sixteen times 

 as much as at 32°, eight times as much as at 53°, or four times as 

 much as at 75". 



It is customary to speak of this as the solvent power of the atmos- 

 phere, as if the moisture were dissolved in the air as sugar may be 

 dissolved in water ; but it is more in accordance with fact to consider 

 that as water requires a certain amount of heat to keep it from 

 passing into a state of ice, so it requires a certain amount of heat to 

 keep it in a state of vapour and prevent it from passing into a state 

 of water. 



In accordance with this, it happens that if air of a higher temper- 

 ature, having as much moisture in it as can be sustained in a state of 

 vapour, be cooled down some degrees, a quantity of moisture, which 

 can no longer be maintained in a state of vapour, will assume the 

 form of water in minute drops, and appear as fog, or cloud, or mist ; 

 but let the temperature be again raised to the same elevation as 

 before, and this water will again assume the state of vapour, and the 

 air become transparent, the fog or cloud having melted away. In 

 illustration of this fact, I may i-efer to what is seen in the laying of 

 the cloth on Table Mountain when the south-east wind blows. The 

 temperature on the top of Table Mountain is lower than that of the 

 wind blowing over its summit, and the mountain cooling down the 

 air, a quantity of the vapour intermixed with it is deposited in the 

 form of a cloud which is blown over the face of the mountain, and 

 falls, threatening to bury the city below ; but before it can reach 



