The Weather. 103 



An instrument well adapted to illustrate this peculiar property 

 of aqueous vapour is the cryopkorus. This is an instrument 

 composed of two glass globes connected together by a bent 

 glass tube. The whole is completely exhausted of air, and the 

 globes are partly filled with water. In this condition the 

 cryophorus resembles a kind of " water-hammer/'' for when the 

 water is emptied from one globe into the other, it rattles with 

 a noise like falling ice. To use the instrument, the emptied 

 globe is placed in a freezing mixture of pounded ice and salt. 

 The other globe, containing water only, is exposed freely to 

 the air. A brisk evaporation commences immediately from the 

 surface of the water, and the vapour passing over to the bulb 

 imbedded in the freezing mixture is condensed. While water 

 is converted into vapour, a large quantity of sensible heat is 

 rendered latent. If this is not supplied from without, the 

 effect of evaporation is to lower the temperature of the bulb, 

 and at last to freeze the water it contains. On this account 

 the instrument is called a cryophorus (/cpvos, cold; 6epa>,Icarry) ; 

 because cold, in effect, is carried from the freezing mixture to the 

 other bulb. The principles illustrated by this experiment of 

 the cryophorus are those of the condensing steam-engine, and 

 of the exhausted evaporating pans employed by refiners for 

 crystallizing sugar. The boilers in these machines are supplied 

 with external heat, and the condensers are kept cool by run- 

 ning water. 



At high temperatures great pressure is required to reduce 

 a gas to its liquid form, or, in other words, the greater thetem- 

 perature of a liquid the greater is the pressure of its vapour. 

 Boilers are thus replenished by vapour, of a pressure corres- 

 ponding to the temperature of the water. The space within 

 the boiler is in this case said to be saturated with vapour. At 

 a high pressure (and temperature) great mechanical effects can 

 be produced. In the cryophorus, above described, and in 

 performing distillation at low temperatures, the saturating 

 pressure of the vapour is also low. 



Maury has actually described the atmosphere as ee incompar- 

 ably the greatest steam-engine with which we are acquainted." 

 The temperature of the air at the equator is 83° Fahrenheit, 

 and the mean temperature at the coldest places near the Poles 

 is little above the zero point of Fahrenheit's scale. A similar 

 depression of temperature is found in ascending 50,000 feet 

 into the air. The atmosphere therefore more nearly resembles 

 an immense cryophorus, or vast distilling apparatus, of which 

 one boiler is placed at the equator and its condenser at the 

 poles, and another boiler is placed at the base and its condenser 

 at the summit of the atmosphere. Were the air at rest, an 

 universal cloud- canopy would envelope the earth by reason of 



