THE CYCLONE IN THE UNIVERSE. 211 



warm stratum along the surface of the earth rushes toward the open- 

 iiio;, and there ascends, while the colder air above descends to take its 

 jalace. If the ascending column of heated air remained Btationarj'-, it 

 is evident that its supply of warm air would soon be exhausted, and the 

 process therefoi-e speedily come to an end. But this is not so. It 

 moves forward to where there is more heated air, just as one might 

 fancy a chimney to travel after a moving fire. The operation can thus 

 iro on for a considerable time. 



The ascension of air with a higher sensible temperature would not 

 alone suffice to supply the tremendous power of the cyclone. The 

 diiference of temperature, even in extreme eases between lower and 

 higher strata, is wholly insufficient to account for the enormous energy 

 developed by our cyclones of hundreds of miles in diameter. We 

 therefore require another source of power. Nor have we far to seek 

 it. The sun's rays falling upon dry earth heat it, and thus raise the 

 temperature of the air in contact with it. But if they fall upon our 

 oceans, lakes, and rivers, or upon moist earth, there is another result, 

 of a somewhat different though equally familiar kind. It is this, that 

 some of the water is converted into steam or vapor. Now, every one 

 knows that no amount of heat can raise the temperature of boiling- 

 water if it is unconfined. Where, then, does the heat go to? Plainly 

 it is carried off by the vapor in an insensible or latent condition. It 

 is a demonstrated fact that it requires as much heat to convert a quan- 

 tity of water into steam as it takes to raise the same quantity 1,000 

 of temperature. The same amount is required to evaporate water 

 vvithout boiling it. Consequently, when the sun's rays evaporate 

 water, a vast amount of heat becomes insensible to our thermometers. 

 It is not annihilated, however, and all that is required in order to 

 make it manifest is simply to condense the vapor into water again. 



When the heated air, as already described, rushes up in a column, 

 it becomes subjected to less and less pressure, because thei'e is less and 

 less air above it. Since air in expanding under pressure produces 

 work, and since heat is an equivalent of work, it expends heat in so 

 doing, and is thus lowered in temperature. Consequently, the as- 

 cending air rapidly cools as it rises. Now, this air is carrying large 

 quantities of vapor of water with it, which likewise is cooled by ex- 

 pansion. But you cannot cool vapor at any tension below a certain 

 temperature without condensing it ; and so, indeed, it happens. The 

 steam carried up by the cyclone is condensed into rain, snow, or hail, 

 and falls to the earth. In condensing it gives forth the enormous 

 quantity of insensible heat which it received from the sun. This heat 

 is imparted to the ascending current, and thus keeps it warmer and 

 therefore specifically lighter than the strata through which it is ris- 

 ing. The heat of the sun, which had been potential in the vapor, is 

 converted into the energy developed by the cyclone. 



We thus see that the cyclone is really a kind of vast steam-engine. 



