warm stratum along the surface of the earth rushes toward the opening, and there ascends, while the colder air above descends to take its place. If the ascending column of heated air remained stationary, it is evident that its supply of warm air would soon be exhausted, and the process therefore 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 go 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 difference 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 quantity of water into steam as it takes to raise the same quantity 1,000° of temperature. The same amount is required to evaporate water without 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 there 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 ascending air rapidly cools as it rises. Now, this air is carrying large quantities of vapor of water with it, which likewise is cooled by expansion. 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 rising. 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.
