"You use the words which are commonly employed in explaining the origin of winds, and very likely your idea is right, but the language needs a little correction. The warm air does not rise of its own accord, so to speak, but is pressed upward. The warm air is expanded; it presses outward and upward; the same weight of warm air occupies more space than cold air; the warm air rises and overtops the surrounding air, and then flows off in order to reach the common level. The column of warm air is lighter than the cooler air, and cannot balance it; consequently, the cold air sinks down, pressing the warm air upward. In this manner an ascending current of warm air is formed, and also currents of cold air flowing from every direction toward the warm centre. These currents continue until the temperature of the air is equalized. "The atmosphere is commonly believed to be forty-five or fifty miles in height, though some men have estimated its height as very much less than this, while others believe it to be six or seven hundred miles in height. Are we to suppose that the column of heated air reaches to the top of the atmosphere?" "I think not," answered Mr. Hume. "The rarefaction of the lower part of the column renders the whole column lighter than the air around, and the warm air, as we know by the movements of the clouds, after rising a little way, spreads off in every direction, forming upper currents corresponding to the currents below, but moving in the opposite direction." "Only a few days ago," remarked Peter, "I saw in the same part of the sky clouds moving in exactly opposite directions, and others which seemed to be standing still. I knew how one layer of clouds might be moving north and another layer moving south, but I did not understand why some should be standing still." "Do you imagine, Peter, that the upper and lower currents of air, moving in opposite directions, come sharply together, the one sliding against the other?" "I think not," said Peter. "Supposing, then, as is certainly true, that a stratum of still air lies between the upper and lower winds, does not that explain how certain clouds might be standing still while the others were moving?" "I might have thought of that myself." "But how does this carry heat from the warmer region to the colder regions around?" asked Ansel. "I see how the colder air coming in would cool the warm region, and how the warm ascending air would carry away the excess of heat, but how do the cooler regions get the advantage of this heat?" "That is just what I was on the point of explaining. Do you remember what was said about the production of cold by expansion and of heat by compression?" "I remember that if air be rarefied by removing pressure from it, its temperature falls: I think you said that a part of its sensible heat becomes latent; and if air be compressed, its temperature rises. I have seen experiments with the air pump and condenser to prove this."