Page:Popular Science Monthly Volume 78.djvu/38

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earth's surface it would be wanned up by compression so as to be insufferably hot and dry. This is the method of the formation of the hot, dry southwest winds of Kansas, Arkansas and Oklahoma where "corn is roasted on the stalk." Our North American cold winds from the northwest represent one step in the general condition of the whole atmosphere; they are undoubtedly descending winds, but descending so slowly and rolling along the earth to such great distances toward the equator that the air has time to cool by radiation before it reaches us. This is the formation of our areas of "high pressure" and cool, dry, clear weather.

In close analogy to the steam engine driven by heat that is derived from fire but is lost in the condenser, so the motive power in the atmosphere is the heat received from the sun, but lost by radiation from the earth. As to quantity and quality of this solar heat we are still at the beginning of our knowledge. Eminent authorities adopt figures ranging between two and four calories per minute per square centimeter. Every effort must be put forth to determine more accurately this fundamental datum.

Very many insist on searching our climatological records for periodic phenomena such as solar rotation periods and sunspot periods and lunar periods. Br├╝ckner's period of 35 years is quite famous. We have as yet very little data on which to base satisfactory researches into these questions, but the trend of our present knowledge is to show that in so far as these periods depend upon external or cosmical influences they are too feeble to be of importance; in fact, too feeble to be clearly recognized.

On the other hand, in so far as they depend on the internal mechanism of the atmosphere, they die out in a short time after they have been started, and are not permanent or steady periods in the proper sense of the word, but are driven and imposed on the atmosphere by conditions outside of it. Just as we see ripples standing in the rear of a stone in a shallow stream of water, so we have waves and clouds in the atmosphere on the leeward side of every obstacle. The annual periodic changes in the declination of the sun and in the resulting monsoons are undoubtedly accompanied by great reactions in our atmosphere extending like waves around the whole globe; but these again die out in a few years. Almost the only periodic phenomenon due to the internal mechanism of the atmosphere, one that is permanent and appreciable, is the semi-diurnal change of pressure which appears likely to be an internal phenomenon of resonance maintained by the regular diurnal change of temperature. But these questions are not settled and remain for further investigation.

I think the great climatic changes that seem to have taken place during geological history must be explained in connection with the