34 THE POPULAR SCIENCE MONTHLY 



earth's surface it would be warmed up by compression so as to be insuf- 

 ferably 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 atmos- 

 phere 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 rang- 

 ing between two and four calories per minute per square centimeter. 

 Every effort must be put forth to determine more accurately this fun- 

 damental datum. 



Very many insist on searching our climatological records for peri- 

 odic phenomena such as solar rotation periods and sunspot periods and 

 lunar periods. Bruckner'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 mech- 

 anism 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 mon- 

 soons 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 



