194 DISCUSSION OF THE DESICCATION QUESTION. 



the increase of tempei'ature the elastic force of the vapour in the atmos- 

 phere is increased, and Avith it the rate of evaporation." * Hence, as a 

 general rule, the warmer regions of the earth are the areas over -wliich pre- 

 cipitation is greatest. No one can doubt for a moment that a diminution of 

 the mean temperature of the earth's surface, other conditions remaining the 

 same, would bring about a diminution in the total quantity of rain and snow 

 precipitated. This self-evident fact is to be kept in mind during the present 

 discussion, since it forms, so to speak, the basis on which the present chapter 

 rests. 



It needs, however, but little experience in the study of physical geography 

 and meteorology, or but a slight examination of the rain-charts of the world, 

 to convince the inquirer into this subject that the distribution of rain-fall on 

 the earth's surface is extremely irregular and — as it would, perhaps, at first 

 sight appear — even capricious. While the tropical regions, on the whole, 

 receive a very heavy precipitation, certain countries where the mean tem- 

 perature is very high are included within tlie almost or quite rainless areas, 

 while other regions, in high northern latitudes, where we know that the 

 mean temperature must be quite low, are abundantly supplied either with 

 rain or snow, or with both. We must therefore proceed to investigate what 

 other cause or causes influence precipitation on the earth's siu'face as a 

 whole, and then pass on to inquire what are the conditions by which its dis- 

 tribution over various regions is mainly influenced. 



Evaporation takes place from the surface of water, from moist surfeces 

 generalh^, and even from snow and ice. But the essential point is this : that 

 with a given temperature the larger the surfiice of the water exposed to the 

 atmosphere, other conditions being equal, the larger the amount of moisture 

 which will be taken up, while the quantity precipitated will, somewhere on 

 the earth's surface, be proportionately increased. 



For reasons, the ultimate cause of which we are quite unable to fothom, 

 we find the solid crust of the globe to be covered, to a large extent, with 

 water,! holding in solution a considerable quantity (about three and a half 



* A. Buclian, in "Handy Book of MetPorology." Second edition. Edinburgh and I^ondon, IStiS, p. 148. 



t Tlie ordinary statement of the i-atio of the surface of the laiul to that of tlie water on the glolic, at the 

 present time, is — in round numbers — four to eleven, the land occupying a little moie than one fourth of the 

 earth's surface. Behm and Wagner promise (in Erganzungsheft, No. 62, to Pelermann's Jlittheilungen, ISSO) a 

 complete revision of their former figures, relating to the areas of land and water surface on the globe ; but, so far 

 as known to the writer, these results have not yet appeared. The figures given by Kriimmel, in his " Versuch 

 einer Morphologic der Meeresraume," are declared by them to be incorrect. Kiiimmel made the entire ocean sur- 

 face equal to 3G6,50l),106 .square kilometers, or about 14H million square miles. 



