156 EFFECTS OF FORESTS ON RAINFALL. 



Of the moisture thus absorbed by the air from the wide ocean 

 and the inland sea, and borne onwards to the tropics, no trace is 

 seen in the atmosphere till the air is cooled below the. temperatare 

 at which it can keep it so sustained. This happens when it is first 

 raised aloft, and when, on its return towards the poles, it impinges 

 on some mountain range or solitary peak. 



But while the course detailed may be considered the normal 

 course of the air in its current to and from the equator, it is liable 

 to many disturbances. 



It is not usual for two currents of water flowing in difl:erent, and 

 to some extent contrary directions, to glide past each other, each 

 pursuing its own way ; there is friction, and asaresult of this friction 

 there are eddies and whirlpools. It is so when they flow side by side ; 

 it is so when they flow the one over the other ; and it is so with 

 these aerial currents : the upper current coming completely through 

 the lower, and producing those westerly winds, of which, otherwise, 

 we should have none in medium latitudes; and it is reasonable to 

 conclude by inference that there may be eddies of the lower current 

 passing into the uppei\ Every one of these disturbances must 

 aflfect the temperature, and consequent capacity of a great mass of 

 air to retain the moisture with which it is charged, and that in 

 different ways : for instance, the eddy of the warm upper current 

 may be so cooled down that it must deposit a great quantity of the 

 moisture which it held, or the eddy of the lower current cooling 

 down a portion of the upper current penetrated by it may render it 

 incapable of retaining all the moisture with which it was charged ; 

 and in each of these cases rain or snow, or hail, according to circum- 

 stances, may be the consequence. The eddy moreover may take a 

 wide sweep, developing into a whirlwind or cyclone, with a radius of 

 a hundred miles or more, and advancing onwards thousands of miles, 

 carrying disturbance of temperature, and of hydrometric conditions 

 wherever it goes. Similar whirlwinds I have seen produced upon a 

 smaller scale, with all the indications of capability of being in like 

 manner developed on a scale as gigantic, simply by the sun beating 

 on arid ground, or on ground from which water has evaporated. 



In the centre of such a whirlwind the air of the district over which 

 it passes may be raised to such a height as to be cooled so low that 

 much of its moisture is precipitated, and falls in two parallel lines 

 of rain, or hail, marking the lateral limits of its course ; and over the 

 whole area of the ground traversed by it, embracing hundreds of 

 thousands of square miles, and over all the area of the ground 



