152 EFFECTS OP FORESTS ON RAINFALL. 



than that of the spot on which they strike, they seem to come from 

 some westerly point nearer to the equator. 



Water in contact with air appears to be absorbed by it in a state 

 of vapour up to a point of saturation, varying with temperature, 

 beyond which it cannot either absorb or retain more. As the air 

 advances from the polar regions towards the equator, its temperature 

 and consequent capacity of absorption are raised, and ere it reaches 

 the equatorial region it may have absorbed a quantity it cmild retain 

 where it is, but could not retain either at a greater height or in the 

 polar region whence it had come. Accordingly we find that in the 

 equatorial regions, when it is raised to a great elevation, and is thus 

 cooled, the air can no longer retain it all dissolved, and the surplns is 

 deposited in clouds. The moisture thus deposited, attracted by 

 gravitation, passing through lower strata of a higher teimperature, 

 not saturated, may be again absorbed ; and thus may be explainsd 

 the phenomenon of a continuously clouded sky unaccompanied by 

 rain, which is seen so frequently at sea, within the tropics, as almost 

 to be the characteristic of the sky in these regions. 



The current returning to the polar regions from the equator, in 

 passing over mountains on the summits of which a polar temperature 

 prevails, being thus cooled down, and unable to retain more than a 

 little of the moisture with which they are charged, the surplus is 

 there deposited in the form of rain, or sno'w, or sleet, or hail. 



To this also is attributable the humidity of the climate of England. 

 It may be traced to the prevalence of southerly and westerly winds, 

 which come to us laden, not only with moisture which they may have 

 absorbed in passing over the Atlantic in their passage thither, but 

 with moisture previously absorbed in their passage from the polar 

 towards the equatorial r^ions. 



In England, the southerly and westerly winds are to northerly and 

 easterly winds in the proportion of 196'4 to 135. 



There are waves of wind which come sweeping over the Atlantic 

 from the north-west, but these are comparatively rare. Those which 

 come blowing from the south and the west, and more especially those 

 which cfme blowing from the south-west, have come at a higher 

 level, with an under current, between them and the ocean, flowing in 

 an opposite direction ; and these are the most prevalent winds in 

 England. The following are given by Steinmitz, in his little populai- 

 work, entitled " Sunshines and Showers," as the mean of several 

 years as the number of days on which the following winds were 

 observed : — 



