III.] RAIN AND DEW. 47 



by differences of tint, which are sufficiently explained by 

 the accompanying index. 



In examining the distribution of rain, it will be found to 

 be regulated partly by the physical features of the country, 

 and partly by the character of the prevailing winds. In the 

 neighbourhood of mountains, the rainfall is increased, since, 

 as has already been pointed out, a mass of moist air, when 

 forced up the side of a mountain, is chilled in the ascent, 

 and its moisture consequently discharged. Among our 

 western counties, in the neighbourhood of hills, the rainfall 

 rises to eighty, or even to a hundred, inches, and upwards ; 

 while away from hills, though still in the west, it is only 

 from thirty to forty-five inches. A table-land, or high plain 

 surrounded by mountains, will generally receive but little 

 rain, since the winds which reach it have been more or less 

 drained of moisture in sweeping over the surrounding hills. 

 For a like reason, but little rain is likely to fall on the lee 

 side of a high hill, and many mountains, consequently, have 

 a wet and a dry side ; the wet side being, of course, that 

 towards which the predominant winds blow. As regards 

 the influence of winds on rain, it is evident that, when air 

 has blown over a large expanse of warm water, it must have 

 become laden with moisture, which will be readily precipi- 

 tated on exposure to refrigerating influences. Hence, as in 

 Britain, so in the greater part of Europe, the southerly and 

 westerly winds bring rain ; and most rain falls in the exposed 

 westerly parts, such as the coasts of Portugal, Spain, France, 

 Britain, and Norway. There are certain conditions, how- 

 ever, under which rain is brought to our islands by easterly 

 rather than by westerly winds. 



It is in those regions in which the sun's heat is intense, 

 and powerful currents of highly-heated air, saturated with 

 watery vapour, are raised into the atmosphere, that the 



