PRECIPITATION 21 



its moisture appear as a cloud, exactly in the same way as that in 

 which the frost of winter causes the moisture of our breath to 

 appear as a vapour cloud. In this second excursion we may go 

 even a little further. We think of the warm air rising from the 

 valley, loaded with the moisture which it has gathered from the 

 river. In imagination we see it cooling as it rises, throwing down 

 its load of vapour as the mountain mist, and then being displaced 

 by new gusts of warm damp air coming up from the low ground. 



We notice that even though it be perfectly still on the low 

 ground there is nearly always a breeze on the hill, and we try to 

 >icture to ourselves the meaning of this breeze, how it is always 

 trrying the warm damp air upwards. We examine the trees or 

 mshes on the slopes of our hill, and we try to find from them 

 or even from the way the sheepfolds are built whether or not the 

 ind blows more frequently from one direction than another. 



If our examination is conducted on a real hill, and not an im- 

 'inary one, we shall probably soon find facts which lead us to 

 suspect that just as in the valley, so on the hill, the west winds are 

 tost frequent. That means that the warm damp air blows up one 

 >ide of the hill more frequently than up the others. As it sweeps 

 ip it cools and its moisture is condensed. But it sweeps on and 

 down the other side. As it sweeps down it gets warmer, just as it 

 got colder in ascending. In ascending it threw down its vapour, 

 therefore it has but little left to throw down on this side, which 

 ill be drier than that facing the wind. 



All these statements are of course only very partially true for an 

 isolated hill, but they are sufficiently true for us to pass easily to 

 the great deduction that if a long range of hilly ground runs north 

 and south in a country where the prevailing winds are westerly, 

 then the side which faces the west will be wetter than the side 

 facing east. 



Once again, we stand on our hill and look down to the valley 

 or plain below. However small absolutely our hill is, we try to 

 picture to ourselves the air always carrying up moisture from the 

 river to the top of the hill, always being compelled by the hill to 

 throw down its burden, and yet always recommencing the task. 

 But the river remains in spite of the robber who is always at work ; 

 what becomes of the spoil which the mountain reclaims from the 



