A SHOWER OF RAIN. 41: 



sand and mud, were piled together, they would make a great 

 mountain chain. 



Running water, when provided with a due amount of 

 sand or gravel, is capable of undermining and rounding the 

 hardest rocks, as we shall see more particularly when we 

 come to speak of the rivers, but we have first to say some- 

 thing of their mother, the rain. 



What happens after an ordinary shower ? 



For a time the surface is wet and there may be puddles in 

 the hollows, but if the soil be a light one, all traces of the 

 rain soon disappear, and we say it has " dried up." Every 

 drop that has fallen is somewhere, however, and it has 

 certainly not all gone back to the air in the form of vapour, 

 though in hot or windy weather so much may do so that the 

 soil will benefit but little. 



The hotter the air the more water it can take up and hold 

 in the form of invisible vapour, but it can never hold more 

 than a certain quantity at any given temperature, and when 

 it has taken up this utmost quantity it is said to be saturated. 

 In our damp island the air is often saturated with moisture, 

 which is not an agreeable state of things ; but more un- 

 pleasant still is it when the air contains less than half the 

 saturating quantity, for it will suck up moisture wherever it 

 finds it, and therefore takes it, not only from the ground but 

 from vegetation, and even our bodies, making us feel " dried 

 up" and wretched. In the African deserts, where the very 

 air, as well as the ground, is parched with thirst, the traveller 

 finds that his lips crack and bleed, his whole body burns, 

 and his skin is dried till it bursts in a hundred places. 

 Even then, however, the air is not absolutely dry, though it 



