Masterpieces of Science 



this rain. These uplands retain the cold of 

 winter, and during night they pour forth their 

 heat by radiation through the thin air, with more 

 rapidity than the lower lands, which are covered 

 beneath a thicker blanket of atmosphere. 



When the drop of rain falls to the earth's 

 surface, if it be of ordinary size, it gives a sensible 

 blow. If that surface be covered with a thin 

 layer of scattered sand-grains or small pebbles, 

 we may observe that the bits of rock dance about 

 and thus apply a little of the force which comes 

 from the drop, to rub the stone on which they 

 lie. At first, the water spreads over the earth's 

 surface as a thin sheet, but as that surface is 

 never perfectly level, it is, provided the rock be 

 bare, quickly gathered into rivulets; or if it be 

 covered with mosses, or the thin layer of porous 

 soil common to mountain-tops, it may for a mo- 

 ment disappear from sight in the spongy mass; 

 but a little farther down, we find that it is gath- 

 ered in rivulets, which quickly join together, so 

 that in descending even a hundred feet below 

 the summit, in a time of rain, we find a number 

 of shallow valleys, each occupied by a little 

 rivulet. The union of these streams gives us 

 one of more power, which may be taken as a 

 typical mountain torrent. We observe that 

 such a stream descends with considerable rapid- 

 ity; it is rare indeed that it does not have a fall 

 of more than fifty feet to a mile. The rate of 

 fall in steep-faced mountains often amounts to 

 as much as five hundred feet in that distance. 

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