Rivers and Valleys 



laws, will obtain a clew to a very large part of 

 the earth's machinery. 



To see the actual beginning of the river under 

 the conditions which are best for our inquiry, we 

 must observe the surface at some point on the 

 dividing line between two streams where they 

 head together, near the crest of a mountain, in a 

 time of rain. All that is visible are the drops of 

 rain which slip out of the air and patter on the 

 surface of the earth. We must be prepared at 

 the outset to look past this simple fact of rainfall 

 and to conceive the physical history of the drop 

 of water since it left the surface of the earth in 

 its journey through the clouds and back to 

 earth again. 



The story of the rain-drop before it comes to 

 the earth is very simple. The heat from the 

 sun, aided in a small measure by the heat from 

 all the stars, evaporates the water from the 

 earth's surface, mainly from the sea, and removes 

 it in the state of vapor to a height of many thou- 

 sand feet above the earth's surface. It is main- 

 tained there by the heat which it has absorbed, 

 and thus the main spring of the rain is in the 

 sun. After abiding awhile in the upper regions 

 of the atmosphere, by some of the many chances 

 which beset the clouds, the vapor is cooled; it 

 condenses from the loss of heat, and falls as rain 

 or snow. The circumstances of our imaginary 

 mountain top, if that summit be at a consider- 

 able height above the sea, favour the cooling 

 of the cloud and therefore the precipitation of 

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