64 Raivfall, Natural Drainage, 



The reader must be content to accept this statement, and the 

 conclusions and inferences that result from it. Without some 

 definite notion of the earda's structure and history the circulation 

 of water cannot be understood. But this history is written only 

 in the rocks of which its surface is made up, and that surface is 

 but a thin crust. This is all we have to deal with, for it includes 

 all that we can examine ; and it is amply sufficient for our 

 purposes, for as all known rocks and minerals were formed by 

 and with water and air, always Avithin such limits of temperature 

 as made a complex condition possible, this view of rocks simpli- 

 fies the question, and enables us to proceed at once a step 

 further. 



I. Eainfall : ITS Distribution and Quantity. 



Owing' to the form of the earth and its position and move- 

 ments with respect to the sun, and owing also to the fact that the 

 surface of the earth is only partially covered by water and 

 entirely by air, there is a perpetual movement of the water in its 

 bed and a constant circulation of air above. The circulation of 

 the air is effected partly by the varied influence of the sun, partly 

 by the motion of the earth. There are certain very important 

 atmospheric currents that are constant and on a very grand scale, 

 and many others that are variable or only periodical. Of the 

 former the Trade-winds are examples. The latter are familiar 

 to every one. With regard to the Trade-winds, we perceive but 

 a small part of their regularity. They are only parts of currents 

 that rise in the tropics, range steadily at vast elevation, and 

 return from the Poles in a never-ceasing circuit. Many other 

 currents exist, but most of them are so constantly interfered with 

 and localised near the earth that we regard the winds as in- 

 constant. Like all the great operations of nature, they obey 

 certain laws, and are really constant in the highest and most 

 important sense of the term. It is one result of the action of 

 these laws that within the limits of temperate latitudes there is 

 a region especially characterised by variable winds. 



The circulation of air by currents is the cause of a grand 

 system of circulation of water. Air dissolves water as water 

 dissolves salt. It takes up a certain portion without losing its 

 transparency. The quantity taken up is larger or smaller accord- 

 ino- to the temperature, and when the temperature is lowered the 

 water continues in the air in a visible state as cloud. Thus, 

 for example, a very large quantity of water is dissolved by and 

 rises with the hot air that is about to commence its circuit 

 from the regions near the equator. This hot air gets both cooler 

 and less dense as it rises, and is thus gradually less and less 



