and SuUerranean Water Storage. 65 



able to sustain so large a quantity of water. Some may sliortly 

 come back again in heavy showers, and part of the water lilted 

 is carried on in a visible form until it can no longer be retained 

 owing to the altered condition of the air. Visible vapour is 

 mist or cloud ; from mists or clouds large quantities of water 

 fall to the earth as rain hundreds or even thousands of miles 

 from the spot whence it started. As rain, it generally falls 

 gently on the soil. Some of it runs off in water-courses, which 

 in time, by union with millions of similar streamlets, become 

 mighty rivers, and run back to the parent ocean. Some is at 

 once made use of by thirsty vegetable or animal life, and is con- 

 verted into leaves, stems, flowers or forest-trees, or enters into 

 the flesh and tissues of animals by combining with carbon and 

 a few other solid elements. Another part sinks into the 

 earth. 



But each part — including this last part — is greatly subdivided, 

 and has a distinct history. Thus, one group of those drops that 

 remain at the surface may trickle into a shallow pool and be re- 

 evaporated directly ; other drops may form part of a rivulet and 

 be conveyed to a deep lake ; and others, again, may return to the 

 sea by some river with as little delay as possible. So, again, of 

 that water that enters the earth the history is very varied. Such 

 considerations form the subject-matter of important chapters 

 of physical geography. The actual quantity of the rainfall in 

 a'' given period, and its distribution in days, months, and years, 

 is another inquiry altogether. The facts involved in this latter 

 inquiry cun only be determined by careful observation ex- 

 tending over a long period of time. The causes are extremely 

 varied, and depend on a multitude of peculiarities in the phy- 

 sical condition of the earth's surface. As these vary, so the 

 quantity and distribution vary ; no two localities are precisely 

 alike in this respect, and it is only as a matter of average and 

 approximation that we can consider the question or come to 

 a conclusion. It is not difficult to understand in a general way 

 the causes of difference, but in any particular case it is almost 

 impossible either to explain fully the result when we know it, or 

 anticipate the result by theorising. 



Still it is a very important question to all practical men, as 

 well as a very interesting inquiry in general science, to decider 

 what is the average annual rainfall in a particular place or 

 district. Not less is it important to know the causes to which 

 the quantity of rainfall is due so far as they are local. The ordi- 

 nary or extraordinary limits within which the rainfall varies 

 monthly, seasonally, and annually ; the existence or non-existence 

 of periods of larger or smaller supply ranging over many years ; 

 and finally the definite and permanent changes that seem to take 



VOL. II. — S. S. F 



