DEPOSITION OF DEW 55 



inches above the bottom. The depth of soil in the barrel was 

 3 ft. 6 in. With this large and permanent supply of water 

 very luxuriant crops were obtained, far exceeding those pro- 

 duced in the surrounding field. With oats and barley a yield 

 of 10,000 Ib. of dry matter per acre was obtained ; with 

 potatoes 1 2,000 Ib. ; with maize over 20,000 Ib. The water 

 consumed would average about 24 inches per acre ; a quantity 

 of course far exceeding the summer rainfall. Average English 

 crops, yielding about 4,000 Ib. of dry matter per acre, will 

 probably evaporate five or six inches of water during their 

 growth. 



There is not enough evidence to point out any particular 

 crop as especially wasteful or economical in its consumption 

 of water ; the figures in the table however certainly suggest 

 that the cruciferae should be ranked with those requiring 

 most water, and maize with those requiring least. The sub- 

 ject is -a promising one for further investigation. 



The principal source of soil water lies of course in the rain, 

 hail, and snow precipitated from the atmosphere ; the amount 

 of this precipitation in any place, and its distribution through 

 the different seasons of the year, are facts which fundamentally 

 affect the fertility of the land. The amount and distribution 

 of the rainfall is however a subject lying beyond the scope 

 of the present lectures. Of the underground water supply, 

 which in some localities is of very considerable agricultural 

 importance, we shall have something to say by-and-by. It 

 will be convenient before proceeding further to notice two 

 sources of water supply to the soil which are of minor im- 

 portance, but which fairly come under the physical properties 

 of soil. 



Deposition of Dew. If air containing water-vapour is 



