THE NEW AGRICULTURE. 85 



circulation, you will at once see that the quantity of water upon 

 the earth's surface must be constantly and visibly diminishing. 

 But no such changes, so far as can be seen, are really taking 

 place. In spite of the rain which disappears into the ground, the 

 circulation of water between the air, the land, and the sea contin- 

 ues without perceptible diminution. 



"You are driven to conclude, therefore, that there must be some 

 means whereby the water underground is brought back to the 

 surface. This is done by springs, which gush out of the earth, 

 and bring up water to feed the brooks and rivers, whereby it is 

 borne into the sea. Here, then, are two distinct courses which 

 the rainfall takes one below ground, and one above. It will be 

 most convenient to follow the underground portion first. 



" A little attention to the soils and rocks which form the surface 

 of a country is enough to show that they differ greatly from each 

 other in hardness, and in texture or grain. Some are quite loose 

 and porous, others are tough and close-grained. They consequent- 

 ly differ much in the quantity of water they allow to pass through 

 them. A bed of sand, for example, is pervious ; that is, will let 

 water sink through it freely, because the little grains of sand lie 

 loosely together, touching each other only at some points so as to 

 leave empty spaces between. The water readily finds its way in 

 among these empty spaces. In fact, the sand bed may become a 

 kind of sponge, quite saturated with the water which has filtered 

 down from the surface. A bed of clay on the other hand, is im- 

 pervious; it is made up of very small particles fitting closely to 

 each other, and therefore offering resistance to the passage of 

 water, which, unable to sink through it from above on the way 

 down, or from below on the way up to the surface again, is kept in 

 by the clay, and forced to find another line of escape. 



" Sandy soils are dry because rain at once sinks through them ; 



