TILLAGE AND EVAPORATION 153 



if two are opened, nearly twice as many may be ad- 

 mitted in the same time ; if more gates are opened, 

 the passenger's will be able to enter the train more 

 rapidly. The water in the lower layers of the soil is 

 ready to move upward whenever a call is made upon 

 it. To reach the surface it must pass from soil grain 

 to soil grain, and the larger the number of grains that 

 touch, the more quickly and easily will the water 

 reach the surface, for the points of contact of the soil 

 particles may be likened to the gates of the railway 

 station. Now if, by a thorough stirring and loosen- 

 ing of the topsoil, the number of points of contact 

 between the top and subsoil is greatly reduced, the 

 upward flow of water is thereby largely checked. 

 Such a loosening of the topsoil for the purpose of 

 reducing evaporation from the to])soii has come to 

 be called cultivation, and includes plowing, harrow- 

 ing, disking, hoeing, and other cultural operations by 

 which the topsoil is stirred. The breaking of the 

 points of contact between the top and subsoil is un- 

 doubtedly the main reason for the efficiency of cul- 

 tivation, but it is also to be remembered that such 

 stirring helps to dry the toj? soil very thoroughly, 

 and as has been explained a layer of dry soil of itself 

 is a very effective check upon surface evaporation. 

 (See Fig. 35.) 



That the stirring or cultivation of the topsoil 

 really does diminish evaporation of water from the 

 soil has been shown by numerous investigations. 



