124 CAMPBELL'S SOIL CULTURE MANUAL 



But it is also true, and this needs no demonstration, 

 that even in the humid sections of the country they suffer 

 from drouth. Down on the Atlantic coast there are many 

 places where there is an average of one rainy day in every 

 three during the growing season and right there you will 

 find the old settlers telling about how they lost a crop by 

 drouth. And if they do not suffer from drouth they are 

 likely to suffer equally as much by having rain when 

 they do not want it. 



RAPIDITY OF EVAPORATION. 



It is stated that the best estimate based on experiments 

 as to the extent of evaporation from the soil in the humid 

 regions shows that fully fifty per cent of the rain water 

 which falls is returned to the air directly in vapor. But 

 this is not true of the semi-arid region, where a much 

 smaller proportion is returned to the air in that way. 

 And where there is cultivation with a special view to pre- 

 venting this evaporation from the surface the evaporation 

 is still less. Prof. Whitney tells of an experiment by the 

 Department of Agriculture to test the rapidity of evapo- 

 ration under different conditions. Two cylinders six feet 

 long were filled with soil and placed erect in water so that 

 the soil was kept damp. Then over the exposed end of 

 one tube a draft of air was blown to hasten evaporation, 

 while over the other a similar blast of air was blown, but 

 in this case the air was heated. It was evident that the 

 heated air would of itself take up the water faster than the 

 cold air. For a time the evaporation from the tube where 

 the heated air was operating seemed to be much faster 

 than from the other. But the surface soil was soon dried 

 out and this checked evaporation. During the time the 

 experiment was conducted it was found that the evapo- 

 ration from the tube with heated air was very much less 

 than from the tube with cold air. 



