186 Irrigation and Drainage 



volume of air stated above. Now, let this structure be 

 provided with a ceiling without weight, which is lifted 

 as the corn grows in height. This imaginary ceiling is 

 to separate the volume of air stored above from the 

 moving air in the corn field below, and to admit 

 through a changing doorway a steady stream whose 

 cross - section is that of the transverse section of the 

 room occupied by the corn. How rapidly must this 

 stream of air flow in order to discharge 80 per cent of 

 the volume contained in the structure in the sunshine 

 hours of 50 days ! The maximum number of sunshine 

 hours in the latitude of New York is about 623. If we 

 suppose the corn to be 1 foot high July 1 and 10 feet 

 high on August 19, the ceiling to have risen uniformlj* 

 in the meantime, so that the stream of air increased in 

 depth from 1 foot to 10 feet ; then, taking the mean 

 depth of the moving air current at 5.5 feet, its hourly 

 velocity, in order to convey the 80 per cent of air 

 across the field, must have been 1.167 miles. On the 

 other hand, let us suppose the corn field to be square, 

 so that the area is as compact as possible, so that a 

 stream of air now about 13 rods wide instead of 1 is 

 passing across it. The required velocity to convey the 

 80 per cent of air across the field is now only one- 

 ninth of a mile per hour and less than 10 feet per 

 second. Since the yield of dry matter per acre is the 

 largest we have yet raised under field conditions, and 

 the computed velocities above are so small, it does not 

 appear likely that an insufficiency of carbon dioxide in 

 the air can ever be a serious limiting factor to the 

 closeness of planting when irrigation is practiced. 



