PRESERVATION OF CROPS FROM INJURY BY FROST. 339 
is due first to convection and horizontal translation of the air, then by 
contact with the soil cooling rapidly by free radiation. 
Frosts are recurrent phenomena, and may reasonably be expected 
within certain dates. The cumulative losses are considerable. To be 
serviceable the protection must be available for several nights in suc- 
cession, for there is no profit in saving a crop one night and losing it 
on the following night. The effort to protect is worth while. 
