SCIENCE OF THRESHING. 35 



casing. 



This air is replaced by a current which enters a 

 central opening at the ends of the casing. 



Fans are overblast when the upper blades travel 

 towards the outlet; they are called underblast when 

 the lower blades move toward the mouth. 



THE BLAST. 



The blast is an important feature of the cleaning 

 mill. It should lift the chaff and light matter and 

 prevent it from falling through the sieve openings, 

 and at the same time maintain a continuous flow 

 through the sieve. It should be strongest in the 

 sieve openings, and light above the sieve surface. 

 When these conditions are secured, if the blast lift a 

 kernel, it will ascend but a short distance before it 

 again returns to the sieve; if the blast were as strong 

 above the sieve as at the meshes, the kernel would 

 be blown out with the blast and wasted. 



To obtain these conditions, the solid or blind 

 portions of the sieve should be in proper proportion 

 to the openings; practice establishes this at about 

 five to seven. That is to say, the solid portion of the 

 sieve should be about five-twelfths of the surface, and 

 the openings should be, in, the aggregate, about seven- 

 twelfths of the surface. It will be observed in using 

 a sieve of these proportions, that the blast will be 

 only seven-twelfths as strong just above the surface 

 as in the meshes. It is to be noted that this state- 

 ment is more strictly correct when the direction of the 

 blast is at about right angles to the sieve. 



