SCIENCE OF THRESHING. 39 



somewhat, causing it to give the grain a slight 

 upward motion at each stroke. This motion varies 

 in different places on the surface of the sieve, being 

 nearly parallel thereto at the center. 



The blast and sieve motion should be properly 

 gauged one by the other, and by the working condi- 

 tions. It is necessary for a shoe using a weak blast 

 to be given a greater motion to assist in carrying off 

 the refuse and chaff, when the blast is overtaxed, as 

 when an undue amount of chaff and cut straw loaded 

 with dust isdelivered to it, or in damp threshing. 



