SCIENCE OF THRESHING. 29 



motion thus attained will cause it to strike the rack 

 with a sharp jarring motion at the midway point 

 before mentioned ; if the rack swing has been properly 

 timed the weight of the falling straw will add to the 

 shock, while the stiffness of each straw will cause it 

 to move relatively to its neighbor ; as soon as released 

 at the up end of the stroke, the mass will expand to its 

 normal condition. This jarring, compressing and 

 expanding motion accomplishes the desired result in a 

 remarkably perfect manner when the layer of straw is 

 not too thick or bulky. If it is so deep that the 

 jarring motion is not felt in its upper part the result 

 is not so good. The upper part will float along 

 undisturbed because of the elasticity of the inter- 

 vening straw; if the rack is vibrated so as to throw 

 the straw quite high, it is liable to toss the grain too 

 far and the latter, being heavier, will go higher than 

 the straw because of its greater momentum, and 

 retard the separation. 



Thus the straw should be made to pass over the 

 rack in a layer of even thickness of just the depth to 

 be thoroughly and quietly agitated from top to 

 bottom, and the motion of the rack should be such as 

 to strike it upward blows at the most effective point; 

 that is, midway of the upward stroke. 



The stroke of the rack is usually from five to nine 

 inches in length and the number per minute varies 

 inversely as the length, the long, slow stroke passing 

 as much straw along as the short stroke of higher 

 rate. 



