SCIENCE OF THRESHING. 59 



umns of air rushing in from either end meet at about 

 the center. This meeting of the opposing blasts 

 increases the pressure somewhat at this central point, 

 consequently the fan delivers more freely at the center 

 of the outlet; further, the air rushes in more freely 

 behind the ends of the wing as it passes the outlet as 

 the pressure or vacuum is greater there than at the 

 center of the wing; therefore, the air passes from the 

 sieves into the fan near the ends of the wings, instead 

 of from the fan through the sieves. This motion is 

 more marked in long than in short fans, therefore in 

 wide than narrow machines. 



A further explanation of this usually unrecognized 

 phenomenon is this : As the air rushes from the ends 

 towards the center of the fan, there is little side pres- 

 sure towards the sieves at the ends of the fans and the 

 extra pressure in the sieves forces the air back into the 

 fan; that is, the air, for a little distance from the fan 

 ends, travels in the wrong direction ; as the air comes 

 from the center of the fan, it thus swings back upon 

 itself, forming an eddy. Should this counter current 

 be strong enough, it results in carrying some of the 

 grain along with it into the fan. This effect is so 

 marked in some machines that the corners of the fan 

 are worn by striking the grain blow r n in by this 

 motion of the blast. This effects the perfect cleaning, 

 as the direction of the blast is down at the corners 

 of the sieves, thereby carrying the chaff and dirt into 

 the grain. 



This whirling motion can be tested in any machine 

 by taking the sieves out and running the machine 



