58 SCIENCE OF THRESHING. 



to arrange the blast or deflecting boards in such a 

 manner as to cause the blast to strike the front end 

 more forcibly. 



To aid in this adjustment, it will be well to consider 

 first, some of the peculiarities of the fan and its 

 action on the air. As it revolves the wings make the 

 air revolve along with it. As each wing passes the 

 outlet, a small portion of the air is thrown off by 

 centrifugal force, other air coming in through the 

 inlet to take its place in the center of the fan. 



As the fan revolves there is a slight increase of 

 pressure of the air against the front side of the wing, 

 and a corresponding decrease along the rear side, the 

 sum of these plus and minus pressures being nearly 

 equal to the atmospheric pressure. 



As each wing passes the outlet opening in the shoe 

 the air starts to rush in behind the wing from the 

 shoe to fill the slight or partial vacuum on the rear of 

 the wing. A small portion thus enters the fan casing; 

 the next wing as it comes round, has a tendency to 

 force this intruding portion back before it has entered 

 very far. This alternate rarefaction and compressing 

 action of the fan causes the low vibrating or humming 

 sound heard in fast running fans. Now this action of 

 the air is not uniform along the entire length of the 

 wing. The most air is forced out at the center of the 

 shoe, which materially affects the action of the blast 

 through the sieves. 



When a fan whose casing is open at both ends to 

 admit air, is in motion, the two opposite moving col- 



