60 SCIENCE OF THRESHING. 



empty. When the machine is in motion, take a cane 

 or stick of -convenient length and fasten one end of a 

 ribbon four or five inches long to one extremity. If 

 held in the blast near the fan outlet, the ribbon will 

 indicate the way the current of air is moving. 



Further study of the blast will show that its direc- 

 tion is not in a direct line from the point of delivery 

 of the fan along the stationary blast boards at the top 

 or bottom, towards the sieves, but tends to follow the 

 curve of the fan casing until it comes against the 

 blast board. Furthermore, in the case of an under- 

 blast fan, the blast does not leave the fan and travel 

 in a straight line along the bottom board towards the 

 sieves, but extends in a thin sharply defined current 

 along near the fan, and increases slightly in thickness 

 until it reaches the top blast board, from which it is 

 deflected down and back. 



The slight compression of the air at the center, 

 makes it veer off at an angle towards each corner, so 

 that it blows harder at the rear corners than it does 

 at the central portion. 



Two or three narrow wind boards one or two 

 inches wide, placed parallel to each other and the fan 

 in the forward part of the shoe below the sieves, 

 greatly aid in equalizing and adjusting the blast. 

 They should be set at an angle which will deflect the 

 blast toward the forward part of the sieves and 

 should be near enough together to require a little 

 pressure to force the blast between them. This 

 arrangement makes the air pass out along the entire 

 length of the fan opening to relieve the pressure on 



