162 MECHANICS. 



raised levers, operating on a knee-joint, and compress- 

 ing the hay into a small and compact mass, the great- 

 est force being given when most needed, at the termi- 

 nation of the pressure. Side-doors are then thrown 

 open, and the hay secured by bands and taken out. 

 Two hundred and fifty pounds of hay may be thus re- 

 duced to a space of sixteen cubic feet, or a little more 

 than half a cubic yard, by a single horse ; and several 

 tons may be pressed in a day. Dederick's improve- 

 ment in this press consists in placing the levers at one 

 end only, compressing the hay into the other end, and 

 thus simplifying the machine. Double levers, press- 

 ing equally against the upper and lower part of the 

 slide or piston, keep it always upright and even, al- 

 though the hay may be unequally compact. These 

 double levers are connected and kept parallel by con- 

 necting hinged bars. 



The power exerted by a rolling-mill, where bars of 

 iron are flattened in their passage between two strong 

 roUers, is precisely like that of the knee-joint. The 

 only difference is, that the rollers, which may be con- 

 sidered as a constant succession of levers coming into 

 play as they revolve, are both 

 fixed, and consequently the 

 bar has to yield between them 

 [Figure 143). The greatest 

 power is exerted just as the 

 bar receives the last pressure 

 from the rollers. The most 

 , ^ , , . . . , powerful and rapidly-workina: 



Principle of the knee-jomt m the ^ i ./ o 



roiiing-miii. straw-cuttcrs are those which 



draw the straw or hay between two rollers, one of 



