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BULLETIN 220;, U. S. DEPARTMENT OF AGRICULTURE. 



jaw returns it grips the stone and crushes it. The maximum size of 

 the product is determined by the distance between the jaw plates at 

 the lower edge. 



The essential feature of the gyratory crusher consists of a per- 

 pendicular iron shaft which revolves with an eccentric motion. 

 Around the top of the shaft, which is conically shaped, is a hollow 

 iron receptacle somewhat resembling an inverted bell. By the 

 eccentric motion of the shaft the conical portion crushes the rock 

 alternately against different sides of the inner surface of the bell. 



Fig. 2.— Model of road grader. 



ROAD GRADERS. 



Figure 2 shows in miniature a road grader which is used in building 

 earth roads and in preparing the subgrade for hard-surfaced roads. 

 The frame supports an adjustable scraper blade, the front end of 

 which may be used to plow a furrow, while the rear end pushes the 

 earth along the surface of the blade toward the center of the road or 

 distributes it smoothly. The blade may be set at any angle or tilted 

 either backward or forward according to the work to be done. 



It is advisable to use the grader when the ground is soft, preferably 

 in the early summer, in order to give the loose earth, time to settle 

 and pack before the fall rains begin. If the work is done in the fall 

 not more than 3 or 4 inches of loose earth should be put on at one 

 working. 



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