IIOAD MODKLS. 
23 
thai marked A in the model. The openings of the second section of 
screen are 2 inches in diameter. The stones which pass through these 
openings into bin B arc called No. 2 stone. The third or last section of 
screen lias 3-inch openings, passing No. 1 stone into bin C. The tail- 
ings, or larger stones, will be forced through the opening at the end 
of the screen, from which they will drop into the tailings conveyor, 
to be crushed finally or eliminated from the work. The jaws of the 
crusher should be set so as to make as few tailings as possible. The 
sixes of these screen openings may vary, according to the quality of 
the stone. For instance, in crushing a hard stone, the openings of 
each section may be of slightly smaller diameter, since it is customary 
Fig. 1. — Model of portable crushing plant in operation. 
to crush a hard stone to somewhat smaller grades than a softer 
material. 
When soft stones are being crushed, a dust jacket having a J-inch 
mesh may be placed over the first section to eliminate dust from the 
screenings. The bins A, B, and C, for receiving the various sizes of 
crushed rock, should be provided with slanting metal bottoms and 
sliding doors, so that the material may be loaded into wagons by 
gravity. ' 
Two types of crusher are commonly used, one the jaw crusher, 
shown in figure 1, and the other the gyratory crusher. The jaw 
crusher is generally used for portable plants. In this machine one of 
the jaws moves backward and forward by means of a toggle joint 
and an eccentric, and the stone descends as the jaw recedes. As the 
