STATE GEOLOGIST. 271 
are arranged through which the fine material escapes and falls 
into a collecting bin. The oversize material is caught by deflect- 
ors and returned to the crushing chamber. The material is fed through 
a hopper at one end and leaves through the screens. The machine is 
also able to do fine grinding if outside of the first screens finer sieves 
are attached, and the oversize of the fine screens again falls back into 
Fig. 42. View of a ball mill, as erected by the Gates Iron Works. 
the crushing space. But the capacity is thus decreased very greatly and 
the efficiency of the machine for this purpose is rather doubtful. The 
machine, however, works more successfully in this direction if, instead of 
the screens, there is a strong current of air blowing through the mill, 
which removes the fine particles constantly as soon as formed, leaving 
the coarser grains to be ground fine. 
The following table gives some data concerning modern American 
ball-mills, made by the Allis Chalmers Company: 
