Crang.| Prospecting, Mining and Milling Machinery. 267 
connected with the main line shaft, drives the crusher. Oc- 
casionally there are two drive pulleys, one on either side, 
thus requiring two drive belts. 
The crusher frame is bolted to an elevated framework or 
foundation, described under ‘‘ Mine Buildings.’’ The floor of 
the crusher room is on a level with the top, Z, of the crusher 
frame, so that the ore can be shoveled or pushed directly into 
the hopper. 
Most mills have but one crusher, yet some of the larger 
mills, constructed recently, have two crushers, and are thus 
prepared to run on double capacity. Occasionally mills 
which are built for double capacity have double or duplex 
' crushers, 7. e., one frame contains the working parts of two 
crushers. The one driving shaft supports both eccentrics, 
which are set diametrically opposite each other. This ar- 
rangement distributes the pressure more evenly upon the 
shaft and prevents the jerking motion usually given by ec- 
centrics. Jets of water are kept running over the eccentric 
bearings to prevent them from becoming too hot, and a jet is 
also run into the hopper to assist in carrying the crushed and 
undersize material through, and so not interfere with the 
working of the crusher. 
The material when crushed passes downward through a 
gravity spout or launder to the first set of rolls. Plate LII. 
Rotus.—The material to be crushed, after having passed 
the crusher, enters the first set of rolls. Rolls are the finish- 
ing machines in the reduction process of this district, and 
there are from two to four sets to a mill. 
The roll is a very simple and effective machine for handling 
the partly reduced ore from the crusher, and with a sufficient 
number, through which the material may pass in order, a 
fairly fine product may be obtained. 
A side view and plan of the rolls in common use in this 
district are shown in figure 50. A roll consists of a frame- 
work, A, which supports a shaft, B, upon which is keyeda 
small cog wheel or pinion, C, a pulley, D, and a balance wheel, 
iE. Two long base castings, G, are bolted to the frame, A, to 
which castings or base pieces are fastened all the working 
18—viii 
