260 University Geological Survey of Kansas. 
The space between the bars varies somewhat with the char- 
acter of the material sized; the usual range in width is, how- 
ever, from three-fourths to one and one-half inches. 
- Parallel bar screens are occasionally used in mills, and are 
placed at the end of the tramway leading from the shaft. 
They are intended to separate the large lumps or boulders, 
which have to be broken by sledge, from the finer and more 
millable dirt. They do not, however, differ essentially in 
structure from the forms already described. 
Shaking Screens.—Shaking screens, or riddles, are seldom 
used in the ordinary milling processes. They are, however, 
nearly always employed in the slime mills, where finer ma- 
terial is treated, as there are certain grades which can be 
handled to better advantage by this form of screen. In 2, 
figure 47, is shown a common form of shaking screen. The 
screen proper, AB, consists of a trough with sides 6 inches 
high. The bottom is wire cloth or perforated metal, sup- 
ported by slats running across transversely beneath the 
screen. The upper end, A, where the material to be treated 
is fed to the screen, is made tapering or V-shaped, narrowing 
down from three and one-half feet at the upper end of screen 
to less than a foot at the point of feeding. The bottom of the 
feed end rises by steps from C to D. The screen is supported 
by the rods, # and F, which are hooked into eye bolts fastened 
into timbers above and into the sides of the screen frame be- 
low. Supported in this manner, the screen has a vertical as 
well as horizontal motion, both being, however, in the same 
plane. A shaking motion is given to the screen by means of 
the pitman fastened to the bottom at G and to the eccentric, 
I, on the shaft, H, which is driven by the pulley, K. The 
box, M, catches the coarser or oversize material, which is 
shoveled into a gravity trough or launder, by which it is 
borne to some other part of the mill for further treatment. 
The fore or feed end of the screen is often of finer mesh 
than the rear or discharge end, B. These screens are of va- 
rious sizes; the particular size that is used depends, however, 
upon the amount of room allowed for the same, also the ca- 
pacity of the mill. 
