262 University Geological. Survey of Kassas. 
three-eighths to five-eighths inch) in diameter, and are en- 
larged or countersunk on the outside when punched. The 
countersinking or enlargement of the holes on the outside 
(so formed by the taper of the punch) is done to prevent 
the material screened from catching in and clogging or 
blinding the holes. If a particle can pass through the first 
part of such a hole, the hole becoming larger will allow it 
to pass readily through the remaining portion; otherwise it 
will catch and clog the screen. The holes are countersunk 
to within about one-thirty-second inch of the inner side, 
and are made about one thirty-second inch wider on the out- 
side. ‘The mesh of the wire screen varies from two to twelve 
to the inch. 
As seen in /, figures 47 and 48, the screen is bolted or 
bound by strap irons to hoops, which in turn are fastened to 
spiders, which consist of cast-iron hubs and wrought-iron 
spokes, or the spiders may be wholly castiron. The spiders . 
are keyed to the driving shaft. The screen is then set in a 
framework which supports the shaft by a boxing at either 
end, and the whole is occasionally housed in by wood or iron 
coverings. fl astened to the lower side of the framework, AB, 
is attached a hopper-shaped spout, CD, to which is connected 
the launder, which bears the undersize material to the jig, 
or some other form of concentrating apparatus. The over- 
size material is caught by another hopper and conducted to 
some other form of reducing apparatus. 
The screen is usually made thirty-seven inches in diameter 
by seventy-two inches long. In the truncated form, the pitch 
is about four inches to the seventy-two inches in length, thus 
making the diameter of the larger end eight inches greater 
than that of the smaller, with conical drums or trommels ; 
when the bearing is level the screening surface of conical 
trommels slopes from the feed to the discharge end, but with 
cylindrical trommels the shaft must be given whatever in- 
clination it is desired to give the screen surface. The driv- 
ing pulley is four inches wide and has a diameter of about 
three feet. Wire screens are probably not used as much as 
the perforated metal screens for trommel coverings, but both 
are In common use. 
