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one may be stationary while the other rotates. The shaft of one disc is
hollow to receive that of the other. They are driven with separate belt
pulleys, or a bevel wheel on a pulley shaft drives a similar wheel on each
disc shaft, in opposite directions.

Riddles and sieves are used to grade sands into coarse and fine varieties,
to separate portions imperfectly pulverized, and, in the case of old sand, to
get rid of cold shots and nails. The former generally consists of a frame with
parallel rods, leaving open spaces of J in. or so, while a sieve has a reticulated

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Fig. 64.—Sand Mixer and Disintegrator
A, Hopper.   B, Revolving prongs electrically driven.
mesh of crossing wires. Hand-operated riddles and sieves reciprocated on
a horse are too slow in action. Any machine is far more economical. The
simplest is that in which the ordinary round sieve is attached and locked to
a light iron frame reciprocated with a belt-driven pulley and crank. This
can make 800 reciprocations per minute, and deal with as much sand as a
man can shovel into it, an output of 3 tons per hour being possible at an
expenditure of from J to |- h.p. Larger machines have sieves and riddles
made to interchange in a rectangular frame driven by cranks and connecting
rods, and sloped at a slight angle from the horizontal to throw the lumps that
will not pass the meshes out at one end. Machines of this class will deal with
quantities ranging from 3 to 14 tons of sand per hour, with J-in. mesh, the
output being less with finer grading. To deal with larger quantities machines
have the sieves arranged on six sides, enclosing the sand, and rotated on a
VOL. I.                                                                                        11