Here the stream first passes over the
"sand-tables" in a continuous flow. These are composed of little troughs
with cross-pieces, and are covered at the bottom with long-haired felt,
to catch any sand or dirt that may still adhere after the numerous
operations to which the pulp has been subjected. The flow is then forced
through the "screen," which is a horizontal piece of metal pierced with
slots. For very fine paper these slots are so small as to be only one
one-hundredth of an inch in width. They are usually about a quarter of
an inch apart. Through these tiny apertures the fibers must find their
way, leaving behind in their difficult passage all lumps, dirt, or
knotted fibers which would mar the perfection of the product toward
which they are tending. A vibrating motion is given to the screen as the
flow passes over it, or revolving strainers may be used.
When the screen has finished its work, the water carrying the pulp in
solution flows in an even stream, the volume of which varies according
to the width of the web of paper to be produced, through a
discharge-cock onto the Fourdrinier or cylinder machine, as the case may
be, each of which will be duly described.
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