248	MODERN PAPER-MAKING
pulp, or of stock of any colour. Such papers are known as duplex, which
means that they are two-sided or two-ply. The width of the web which is
to be made on the drum is regulated by stopping up the meshes of the wire
by tapes, which may be wound on at either side. This obviously has the effect
of closing the meshes of the wire cloth, so that water cannot run freely through
and cause the formation of a web of fibres.
The water which passes through is run away to a back-water box, whence
it is pumped back again for further use in furnishing the beaters. The thick-
ness of the web which will form on the cylinder is obviously limited, for, as it
becomes thicker, less and less water can pass through, and so the number of

[Banning and Seybold
fig. 103.—sheet-forming vats of a multiple-vat board machine
fibres deposited becomes less. The thickness of the various webs which form
the board is regulated by the consistency of stuff going to the vat, and by the
speed of the machine.
In order, however, to produce a thick board suitable for making folding
boxes, or a paper which has to be very thick and stiff, the number of vats and
making drums may be increased to four or even sk or eight, and all the webs
pressed together to form one thick sheet (Figs. 103 and 104).
ib this way boards may be made with a different-coloured surface on each
side aikl with a 'middle' of entirely different colour and quality. To do this
it wiH he seen that different sets of beaters will be required, also a number
of staff dbest^ mixing boxes, and strainers, to correspond with the number of
qualities of stuff to be used to compose the finished board.