244	MODERN PAPER-MAKING
exactly under the centre of the cylinder, so that the nip given to the web of
paper and felt is as direct as possible.
These press rolls were at one time made of iron, but they are now more
generally covered with rubber, as they give almost equally good results and
are much less hard on the felts.
On some machines the press roll is placed either in front or behind the
centre of the cylinder, so that the pressure of the weighted lever does not press
directly to the centre of the cylinder, but to a point between the centre and
the circumference, and has a dragging action. Our experience has led us to
favour the former method, and we are of opinion that the paper must stick
instantly to the cylinder and remain fixed in close contact with it during the
whole of the time it is passing round.
On other machines two cylinder press rolls are fitted, but we cannot under-
stand the necessity for these, in spite of the fact that they are presumably giving
good results, and they are being fitted to some of the latest machines. It
would appear to be highly probable that the second press roll to come in contact
with the web would interfere with the work done by the first one, in sticking
the web to the cylinder,
We believe that two press rolls are the cause of a lot of trouble on M.G.
machines. It is asserted that by having two rolls the pressure on each can be
varied at will to suit the special requirements of the paper being made. We
think this is very doubtful.
The overfelts are usually washed continuously after they leave the press
roll, or if no washer is available they are sprayed with cold water to cool and
clean them before they again come in contact with the web.
An important factor in determining the finish of both sides of the paper
is the amount of water left in it at the wet press rolls. If a good finish is re-
quired, the web should be fairly moist on reaching the cylinder; and if a
pronounced rib or stripe is required, or a rough under side, die wet press rolls
should be let down very lightly or they may be hung up altogether. This
will leave the web bulky and impressionable, so that the full effect of the over-
felt can be produced.
The furnish of the paper and the way it is beaten have, of course, a very
marked effect on the finish, as obviously very long and wild stuff will not take
on such a good finish as finer stock, or a common brown as will a pure sulphite.
Hie state of fibrillation or wetness also has another important effect, in that
if the stuff is too wet it may be impossible to get it to stick to the cylinder at all.
For baft papers and sulphites it is usual to try to get the highest possible
finish, while for Httos and certain other papers too high a gloss is not
desirable.