Printing

the cylinder. The bod with the form still
travelled backwards and forwards, but the
improvement in feeding allowed it to move
at ten times its former speed without any
difficulty in keeping it supplied with paper.
Cylinder presses, improved from Konig's
model, remain today the commonest form of
printing press. It should be noted that there
are two kinds of cylinders in use in printing
presses—'impression cylinder' and 'form' or
'type cylinders.5 The first cylinder presses,
printing one side of the paper only, had one
impression cylinder, the form being flat; the
perfecting press, printing both sides, had two
impression cylinders, the form being still
flat. Rotary presses have the form curved
around a form cylinder. They have either
one impression cylinder and one form cylin-
der, and print one side of the paper, or two
of each, and print both sides. The cylinder
press provided for an 'impression cylinder'
with a flat form. Later came the further de-
velopment of a 'form cylinder,' with stereo
or electro plates curved round it, the two
cylinders — impression and type — running
continuously in gear, and the paper being im-
pressed between them. This movement has
greatly increased the possible speed and out-
put of printing machinery. The first press of
this type, the rotary press, was invented by
Thomas Nelson of Thomas Nelson and Sons,
Edinburgh, and exhibited, 1851. Almost
every rotary machine is designed specially,
and no general description is possible.

LITHOGRAPHY (commonly called 'litho').—
This process was developed through the dis-
covery of certain properties of a German
limestone, called lithographic stone. The
lithe process depends on two properties of
the stone—its absorption of grease and the
fine polish which its surface will take—and on
the mutual antipathy of grease and water.
The design to be printed is drawn on the
stone in reverse with a greasy ink, The stone
is then fastened in a machine resembling an
ordinary flat-bed press* The printing is (lone
as in the latter, with the exception that a
water-roller goes over the whole stone before
the ink-rollers are applied* The greasy ink
of the design repels the water, and none can
settle on it; but where there is no design,
the water damps the finely-polished surface
of the stone. The effect of the ink-roller is
exactly the opposite. The greasy design free-
ly takes additional ink; the moist, polished
surface repels it. In this way only those por-
tions of the printing surface covered with the
design receive the necessary ink for printing.

3832___________________________Printing

The aluminum rotary printing press for
printing one side of the paper from a sheet

of that metal bent around the cylinder is
rapidly displacing the ordinary lithographic
press, as it more than doubles the output.
An aluminum perfecting press has also been
invented. Lithography was, until thirty
years ago, practically the only method of
printing designs and work in colors. It is
now, however, possible to make a raised
'plate1 from any design (see PROCESS WORK),
and print from it by the letterpress method,'
which is cheaper and quicker for large edi-
tions. Lithography by the photo-litho-
graphic process, and particularly when print-
ed by the off-set process, has recently made
great strides and is competing on even terms
with letterpress printing. Off-set printing is
now by far the most important branch of
lithography.

LVTAOLIO PRINTING.—This method is the
costliest form of printing. The surface con-
sists of a smooth plate of copper or steel,
in which depressions are either cut by hand
with a 'graver* (a fine engraver's chisel)
or bitten out by add. An 'ink-ball' (leather
pad with a handle), thickly coated with
ink, is rubbed over the surface of the plate
until the depressions are well filled with
ink. The surface is then rubbed clean, the
paper Ls placed in contact with the plate and
put into a cylinder press. The process is
superior to all others for giving great den-
sity of color in the dark parts of the plate
and variety and contrast in the lighter tints.
Music may be sc*t up in type and printed
as letterpress, but the finest music is en-
graved on a metal plate.

THREE-COLOR WORK OR TRICHROMATIC
PRINTING.'—Fully-colored pictures may now
be reproduced in three printings from three
plates printing yellow, red, and blue re-
spectively. (For the preparation of these
plates, see PROCESS WORK). The most suc-
cessful fast color-printing press is an Amer-
ican aluminum rotary, printing seven colors
in perfect register. See also BOOKBINDING;
COPYRIGHT; MAGAZINES; PUBLISHING and
BOOKSELLING.

Printing Ink, an ink made from oils or
varnishes, mixed with lampblack or other
pigments, and thereafter thoroughly milled
or ground through steel, granite, or porce-
lain roller mills, The principal oils used are
linseed, poppysecd, hcmpsced, resin, and
mineral oils. The two last, after being re-
flned and purified with steam, are chiefly
mixed with ordinary lampblack; and this