mistakes have been corrected. Such a correction is effected by turning
the plate on its face on a hard substance, hammering on the back until
the surface is driven up smooth and then entering the design anew.

A number of very delicate machines are used as aids to the engraver,
though much more for bank-notes and large pieces of work than for
postage stamps. These are called ruling machines, medallion rulers,
cycloidal and geometric lathes. Ruling machines are used to make the
backgrounds of portraits, the shadings of letters and similar work.

[Illustration: Coin Stamp, "New South Wales", 5 shillings]

Here is a very pretty example of ruling, in the so-called "coin" stamp
of New South Wales. These machines rule either straight or curved lines.
They can be adjusted to rule several thousand lines to an inch, but that
is only done for microscopical work, not for engraving. The general
principle of a medallion ruling machine is a rod, fixed on a pivot, at
one end of which is a pin which is drawn across a medallion, while at
the other end a graving point traces a corresponding line on the steel.
The large stamps issued in the United States in 1865, for the payment of
postage on newspapers and periodicals, are examples of this work.

Cycloidal ruling in its simplest form resembles a series of loops. It is
produced by a fixed point which is held against a plate while the latter
is moved in a circle and, at the same time, forward. By altering the
size of the circle and the speed of the forward movement a great variety
of results are obtained. By cutting one series of loops over another,
lace-like effects are produced. The process is still further varied by
the use of eccentrics.