et. The third resembles the lines in watered silk and is called
_moiree_. It covers the entire back of the sheet. Sometimes the stamps
are covered with a network which only becomes visible on the application
of certain chemicals. In this country the experiment has been tried of
breaking the fibre of the paper by pressing into the stamps a group of
tiny pyramids, called a grill. The idea was that the cancelling ink
would penetrate the broken paper and could not be removed.

We cannot finish our study of the material side of stamps without
reference to another feature, i.e., surcharges. Correctly speaking, a
surcharge is an added charge, but in philately the term is applied to a
variety of overprints, the majority of which indicate a reduction rather
than an increase in value. Years ago the word surcharge usually
suggested a makeshift, something of a temporary nature prepared to meet
an emergency and, therefore, interesting and likely to become valuable.
But our little weaknesses are now well understood by those who are
exploiting the commercial side of postage stamps and we have reason to
fear that many recent surcharges were made for revenue only and not from
any real necessity. The majority of surcharges are made to supply a
value which has been temporarily exhausted. For example, many of the
British Colonies obtained their supplies of stamps in London. It may
happen that an order is not placed early enough or there is delay in
filling it and delivering the stamps. Owing to this, the values most in
use may be exhausted. Under such circumstances, it is customary to
provide a temporary supply by printing the needed value on some other
stamp, usually one of higher value. To use a lower value would tempt the
counterfeiting of the surcharge, for the profit to be made through the
increased value.

[Illustration: Stamp, "Mauritius", surcharged 4 cents]

[Illustration: Stamp, "Gibraltar", surcharged 5 centimos]

There are, however, a variety of other surcharges, a few of which may
interest you. The first two stamps indicate a change in the form of the
currency of the country, from pence to cents in Mauritius and from the
English half penny to its Spanish equivalent in Gibraltar. The
Seychelles stamp was prepared to meet a change in the rate for letters
to countries in the International Postal Union.

[Illustration: Stamp, "Seychelles", surcharged 8 cents]

[Illustration: Stamp, "St. Helena", surcharged 1 penny]

The fir