butterflies' wings of the entomologist. Some examples of secret
sketches of fortresses which have been used with success are shown on
the following pages.

[Illustration: _This sketch of a butterfly contains the outline of a
fortress, and marks both the position and power of the guns. The marks
on the wings between the lines mean nothing, but those on the lines
show the nature and size of the guns, according to the keys below._]

[Illustration: The marks on the wings reveal the shape of the fortress
shown here and the size of the guns.

[Illustration: FORTRESS GUNS.]

[Illustration: FIELD GUNS.]

[Illustration: MACHINE GUNS.]

_The position of each gun is at the place inside the outline of the
fort on the butterfly where the line marked with the spot ends. The
head of the butterfly points towards the north._]

[Illustration: _A smart piece of spy-work. Veins on an ivy leaf
show the outline of the fort as seen looking west (Point of the leaf
indicates north.)_]

[Illustration: _Shows where big guns are mounted if a vein points to
them._]

[Illustration: _Shows "dead ground," where there is shelter from