But
gradually the Allies employed nets as traps. These were anchored or
dragged by fishing boats. Some submarines have gotten inside, been
juggled around, but have escaped. More, perhaps, have been lost this
way.

Then, when merchant ships began to carry armament, the periscopes were
shot away, so the navy invented a so-called "finger-periscope," a thin
rod pipe with a mirror at one end. This rod could he shoved out from
the top of the submarine and used for observation purposes in case the
big periscope was destroyed. From time to time there were other
inventions. As the submarine fleet grew the means of communicating
with each other while submerged at sea were perfected. Copper plates
were fastened fore and aft on the outside of submarines, and it was
made possible for wireless messages to be sent through the water at a
distance of fifty miles.

A submarine cannot aim at a ship without some object as a sight. So
one submarine often acted as a "sight" for the submarine firing the
torpedo. Submarines, which at first were unarmed, were later fitted
with armour plate and cannon were mounted on deck. The biggest
submarines now carry 6-inch guns.

Like all methods of ruthless warfare the submarine campaign can be and
will be for a time successful. Germany's submarine warfare today is
much more successful than the average person realises. By December,
1916, for instance, the submarines were sinking a half million tons of
ships a month. In January, 1917, over 600,000 tons were destroyed. On
February nearly 800,000 tons were lost. The destruction of ships means
a corresponding destruction of cargoes, of many hundreds of thousands
of tons. When Germany decided the latter part of January to begin a
ruthless campaign German authorities calculated they could sink an
average of 600,000 tons per month and that in nine months nearly
6,000,000 tons of shipping could be sent to the bottom of the
ocean,--then the Allies would be robbed of the millions of tons of
goods which these ships could carry.

In any military campaign one of the biggest problems is the
transportation of troops and supplies. Germany during this war has had
to depend upon her railroads; the Allies have depended upon ships.
Germany looked at her own military situation and saw that if the Allies
could destroy as many railroad cars as Germany expected to sink ships,
Germany would be broken up and unable to continue the war. Germany
believed ship