e of time the submarine would be compelled to
come up for air, and then, if luck were with the destroyer, it might
find its foe before it was seen itself. Having discovered the submarine
the destroyer immediately endeavored to ram, dropping depth bombs at the
point where they supposed the enemy to be.

These bombs were so constructed that at a certain depth in the water
they would explode, and the force of the explosion was so great that
even if they did not strike the submarine they would be sure to damage
it seriously, sometimes throwing the submarine to the surface partly out
of water, and at other times driving her to come to the surface herself
ready to surrender.

In many cases it was not necessary to use the depth bomb at all. The
gunners on board the destroyers had become extraordinarily expert, and
though a shot might destroy the periscope of a submarine without doing
much damage, most submarines carrying extra periscopes to use if
necessary, yet it was soon found that it was possible by the use of
plunging shells to do effective damage. Plunging shells are somewhat
similar in their operation to bombs. Such a shell falling just short of
a periscope and fused to burst both on contact and at a certain depth
was extremely likely to do damage.

In the pursuit of the U-boat the airplane was also extremely effective.
These were sent out to patrol large districts near the Allied coast, and
also, in some cases, from ships themselves. It is possible in certain
weather conditions for the observer on an airplane to detect a submarine
even when it is submerged and the airplane can not only attack the
submarine by dropping depth bombs, but it can signal at once the
location of the enemy to the hurrying destroyers. Indeed, as the
submarine warfare proceeded the main difficulty of the Allies was to
locate the submarines. Many ingenious devices were used for this
purpose, and many of the English vessels had listening attachments under
water which were intended to make it possible to hear a submarine as it
moved. These, however, do not seem to have been very effective. The
submarine itself seems at times to have been fitted out in a similar way
and to have thus been able to hear the sound of an approaching ship.

Many thrilling reports of naval actions against German submarines were
given out officially by the British admiralty from time to time. In most
of these cases the submarine was both rammed and attacked by depth
bom