much greater care to be sure
that it does not explode accidentally or prematurely. The bomb usually
contains from 100 to 500 pounds of tri-nitro-toluol, or T.N.T., as it
is usually called, the most powerful of all explosives. The explosion
of a ship loaded with it in Halifax harbor, December 6, 1917, caused
almost as great a loss of life and property as a volcanic eruption.

When the 500 pounds of T.N.T. is exploded it changes suddenly into
nearly 80,000 cubic feet of gas. Now this amount of gas will fill a
room 160 feet long, 25 feet wide, and 20 feet high. When the bomb
explodes under the water the gas must find room somewhere, and with
tremendous force it pushes the water in all directions. If a hollow
submarine is near the point of the explosion, its walls will give way
easier than the water around it and it is crushed like an empty egg
shell.

Only very swift boats should drop the depth bombs from their sterns,
for the boat must be moving at a rate of at least twenty-five miles an
hour to be sure to escape damage from the bombs dropped behind her.

John Mackenzie, the hero of this story, writes in regard to the
converted yachts used for dropping depth bombs in European waters as
follows: "Only destroyers made speed exceeding 25 knots. There were no
converted yachts operating in European waters capable of making 25
knots. A very few made 15 to 18 and the majority about 12. Of course
we had to take our chances in getting away safely, although we knew
that the chances were about even. That is, we were in about as much
danger from our depth charges as the enemy was." His statement shows
the risks that American sailors were willing to take.

The bomb, of course, weighs over one hundred pounds. It is made with
one end flattened, upon which it will stand, and in the early types its
accidental discharge is rendered practically impossible by a sort of
peg called a safety pin, which must be removed before the bomb is
dropped. The use of depth bombs against the U-boats made fighting in
the German submarines so dangerous and so much to be dreaded, that it
is said, as the war drew to a close, all U-boat crews had to be forced
into service, and that none of them expected ever to return and see
their homes and friends again.

In the early days of the war the bombs were carried in cradles, and
later in racks or run-ways. From most of the bombs the detonator,
which would fire them, was removed; but some were kept