You may set the air-valve fast or slow, and the torpedo will go
accordingly. But if you want to make pretty sure you must get within
less than a mile, with the ship's broadside toward you, set the torpedo
for the right depth, the right pace to keep it going as fast as
possible just long enough to hit, and of course the right aim. Then,
if all goes well, the cap, or "war head" of the torpedo, on hitting the
ship, will set off the fuse that sets off the tremendous charge of high
explosive; and this may knock a hole in the side big enough to drive a
street car through. But there are many more misses than hits.
Yet the German and Austrian raiders, mines, and submarines sank fifteen
million tons of shipping, which is not far short of a third of all the
merchant tonnage in the world; and the submarines sank more than the
mines and raiders sank together. (Ships are measured by finding out
how many cubic feet of space they contain and counting so many feet to
the ton. Thus you get a much better idea of how much shipping a
country has by counting in tons rather than by the number of ships; for
twenty-five ships of one thousand tons each have only half as much
sea-power as one ship of fifty thousand tons.
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