than forty sea-going submarines when the war began. But
nearly four hundred took part, or were ready to take part, before the
war was over, while many more were building.

We have already noted the weak points of submarines. They are "tender"
because they must be thin. An old collier that couldn't steam faster
than you could walk sank a submarine by barging into it, end-on--one
can hardly call it ramming. Submarines are slower on the surface than
dreadnoughts, cruisers, and destroyers; and, after doing a total of ten
or twelve hours under water, they have to recharge their batteries; for
they run by oil engines on the surface and by electricity submerged,
and the crew would be smothered if the oil engines tried to charge
batteries without coming up.

Then, firing torpedoes is not at all like firing big guns. At a range
of five miles a shell will still be making 2000 feet a second or 1400
miles an hour. At the same range a torpedo like those used at Jutland
would be making only 50 feet a second or 35 miles an hour. Thus shells
whizz through the air forty times faster than torpedoes sneak through
the water. A torpedo, in fact, is itself very like a submarine, more
or less cigar-shaped, and with its own engine, screw, and rudder.
Hitting with a torpedo really means arranging a collision between it
and the ship you are aiming at. When you and the ship and your torpedo
and the water are all moving in different ways you can see that hitting
is not so easy. The shorter the range the better. But you cannot see
at all unless your periscope, with its little mirror, is high and dry
out of the water; and periscopes are soon spotted by a sharp look-out
at very short range. The best torpedoes are over twenty feet long and
as many inches through, and they will go ten miles. But the longer the
range the slower the pace and the less the chance of hitting. The
engine is driven by air, which is compressed so hard into the middle of
the torpedo that it actually bulges out the steel a tiny fraction of an
inch. 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, wil