he 'bubble fount,' we passed
over the point below which the U-boat must have been moving, but now he
was so much more deeply submerged than before that no hint of his
outline was visible on either side. We knew he was there, however, and
when we hit the proper place shook loose another shower of 'cans' over
him.

"There is nothing deeply mysterious about the calculations in dropping
depth-charges, for in no sense of the term can it be called an
instrument of precision. Indeed, it is of the bludgeon rather than the
rapier type. If you have a wake to guide, you approximate his speed and
course from that, guess at his depth, set the charge at the
corresponding depth from which you judge its explosion will do most
good, and then, allowing for your own speed and course, release it at a
point which you reckon the target will have reached by the time the
charge gets down on a level with it. It is something like bomb-dropping
from an aeroplane, only rather less accurate, because you don't see your
target as a rule.

"This is more than compensated for, however, by the greater
vulnerability of its target and the fact that the force of an
under-water explosion is felt over a wider area than that of an
air-bomb. That's about all there is to it. Success in 'can-dropping'
depends about half on the skill and judgment of the man directing it,
and about half on luck. Or perhaps I should say that fifty-fifty was
about the way it stood when we started in at the game. Naturally, as we
have accumulated experience, skill and judgment begin to count for more
and luck for less, though we are a long way from reaching the point
where the latter is eliminated entirely.

"Again we circled back to pick up the pieces, and again we found only a
wake of oil and bubbles angling sharply off from where the 'cans' had
been dropped. It was encouraging to note that both oil and bubbles were
rising faster than before, but there was surprise and disappointment in
the fact that they were now streaming along at a rate which indicated
Fritz was hitting an under-water speed of six or seven knots.

"By now it was plain what his method was, however. This was to steady on
his course till his hydrophones, which all U-boats are fitted with, of
course, told him we were bearing down on him, and then to start making
'woggly' zigzags. The captain was doing some deep thinking as we headed
in for the next attack, and I noticed him following his stopwatch with
more than