"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 usual care as he jiggled off the 'cans.' "One of the detonations had a different kick from the others, and I was just speculating if it had been a hit, when up comes Fritz, rolling like a harpooned whale. "We were just turning sharp under left rudder and, not wanting to take any chances, the captain gave orders for all guns fearing to open fire. No. 1 and No. 2 of the port battery got off about five rounds apiece, and when the splashes from the exploding shells had subsided Fritz had gone. It looked like a hundred to one that we had finished him--until we ran into another of those darn wakes of oil and bubbles reeling off at a good five or six knots.