"It's seconds that count in a destroyer attack on a U-boat, and the captain hadn't lost a tick in jumping into this one. The dissolving 'V' which the ducked-in periscope had left behind it was still visible in the smooth water when the _Sherill's_ forefoot slashed into it, and it was only a few hundred yards beyond that a slow undulant upcoiling of currents marked, faintly but unmistakably, the under-water progress of the game we were after. There was no oil-slick, understand, because an uninjured submarine only leaves that behind--except through carelessness--when it dives after a spell on the surface running under engines. Then the exhausts cough up a lot of grease and oil, and a layer of this, sticking to the stern, leaves a trail that rises for some little time after submergence, and which almost any kind of a dub who has been told what to look for can follow. "The spotting of the surface wake of a deep-down submarine, and the holding of it after it almost disappears with the slowing down of the screws that make it, is quite another thing. _That_ takes a man with more than a keen eye--it takes instinct, mixed with a lot of common sense. It's a common thing to say of a successful look-out that he has a 'quick nose for submarines.' The expression is used more or less figuratively, of course; and yet the nose--the sense smell--is by no means a negligible factor in detecting the presence, and even the bearing, of a hunted U-boat. I will tell you shortly how it figured in this particular instance. "That wake was swirling up so strong when we struck it that it was plain the submarine was still only on the way down, and it was no surprise when, a few seconds later, the distinct form of it was visible, close aboard under the starboard side of the bridge. "I don't mean that it was distinct in the sense that you could see details such as the bow or stern rudders, or even the conning-tower, but only that a moving cigar-shaped blob of darker green could be plainly made out. The for'ard end was rather more sharply defined than the after, probably because the swirl from the propellers made uneven refraction about the tail. It was doubtless a good deal deeper than it looked, and the fact that it could be seen at all must have been almost entirely due to the fact that the absence of wind left the surface quite unrippled.