injured, a depth-charged crew became extremely cautious in the future about getting anywhere in the neighbourhood of a destroyer; and among the several influences which ultimately disorganized the _moral_ of the German U-boat service these contacts with depth charges were doubtless the most important. The hardiest under-water sailor did not care to go through such frightful moments a second time. This statement makes it appear as though the depth charge had settled the fate of the submarine. Yet that was far from being the case, for against the ash can, with its 300 pounds of TNT, the submarine possessed one quality which gave it great defensive power. That was ability to make itself unseen. Strangely enough, the average layman is inclined to overlook this fairly apparent fact, and that is the reason why, even at the risk of repeating myself, I frequently refer to it. Indeed, the only respect in which the subsurface boat differs essentially from all other war vessels is in this power of becoming invisible. Whenever it descries danger from afar, the submarine can disappear under the water in anywhere from twenty seconds to a minute. And its great advantage is that it can detect its enemy long before that enemy can detect the submarine. A U-boat, sailing awash, or sailing with only its conning-tower exposed, can see a destroyer at a distance of about fifteen miles if the weather is clear; but, under similar conditions, the destroyer can see the submarine at a distance of about four miles. Possessing this great advantage, the submarine can usually decide whether it will meet the enemy or not; if it decides that it is wise to avoid an encounter, all it has to do is to duck, remain submerged until the destroyer has passed on, entirely unconscious of its presence, and then to resume its real work, which is not that of fighting warships, but of sinking merchantmen. The chief anxiety of the U-boat commander is thus to avoid contact with its surface foe and its terrible depth charge, whereas the business of the destroyer commander is to get within fighting distance of his quarry.