Ordinarily, conditions favour the U-boat in this game, simply because the ocean is so large a place. But there is one situation in which the destroyer has more than a fighting chance, for the power of the submarine to keep its presence secret lasts only so long as it remains out of action. If it makes no attempt to fight, its presence can hardly ever be detected; but just as soon as it becomes belligerent, it immediately reveals its whereabouts. If it comes to the surface and fires its guns, naturally it advertises to its enemy precisely where it is; but it betrays its location almost as clearly when it discharges a torpedo. Just as soon as the torpedo leaves the submarine, a wake, clearly marking its progress, appears upon the surface of the water. Though most newspaper readers have heard of this tell-tale track, I have found few who really understand what a conspicuous disturbance it is. The torpedo is really a little submarine itself; it is propelled by compressed air, the exhaust of which stirs up the water and produces a foamy, soapy wake, which is practically the same as that produced by the propeller of an ocean liner. This trail is four or five feet wide; it is as white and is as distinct as a chalk line drawn upon a blackboard, provided the weather is clear and the sun is in the right direction. Indeed, it is sometimes so distinct that an easily manoeuvred ship, and even sometimes a merchantman, can avoid the torpedo which it sees advancing merely by putting over the helm and turning out of its course. But the chief value of this wake to the submarine hunters is that it shows the direction in which the submarine was located when the torpedo started on its course. It stands out on the surface of the water like a long, ghostly finger pointing to the spot where the foe let loose its shaft. As soon as the destroyer sees this betraying disturbance, the commander rings for full speed; and one of the greatest advantages of this type of vessel is that it can attain full speed in an incredibly short time. The destroyer then dashes down the wake until it reaches the end, which indicates the point where the submarine lay when it discharged its missile. At this point the surface vessel drops a depth charge and then begins cutting a circle, say, to the right. Pains are taken to make this circle so wide that it will include the submarine, provided it has gone in that direction. The destroyer then makes another circle to the left. Every ten or fifteen seconds, while describing these circles, it drops a depth charge; indeed, not infrequently it drops twenty or thirty in a few minutes. If there is another destroyer in the neighbourhood it also follows up the wake and when it reaches the indicated point, it circles in the opposite direction from the first. Sometimes more than two may start for the suspected location and, under certain conditions, the water within a radius of half a mile or more may be seething with exploding depth charges.