The time taken up by these disappearances amounted to only a fraction of the total period consumed in a cruise. Yet the fact that the submarine had to keep itself momentarily ready to make these disappearances is precisely the reason why it was obliged to spend the larger part of its time on the surface. The submarine has two sets of engines, one for surface travel and the other for subsurface travel. An oil-engine propels it on the top of the water, but this consumes a large amount of air, and, for this reason, it cannot be used when travelling under the surface. As soon as the vessel dives, therefore, it changes its motive power to an electric motor, which makes no inroads on the oxygen needed for sustaining the life of its crew. But the physical limitation of size prevents the submarine from carrying large storage batteries, which is only another way of saying that its cruising radius under the water is extremely small, not more than fifty or sixty miles. In order to recharge these batteries and gain motive power for subsurface travel, the submarine has to come to the surface. Yet the simple fact that the submarine can accomplish its destructive work only when submerged, and that it can avoid its enemy only by diving, makes it plain that it must always hold itself in readiness to submerge on a moment's notice and remain under water the longest possible time. That is, its storage batteries must always be kept at their highest efficiency; they must not be wasted by unnecessary travelling under the water; the submarine, in other words, must spend all its time on the surface, except those brief periods when it is attempting to attack a merchant ship or escape an enemy. Almost the greatest tragedy in the life of a submarine is to meet a surface enemy, such as a destroyer, when its electric batteries are exhausted. It cannot submerge, for it can stay submerged only when it is in motion, unless it is in water shallow enough to permit it to rest on the bottom. Even though it may have a little electricity, and succeed in getting under water, it cannot stay there long, for its electric power will soon be used up, and therefore it is soon faced with the alternative of coming to the surface and surrendering, or of being destroyed. The success of the submarine, indeed its very existence, depends upon the vessel spending the largest possible part of its time upon the surface, keeping its full supply of electric power constantly in reserve, so that it may be able to dive at a moment's notice and to remain under the water for the maximum period.