If one will pause to consider that light is necessary to sight and that one hundred feet of sea water makes almost as efficient an obstacle to vision as a stone wall, it will be easy to understand why the submarine is still struggling with difficulties that oppose its perfection. The fanciful illustration seen so often of a submarine diving hundreds of feet deep in the water, swimming about and finally coming up under the keel of a battle ship and destroying it, are as yet the creations of vivid imaginations. For submarine marksmen, like all others, require a fairly clear view of the target--even such a huge target as a battle-ship--to direct their shots with any degree of certainty. The greatest problem now confronting the submarine navigator, therefore, is that of seeing without being seen. At night, and at long ranges, this is not difficult, as the little conning-tower, or tiny periscope tube protruding above the waves, is not easily detected even by strong searchlights, sharp eyes, and marine glasses. But long ranges are of little use to the submarine; and there is always another difficulty--the leviathan battle-ship does not lie still waiting to be stabbed by its sword-fish enemy, but keeps moving about, twisting and turning, at a rate of from fourteen to eighteen knots an hour, while the submarine can only make about eleven knots when submerged. In a stern chase, therefore, the submarine is one of the most harmless of sea-monsters, in the open ocean. For harbor work, however, the case is different. In some recent tests the submarine boats made eighty per cent. in hits while attacking moving vessels in a harbor at night--a far higher percentage than is usually made by surface torpedo boats under the same circumstances. At present the best solution of the problem of steering the partly submerged submarine is offered by the use of a conning-tower elevated five or six feet above the body of the submarine, which can be kept just above the waves, and present an inconspicuous target. The early Holland boats did not have this, although the American Lake boats have had it from the first; but at the present time all boats are being so made. At first these towers were made circular in form; but it was found that towers of this shape made sufficient splash in passing through the water to attract attention at a considerable distance on a still night. This shape was abandoned, therefore, and a boat-shaped one adopted.