As the engine is the heart of the submarine, the periscope is its eye. This is, in its simpler forms, a stiff, detachable tube from fifteen to twenty feet long and about four inches in diameter. On its top is an object glass which takes in all objects within its range and transmits an image of them through a right-angled prism and down the tube. By means of other lenses and prisms an image of the external object is thus made visible to those within the submarine. In this process of transmission there is a certain loss of light, and to allow for that the image is magnified to about one-quarter above natural size.

A Submarine Under Ice.

To obtain in this manner a correct idea of the distance of the object seen proved difficult, but by continued experiment this difficulty has been overcome. Mr. Lake developed an instrument suited to this purpose and one which gave a simultaneous view of the entire horizon. There is one fault in the periscope not easy to obviate. It is an instrument for day use only. When dark comes on it becomes useless, and this does away with the possibility of a successful submarine attack by night.

The periscope is the one part of the submarine scout equipment that is open to vision from the surface. But while the outlook of the undersea captain, aided by his telescopic sights, has a radius of several miles, the periscope tube, of only four or five-inch diameter and painted of a neutral tint, is not easily seen. If the sea is a little choppy it is difficult to discover it with the naked eye at about 300 or 400 yards away, or in a smooth sea at over 500 yards.

The idea that a submarine may be located by an aeroplane is looked upon by Mr. Lake as a fallacy, except in water of crystal-like clearness, like that of the Mediterranean or the Caribbean, and periscopes are now being made to scour the heavens as well as the horizon, so that the presence of an enemy aeroplane can easily be seen. An attack by an aeroplane bomb, therefore, can readily be avoided, in view of the difficulty of hitting such an object from the upper air.

The submarine is the guerrilla of the sea. Its tactics are like those of the Indian who fights under cover or lies in ambush for his enemy. It is the weaker party and can hope for success only through strategy. The old adage that "all is fair in love and war" applies to this new weapon of destruction as to every warlike instrument. It is its invisibility that makes the submarine the terror of the seas. This has been well proved during the European war. The North Sea and the English Channel have been invaded by German submarines which have made great havoc among merchant ships. And it is well to draw attention to the fact that submarines are safe from each other. In no case has a battle taken place between two of these armed sharks except in the one instance reported of an Austrian sinking an Italian submarine. But in this case the Italian boat was on the surface and was at the time practically a surface ship.

During the war the Germans were especially active in the use of the submarine, and did much in making them an effective terror of the seas. With no mercantile marine of their own to guard, they had a free field for attack in the abundant shipping of their foes. The loss of ships was so numerous and become such a common occurrence that little attention was finally paid to them except when great loss of life took place, as in the signal instance of the "Lusitania."

Type of High Speed Ocean-going Submarine.