This heavy oil engine, weighing about five hundred pounds per horse-power, was not adapted to the submarine, and efforts have been made to decrease the weight. These have not as yet had a satisfactory result and experiments are still going on.
The Periscope.
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.