The internal-combustion engine is the heart of the submarine. Steam, with its heavy engine, has been long set aside, and electricity, derived from the storage battery, yet awaits sufficient development. Gasoline succeeded them. The internal-combustion engine became essential from its light weight and the fact that it could be started and shut down instantly. This is of prime importance, as permitting quick submergence or emergence, either to escape from a high-speed destroyer or to capture a merchantman. It weighs less per horse power, takes up less room and requires less fuel per hour than any other reliable motor. It was early used in both the Holland and Lake boats and is still the chief prime motor. [Illustration: A PROTECTOR FITTED FOR EXPERIMENTAL WORK UNDER ICE] The difficulty with the early boats was that they were slow in speed, making only from eight to nine knots per hour. Increased speed was demanded by governments and more powerful engines, within a fixed limit of weight, were demanded. In doing this engines were built of such flimsy construction that they soon went to pieces. The gasoline used also gave off a gas of highly explosive character and one very likely to escape from leaky tanks or joints. Several explosions took place in consequence, in one of which twenty-three men were killed. As a result all the nations demanded that a non-explosive fuel should be used, and builders turned to the Diesel engine as offering a solution to the difficulty. 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. [Illustration: 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.