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 internalcombustion 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.

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.