It will be quite evident that the conditions under which a motor in a submarine works differ according as the boat is running above or beneath the surface; and we arrive at the conclusion that if the same motor is to serve for both conditions special arrangements will have to be made to permit it to work under abnormal conditions. Should this be found impracticable a new method of propulsion will have to be found for under-water travelling. Every heat engine consumes both air and fuel (whether coal, oil, gas, &c.), and the process of absorption of the fuel is accompanied by the giving out of a certain weight of the substance in the form of gas. Whilst the boat is proceeding beneath the water its weight is continually being modified, and it is practically impossible to compensate for this change by the addition of water to the reservoirs. Besides this difficulty the combustion of the fuel not only absorbs a large quantity of the air which is so precious a quantity in a submerged vessel, but also sets free deleterious gases which naturally have prejudicial effects on the health of the crew. It may therefore be asserted that a submarine can only be propelled under water by means of a motor capable of working without combustion or loss of weight. It remains therefore to discover the most suitable method fulfilling these conditions. 1. _By Mechanical Means such as Clockwork, Springs, &c._—The Howell torpedo is driven by means of a heavy flywheel in the interior which is spun up to 10,000 revolutions a minute before discharging by means of special machinery. While all these methods are practicable they must be put aside as unsuitable owing to the slowness of the speed which a boat thus propelled can attain. 2. _Compressed Air._—In order that a submarine may be driven at a high rate of speed for a considerable distance, such a large store of compressed air would have to be carried if this method were adopted, that little space would be left in the vessel for any other purposes. In addition to this such a store of compressed air would be a source of danger. 3. _Manual Power._—The earliest submarines were of course driven by hand power, but no one nowadays would think of adopting this method. 4. _Steam from Heated Water._—Mr. Nordenfelt propelled his boats beneath the surface by means of the steam given off by the heated water in the cisterns, and this was found sufficient for a distance run of 14 knots. He disliked accumulators, and this is not to be wondered at, for in his time they were very far from perfect; were he designing a submarine to-day, however, it is probable that he would choose electricity for sub-surface working. 5. _Chemical Engines._—Dr. Payerne, d’Allest, and others, by means of a _chaudière pyrotechnique_, burnt, in hermetically closed furnaces, combustibles containing in themselves the oxygen necessary for their combustion, and got rid of the products of combustion by ingenious devices.