1. The currents normally produced by the electric motor.
2. The abnormal currents flowing in certain unknown parts of the hull owing to lack of proper insulation.
3. The permanent or transitory magnetisation of the hull if made of a magnetic metal.
The best position for the compass on a submarine has been a much debated point, but it is now generally agreed to be in the centre of the hull. The conning tower of the first British submarine was made of steel, but it was afterwards replaced by one of brass.
M. M. Gaget in his book[[4]] states that so entirely untrustworthy and impracticable has steering by compass in French submarines been found, that the gyroscope has been requisitioned. He inclines to the belief that this instrument is the best indicator of route that has yet been devised, yet he points out the want of some reliable method by which the distance made by a submerged boat could be gauged with accuracy.
[4]. “La Navigation Sous-Marin.”
Motive Power.
The question of the best method of propulsion for submarine boats must be considered under two heads, namely, propulsion on the surface and below the water.
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.