A great deal of work is done, on the submarine, by compressed air, of which large stores are carried in strong steel cylinders. For example, the ballast is ejected from the ballast tanks, when the boat is required to rise, not by pumps but by the action of compressed air from a cylinder. The simple movement of a tap thus suffices to blow out the water in a very short time. The torpedoes, too, are given their initial push which sends them out of their tube into the water by compressed air. In other ways, too, compressed air is employed and to facilitate its use there are many tubes and valves whereby the cylinders and other apparatus are connected. Like all things human, these tubes and valves have their defects, which in this case means that they leak somewhat, but this defect is of value since the leaking air helps to keep pure and sweet the air inside the boat which, when submerged, the men have to breathe. To what extent it is used I do not know, but it is a fact that certain chemicals, caustic soda for instance, have the power to absorb the objectionable carbonic acid which makes tightly-shut rooms seem "close" and uncomfortable, and if something of that sort be employed, it, together with the fresh air which thus leaks in by accident, is undoubtedly enough to enable men to live under water for many hours at a stretch. On the other hand, several instances are on record in which strong healthy young officers have, after a course of service on a submarine, been found to be suffering seriously from chest and lung trouble, brought on, no doubt, by long spells of duty in this unhealthy atmosphere. It used to be the custom to keep some white mice on board a submarine to give warning of the impurities in the air. Being very susceptible to the smell of petrol vapour, which used to be a source of considerable danger, and also to carbonic acid, these little creatures squeaked with anxiety some time before the conditions became really dangerous, thus giving timely warning. There is an instrument, however, which will give an indication of this sort and probably it has been brought in to reinforce the mice if not actually to supplant them. This interesting little instrument, which the gasworks people use for detecting leakage, consists of a metal drum with a porous diaphragm. Normally the pressure of the atmosphere upon the diaphragm is equalled and balanced by the pressure of the air inside the drum, but if there be gas in the air this balance is upset, the diaphragm is bulged in or out and a finger is thereby moved, which movement forms a measure of the amount of gas present.