ing 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.
In conclusion, we may fittingly take a glance at what happens when a submarine founders. Only a few years ago this occurred with lamentable frequency, though now it is quite rare except under the actual stress of warfare. Several interesting schemes were therefore invented to give the men at least a sporting chance of getting to safety. One was to make the conning-tower detachable and water-tight, so that the men could get into it, fasten themselves in and float up to the surface. The practical difficulties in the way prevented this being a success. For example, if sufficiently detachable in an emergency it was difficult to make it sufficiently water-tight in ordinary use.
Another and better device provided the men with small helmets and jackets, like the dress of a diver very much simplified. One of these for each man was stored in an accessible place in the boat and partitions were devised inside the hull itself in order that whatever happened there should be air entrapped somewhere wherein the men could live for a time and put on their helmets in safety. Then, thus provided, they could crawl out through the hatchway and
float up to the surface. Arrived there they could inflate their jackets by blowing into them, open the window of the helmet and float upon the surface in comparative safety until rescued.
This apparatus was largely installed in British submarines and a tank was built at Portsmouth where the men could actually practise with it under water.
A third device may also be mentioned. This takes the form of a buoy fitted into a recess in the boat's upper surface. Sufficient line is coiled up inside it and when the occasion arises it can be released from inside. This does not in itself save the crew but it may go a long way towards ensuring their safety by letting those above know just where the sunken craft is and guiding them in their efforts to raise it.
The torpedo, the weapon without which the submarine would be practically useless, is dealt with in another chapter. Enough has been said here to give a good general idea of these interesting craft, their fittings, their uses and the sort of life which befalls those who man them.