Principle of the Aqualung. The reason there is a limit to the useful length of a snorkel tube is that water pressure increases at the rate of 15lb. per sq. in. for every 33ft. of depth. It is noticeable when treading water with the mouth just above the surface, that breathing requires a slight effort; if the lungs sink 2ft. farther below the surface the pressure of the surrounding water prevents them working at all. In order to counteract this effect, the aqualung is arranged automatically to adjust the pressure of the air it supplies to equal the pressure of the surrounding water. This is done by the demand valve, which also conserves the air in the cylinder by closing when the lungs are not taking in air. The cylinder has a capacity of 0.4cu.ft., and it contains 40cu.ft. of air, compressed to a pressure of 2,000lb. per sq. in., when fully charged. This extremely high pressure has first to be reduced by a valve to about 100lb. per sq.in., and the air is then fed to the demand valve, which performs the dual function of equalising the pressure in the inlet tube to that of the surrounding water and supplying air to the lungs as soon as they start to inhale. Conventional aqualungs terminate the inlet and exhaust tubes correctly close together, but as the complete valve gear is mounted in a single unit on top of the cylinder there is often a difference in pressure between the sensitive demand valve and the diver's mouth. In the normal horizontal swimming position this causes the air to be a little difficult to draw through, but while the diver is swimming on his back the valve is placed well below the level of his face, and causes a leakage of air through his nose and dive mask. The aqualung to be described avoids the worst of this trouble by having separate demand and reducing valves, and the demand valve is worn in the best possible compromise position, high on the diver's chest. The same conditions dictate that the inlet and exhaust tubes should terminate close together at the demand valve; if they were only a few inches apart there would be some positions of the diver in the water where the demand valve would be at a greater depth and pressure than the no-return valve in the exhaust tube, and there would be a continuous waste of air..