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At a depth of 40 metres the water pressure is five times atmospheric pressure – that is, 5 bar – and comprises the 1 bar (atmosphere) on the surface plus 1 bar (atmosphere) for each of the four 10 metres. Any air spaces such as lungs would be compressed to a fifth of the volume they would be on the surface – not good. So, the aqualung again cleverly feeds a diver air that is compressed to five times atmospheric pressure – 5 bar. Again, the air pressure in the diver’s lungs is kept exactly the same as the surrounding water pressure – and the diver’s lungs remain exactly the same size as on the surface.

Boyle’s Law – the law of inverse proportions – governs this effect. When scientists were trying to work out what happened to air underwater, some brave, hardy men would sit in an upturned barrel cut in half, which was lowered into the water. As the barrel was taken below to predetermined depths, the air inside was compressed and the water level rose. Marks would be made on the side of the barrel at different depths. The depths and compression marks were correlated, and the law became clear.