In the sixteen years since the last paper on this subject was published in the Journal (1951, 4, 288), some notable changes have taken place in the design and capabilities of submarines. Nautilus, the world's first nuclear submarine, was laid down in 1952 and completed in 1955. Since then a further 120 nuclear submarines have been commissioned. Now, a large proportion of the world's submarines are ‘true’ submersibles, self-sustaining over periods of months in everything except food; capable of proceeding submerged faster than the average surface ship; and rarely requiring to surface or even to come to periscope depth.