Why don鈥檛 marine mammals like whales get 鈥渢he bends鈥�? (continued)
John Davies
Lancaster, UK
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A previous correspondent explained why whales don鈥檛 get the bends, and went on to describe how the bends were first encountered as caissons disease, ascribing this to deep hard rock mining. He was wrong.
Air pressure varies by about 0.8 kilopascals per 1000 metres. The world鈥檚 deepest mine, Mponeng gold mine in South Africa, stretches to around 4000 metres below the surface, so the pressure at the bottom would be 104 kPa, rather than the 101 kPa at sea level (although the mine entrance is actually at 1500 metres altitude). The miners don鈥檛 experience the bends.
A caisson is a closed cylinder on the seabed, pressurised to keep the water out. It was used when mining had to be done underwater, for instance to build a bridge. A caisson鈥檚 pressure must be equal to or just greater than the water pressure where the caisson鈥檚 lip touches the bottom.
The deepest caissons were used to build the Tokyo Harbour Bridge in Japan, in water 46 metres deep, so the pressure inside must have been at least 5 atmospheres! A diver could stay there for less than 10 minutes, but could surface safely with only a precautionary stop for 3 minutes at 5 metres depth.
John Healey
Adelaide, Australia
A caisson wasn鈥檛 a miner in a hard rock mine. Caisson is French for 鈥渃offer鈥� or 鈥渃hest鈥�. The word refers to the large box, watertight at the top and sides, that was lowered into deep water and used in the 18th and 19th centuries in laying the foundations of bridges.
It was open at the bottom, so air pumped into it was at a pressure equal to the surrounding water. Workers in this box were thus breathing compressed air and experienced the bends on getting back to the surface. The earliest reference to caisson in the Oxford English Dictionary is from 1753.
To answer this question 鈥� or ask a new one 鈥� email lastword@newscientist.com.
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