Hard hat diving
Hard hat diving is a physical hard and demanding profession. The diver performs important tasks in harbours all over the world. Even though the hard hat ...
articles.uwphoto.no/Image_month/previous_Images_of_the_month_19.htm

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Note added at 13 mins (2007-07-15 01:01:35 GMT)
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Helmut (Hard Hat) Diving-Underwater Exploration-University of ...
In the 20th century, hard-hat divers learned that breathing mixed gases, in particular a helium-oxygen mixture, permitted them to dive to greater depths for ...
www.seagrant.wisc.edu/madisonjason11/diving_helmut.html

A standard diving dress consists of a metallic (copper, brass or bronze) diving helmet, an airline or hose from a surface supplied diving air pump, a canvas diving suit, diving knife and boots. An important part of the equipment is the addition of lead weights, generally on the chest, back and boots, to counteract the buoyancy of the helmet and diving suit.

This type of diving equipment is also known as hard-hat equipment or a "John Brown" rig, so-called after the British company that built many of the helmets. It was commonly used for underwater civil engineering, commercial diving and naval diving.