Diving Bell
    The diving bell is used to increase the supply of air available to persons working underwater. The device was invented in the 16th century, and had been used in several explorations of wrecks to recover sunken treasure by the end of the 17th century.

   "The impenetrability of air is shown by the Diving-Bell, ... shaped somewhat line an inverted tumbler... As the vessel descends, the air in it is condensed by the upward pressure of the liquid, and water enters. The lower it gets, the more the air is compressed, and the greater the amount of water admitted. The impenetrability of the air, however, keeps the greater part of the bell clear of water, so that several persons may descend in it to the bottom of the sea." From G. P. Quackenbos, A Natural Philosophy (D. Appleton and Company, New York, 1866), pp 166-167
     At the left, below, is a diving bell model at Marietta College in Ohio. The form of diving bell invented by Triewald is shown in the model at the right, below, at Middlebury College in Vermont.

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The copperplate engraving is from the fifth edition of the Brittanica published in Edinburgh, Scotland in 1815. The three diving bells are those developed by Halley, Spalding and Triewald.