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Boatswain's call
Whistle used for communication onboard naval ships / From Wikipedia, the free encyclopedia
A boatswain's call, pipe, or bosun's whistle is a pipe or a non-diaphragm type whistle used on naval ships by a boatswain.
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The pipe consists of a narrow tube (the gun) which directs air over a metal sphere (the buoy) with a hole in the top. The player opens and closes the hand over the hole to change the pitch. The rest of the pipe consists of a "keel", a flat piece of metal beneath the gun that holds the call together, and the "shackle", a keyring that connects a long silver or brass chain that sits around the collar, when in ceremonial uniform.