New Jersey Scuba Diving

Portholes are used to let light and air into a ship. They are usually constructed of brass for corrosion resistance, which makes them highly desirable as collector's items. The drawing at right shows all the major parts:

• The immovable backing plate, bolted to the vessel's hull, which carries the hinges for the swing plate and deadlight
• The glass swing plate, which is secured shut by large butterfly nuts called 'dogs'
• The deadlight, used when it is desired to black-out the vessel, as in time of war

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