In the side of every vessel, somewhat below the waterline is a large circular manhole, two or more feet in diameter into which fits a steel plate or plug. The plate is fastened to the reinforced sides of the ship by means of bolts arranged at intervals of a few inches around the circumference of the hole. Into this plate fit large pipes which, communicating with the sea, form an intake for salt water. This plug and its manhole are together called the ship’s seacocks.