In a type built with vertical sections each division is complete in itself, and is not directly connected with the next section, but communicates with flow and return drums. A defective section may thus be left in position and stopped off by means of plugs from the drums until it is convenient to fit a new one in its place.
Fusible plugs are little used; they consist of pieces of softer metal inserted on the side of the boiler, which melt should the heat of the water rise above a certain temperature.
At the outgoing end the circuits are multipled on the subscribers' switchboard, while at the incoming end they terminate in plugs on a special incoming junction switchboard upon which the subscribers' lines are multipled in the usual way.
Similarly, in Alaska, women felt great shame when seen without the plugs they carried in their lips.
At Wheal Cock near St Just in Cornwall the protecting roof was so thin that holes bored for blasting more than once penetrated to the bed of the ocean, and wooden plugs were kept on hand to drive into such holes when this occurred.
