the principal part of an organ, and that which makes the whole machine play. This sound-board, or summer, is a reservoir into which the wind, drawn in by the bellows, is conducted by a port-vent, and thence distributed into the pipes placed over the holes of its upper part. This wind enters them by valves, which open by pressing upon the stops or keys, after drawing the registers, which prevent the air from going into any of the other pipes beside those it is required in.

SOUND Board denotes also a thin broad board placed over the head of a public speaker, to enlarge and extend or strengthen his voice.

Sound-boards, in theatres, are found by experience to be of no service; their distance from the speaker being too great to be impressed with sufficient force. But sound boards immediately over a pulpit have often a good effect, when the case is made of a jut thickens, and according to certain principles.