Fig. 144.—A reed pipe.
Pipes are tuned by adjusting their length. The plug at the top of a stopped pipe is pulled out or pushed in a trifle to flatten or sharpen the note respectively. An open pipe, if large, has a tongue cut in the side at the top, which can be pressed inwards or outwards for the purpose of correcting the tone. Small metal pipes are flattened by contracting the tops inwards with a metal cone like a candle-extinguisher placed over the top and tapped; and sharpened by having the top splayed by a cone pushed in point downwards. Reeds of the striking variety ([see Fig. 144]) have a tuning-wire pressing on the tongue near the fixed end. The end of this wire projects through the casing. By moving it, the length of the vibrating part of the tongue is adjusted to correctness.
BELLOWS.
Different stops require different wind-pressures, ranging from 1⁄10 lb. to 1 lb. to the square inch, the reeds taking the heaviest pressures. There must therefore be as many sets of bellows and wind-chests as there are different pressures wanted. A very large organ consumes immense quantities of air when all the stops are out, and the pumping has to be done by a powerful gas, water, or electric engine. Every bellows has a reservoir ([see Fig. 143]) above it. The top of this is weighted to give the pressure required. A valve in the top opens automatically as soon as the reservoir has expanded to a certain fixed limit, so that there is no possibility of bursting the leather sides.
Fig. 145.—The keyboard and part of the pneumatic mechanism of the Hereford Cathedral organ. C, composition pedals for pushing out groups of stops; P (at bottom), pedals; P P (at top), pipes carrying compressed air; M, manuals (4); S S, stops.
ELECTRIC AND PNEUMATIC ACTIONS.