n be coupled to any but the echo
manual.

VARIETIES OF STOPS.

We have already remarked that the quality of a stop depends on the shape
and construction of the pipe. Some pipes are of wood, others of metal.
Some are rectangular, others circular. Some have parallel sides, others
taper or expand towards the top. Some are open, others stopped.

The two main classes into which organ pipes may be divided are:--(1.)
_Flue_ pipes, in which the wind is directed against a lip, as in Fig.
138. (2.) _Reed_ pipes--that is, pipes used in combination with a
simple device for admitting air into the bottom of the pipe in a series
of gusts. Fig. 144 shows a _striking_ reed, such as is found in the
ordinary motor horn. The elastic metal tongue when at rest stands a very
short distance away from the orifice in the reed. When wind is blown
through the reed the tongue is sucked against the reed, blocks the
current, and springs away again. A _free_ reed has a tongue which
vibrates in a slot without actually touching the sides. Harmonium and
concertina reeds are of this type. In the organ the reed admits air to a
pipe of the correct length to sympathize with the rate of the puffs of
air which the reed passes. Reed pipes expand towards the top.

TUNING PIPES AND REEDS.

[Illustration: 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