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.

[Illustration: 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.

We have mentioned in connection with railway signalling that the
signalman is sometimes relieved of the hard manual labour of moving
signals and points by the employment of electric and pneumatic
auxiliaries. The same is true of organs and organists. The touch of the
keys has been greatly lightened by making the keys open air-valves or
complete electric circuits which actuate the mechanism for pulling down
the pallets. The stops, pedals, and couplers also employ "power." Not
only are the performer's muscles spared a lot of heavy work when
compressed air and electricity aid him, but he is able to have the
_console_, or keyboard, far away from the pipes. "From the console, the
player, sitting with the singers, or in any desirable part of the choir
or chancel, would be able to command the working of the whole of the
largest organ situated afar at the western end of the nave; would draw
each stop in complete reliance on the sliders and the sound-board
fulfilling their office; ... and--marvel of it all--the player, using
the swell pedal in his ordinary manner, would obtain crescendo and
diminuendo with a more perfect effect than by the old way."[31]

In cathedrals it is no uncommon thing for the different sound-boards to
be placed in positions far apart, so that to the uninitiated there may
appear to be several independent organs scattered about. Yet all are
absolutely under the control of a man who is sitting away from them all,
but connected with them by a number of tubes or wires.

The largest organ in the world is that in the Town Hall, Sydney. It has
a hundred and twenty-six speaking stops, five manuals, fourteen
couplers, and forty-six combination studs. The pipes, about 8,000 in
number, range from the enormous 64-foot contra-trombone to some only a
fraction of an inch in length. The organ occupies