he
pressure of the atmosphere, but each of these ribs is so arranged that
it supports only one flat side and does not form a means of
communication between one flat side and the other. Thus R|1| supports
one flat side whilst R|2| supports the other. The aluminum shutters
are supported by means of pivot P.

[Illustration: Figs. 14-15. The Vacuum Shutter]

They are very light and can therefore be opened and closed with great
rapidity.

A very thin vacuum shutter forms a better interrupter of sound waves
than a brick wall two or three feet in thickness.

When partially exhausted the aluminum shutters are dipped into a bath
of shellac. This effectually closes any microscopic blow-hole that may
exist in the metal.

The use of Swell boxes of this vastly increased efficiency permits the
employment of larger scales and heavier pressures for the pipes than
could otherwise be used, and enormously increases the tonal flexibility
of the organ.

It also does away with the need for soft stops in an organ, thus
securing considerable economy. Where all the stops are inclosed in
cement chambers (as in the case of recent Hope-Jones organs) and where
the sound-trap shutters are employed, _every_ stop is potentially a
soft stop.

CHAPTER VIII.

A REVOLUTION IN WIND SUPPLY.

Prior to the construction of the above-named organ at Birkenhead,
England, it had been the custom to obtain or regulate the pressure of
wind supplied to the pipes by means of loading the bellows with
weights. Owing to its inertia, no heavy bellows weight can be set into
motion rapidly. When, therefore, a staccato chord was struck on one of
these earlier organs, with all its stops drawn, little or no response
was obtained from the pipes, because the wind-chest was instantly
exhausted and no time was allowed for the inert bellows weights to fall
and so force a fresh supply of air into the wind-chests.

BELLOWS SPRINGS VERSUS WEIGHTS.

In one of Hope-Jones' earliest patents the weights indeed remain, but
they merely serve to compress springs, which in turn, act upon the top
of the bellows.

Before this patent was granted he had, however, given up the use of
weights altogether and relied entirely upon springs.

This one detail--the substitution of springs for weights--has had a
far-reaching effect upon organ music. It rendered possible the entire
removal of the old unsteadiness of wind from which all organs of the
time suffered in greater