ce of the man standing in room 1
will be lost. Any little tone, however, that may remain will become
dissipated in room 3, and it will not be possible for a person standing
in room 4 to hear the voice.

[Illustration: Fig. 12. The Principle of the Sound Trap]

This plan illustrates the principle of the sound trap joint.

Figure 13 shows in section the joint between two Swell shutters. A
small proportion of the sound waves from inside the Swell box striking
the sound trap joint, as indicated by the arrow, will pass through the
nick between the two shutters, but these sound waves will become
greatly weakened in charging the groove A. Such of the sound waves is
pass through the second nick will become attenuated in charging the
chamber B. They will be further lost in the chamber C, and practically
none will remain by the time the chamber D is reached.

It is Hope-Jones' habit to place the shutters immediately above the
pipes themselves, so that when they are opened the Swell box is left
practically without any top. It is in such cases not his custom to fit
any shutters in the side or front of the Swell box.

[Illustration: Fig. 13. Sound Trap Joint]

To relieve the compression of the air caused by playing for any length
of time with the shutters closed, he provides escape valves, opening
outside the auditorium. He also provides fans for driving all the cold
air out of the box before using the organ, thus equalizing the
temperature with the air outside--or he accomplishes this result
through the medium of gas, electric or steam heaters, governed by
thermostats.

The Hope-Jones Vacuum Swell Shutters, with sound-trap joints, are shown
in Figures 14 and 15.

It is well known that sound requires some medium to carry it. Readers
will doubtless be familiar with the well-known experiment illustrating
this point. An electric bell is placed under a glass dome. So long as
the dome is filled with air the sound of the bell can be heard, but
directly the air is pumped out silence results, even though it can be
seen that the bell is continuously ringing. As there is no air
surrounding the bell there is nothing to convey its vibrations to the
ear.

That is why the hollow swell shutter, from the interior of which the
air has been pumped out, is such a wonderful non-conductor of sound.

The shutters shown in Figures 14 and 15 are aluminum castings.

Ribs R|1| and R|2| are provided to support the flat sides against t