us stops at different pitches, but the various groups may be
altered in power of tone independently of each other. At one moment
the foundation tone may entirely dominate, by moving the swell pedals
the strings may be made to come to the front while the foundation tone
disappears; then again the woodwind asserts itself whilst the string
tone is moderated, till the opening of the box containing the brass
allows that element to dominate. The variety of the tonal combinations
is practically endless.

The adoption of this principle also saves needless duplication of
stops. In the organ at St. George's Hall, England, there are on the
manuals 5 Open Diapasons, 4 Principals, 5 Fifteenths, 3 Clarinets, 2
Orchestral Oboes, 3 Trumpets, 3 Ophicleides, 3 Trombas, 6 Clarions, 4
Flutes, etc., etc. In the Hope-Jones Unit organ at Ocean Grove effects
equal to the above are obtained from only 6 stops. The organist of
Touro Synagogue, New Orleans, has expressed the opinion that his
ten-stop Unit organ is equal to an ordinary instrument with sixty stops.

SYMPATHY.

A strong reason against the duplication of pipes of similar tone in an
organ is that curious acoustical phenomenon, the _bete noir_ of the
organ-builder, known as _sympathy_, or interference of sound waves.
When two pipes of exactly the same pitch and scale are so placed that
the pulsations of air from the one pass into the other, if blown
separately the tone of each is clear; blown together there is
practically no sound heard, the waves of the one streaming into the
other, and a listener hears only the rushing of the air. That the
conditions which produce sound are all present may be demonstrated by
conveying a tube from the mouth of either of the pipes to a listener's
ear, when its tone will be distinctly heard. In other words, one sound
destroys the other. Helmholtz explains this phenomenon by saying that
"when two equal sound waves are in opposition the one nullifies the
effect of the other and the result is a straight line," that is, no
wave, no sound. "If a wave crest of a particular size and form
coincides with another exactly like it, the result will be a crest
double the height of each one" (that is, the sound will be augmented).
* * * "If a crest coincides with a trough the result will be that the
one will unify the other," and the sound will be destroyed.[1] That is
why in the old-style organs the builder, when he used more than one
Diapason, tried to