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WEAD. 
trary analytical effect is observed in some cases of echoes, as 
from a forest, where the sound of the voice seems to come 
back raised an octave. 
The relative absorbing power of various fabrics has lately 
been measured by Sabine. From his data I calculate that 
for a note of 256 d.v 0.33 of the energy falling on a sheet of 
hair-felt one-half inch thick is absorbed. 
A few instruments have been devised to produce a tone of 
definite reproducible intensity, and other instruments to in¬ 
dicate or measure the intensity of vibration at a given point. 
In Wien’s beautiful manometer the minute yielding of a part 
of the wall of a resonator is measured by mirror and scale to 
one-five-hundredth part of its maximum Amount, while the 
absolute value of the scale readings is determined to within 
a few per cent. 
The study of Forms of Vibration or Quality of Sound has 
been prosecuted both synthetically and analytically. Koenig 
many years ago challenged Helmholtz’s conclusion that the 
quality of sound depended only on the strength of the over¬ 
tones, not on their relative phase, and he invented his wave 
siren to prove his position. In this instrument the flow of 
air through several slits is carved into waves by several 
rotating discs, whose edges are cut into harmonic curves. 
More recently he has greatly perfected it and attempted to 
meet various criticisms made against his earlier work; but 
so many lines of argument support Helmholtz’s view that I 
do not think this brilliant attack will generally be admitted 
to have conquered the field. A more reliable means of syn¬ 
thesis than the wave discs is found in Appunn’s sets of organ 
pipes. These furnish a great number of harmonics of one 
fundamental, and for each harmonic there are two pipes—a 
weak-toned and a strong-toned one. 
Analytically the problem of form of vibration has been 
attacked in various ways, especially by photography. If 
the vibration to be examined is in the air, either the Koenig 
flames connected to a set of resonators may be photographed 
(Hallock), or a little mirror on a convenient speaking tube 
may throw a spot of light on the sensitive plate. If the 
