MODERN PROBLEMS IN ACOUSTICS. 133 
sounding body is a wire, it is mounted to vibrate before a 
transverse slit, through which light falls on the sensitive 
surface moving parallel to the string. Compound curves 
produced in either way are then subjected to harmonic 
analysis. This method of studying vowels was first used by 
Boltzman in 1882, and a few years later Mach obtained pho¬ 
tographs of sound waves in air. 
The superposition of two waves has been further studied 
with reference to the pitch actually observed when two notes 
are beating. The old theory of combination tones has been 
rudely shaken and their objective existence proved in cer¬ 
tain rare cases. Mechanical superposition of harmonic 
motions has been obtained by many elaborate forms of har- 
monographs or curve-tracers (Michelson). The complex 
curves due to vowels and other speech sounds have been 
much studied, especially by the aid of the phonograph. 
Two or three matters of industrial as well as scientific 
importance may also be noted, viz., the enormous develop¬ 
ment of speaking instruments—phonograph, graphophone 
(C. A. Bell), gramophone; the adoption by the Piano Makers’ 
Association of the United States of the French standard 
tuning fork, giving A == 435 d. v., and the many improve¬ 
ments in organ pipes and reed stops that show a practical 
control over the wind sheet such as the older builders had 
not obtained. 
IV. Some of the problems remaining to be solved are 
these: 
(1.) In pure physics: the simplification of the means for 
the precise determination of pitch in ordinary practical cases; 
the establishment of convenient standards of intensity, and 
the perfecting of experimental means of measuring intensb 
ties; the development of means for the thorough analysis of 
sounds. 
(2.) In connection with instruments: the thorough study 
of the action of the sounding board of a piano, of reeds as 
actually used in common instruments, and of the laws of the 
perforated tube as applied in flutes, etc.; the determination 
of the quality of tone produced by our common instruments 
19—Bull. Phil. Soc., Wash., Vol. 14. 
