734 



SCIENCE. 



[N. S. Vol. XI. No. 280. 



at which it could be heard, and so the 

 energy per second required at the limit of 

 hearing. More recently a Swedish experi- 

 menter found, by periodically thrusting a 

 thermopile, mounted on a tuning-fork prong, 

 into a sounding pipe at the node, a rise of 

 0.1° C, due to the adiabatic compression. 

 Since 1870 experiments on the energy of 

 organ pipes, etc., have been multiplied and 

 refined. Similarly, determinations on the 

 intensity of telephone currents and the 

 movement of its disc have testified^ to the 

 incredible sensitiveness of the ear. An 

 amplitude of vibration of air particles of 

 only one fifteen millionth of a millimeter in 

 the region of 440 d. v. produces sensation. 



Other investigations have traced the ex- 

 penditure of the energy once stored in a 

 vibrating mass ; so the rate of damping of 

 forks under various conditions has been ob- 

 served ; also the dissipation of energy in a 

 resonator and the decay of sound in free air: 

 it has been noted that a sound reflected re- 

 peatedly through a tunnel changes in qual- 

 ity, owing to the more rapid absorption of 

 the overtones of high pitch. A contrary 

 analytical effect is observed in some cases 

 of echoes, as from a forest, where the sound 

 of the voice seemed 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 dv. 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 in- 

 tensity, and other instruments to indicate 

 or measure the intensity of vibration at a 

 given point. In Wien's beautiful manom- 

 eter 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 Form of Vibration or Quality 

 of Sound has been prosecuted both synthet- 

 ically and analytically. Konig many years 

 ago challenged Helmholtz's conclusion that 

 the quality of sound depended only on the 

 strength of the overtones, not on their rela- 

 tive 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 gen- 

 erally be admitted to have conquered the 

 field. A more reliable means of synthesis 

 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. 



Analyticallj' the problem of form of vibra- . 

 tion has been attacked in various ways, 

 especially by photography. If the vibra- 

 tion to be examined is in the air either the 

 Konig's flames connected to a set of re- 

 sonators may be photographed, or a little 

 mirror on a convenient speaking tube may 

 throw a spot of light on the sensitive plate- 

 If the sounding body is a wire, it is mounted 

 to vibrate before a transverse slit through 

 which ligbt falls on the sensitive surface 

 moving parallel to the string. Compound 

 curves produced in either way are then 

 subjected to harmonic analysis. In pass- 

 ing it may be noted that Mach obtained a 

 photograph of a sound wave in air as far 

 back as 1888. 



The superposition of two vibrations 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 ob- 

 jective existence proved experimentally in 



