426 swan. 



Wead 4 followed with a series of experiments on tuning forks. An 

 observer was stationed at a fixed distance, and the vibrations of the 

 fork were allowed to die away, the amplitude being noted at the in- 

 stant the observer ceased to hear the sound. The rate of emission of 

 energy at the moment was calculated and thence the minimum audible 

 energy. A somewhat similar method was employed by Rayleigh 5 a 

 few years later. 



Investigations in more recent years were made with telephone 

 diaphragms. The experiments of Rayleigh, 6 Franke, 7 Shaw, 8 and 

 Abraham, 9 along this line are fragmentary and do not call for extended 

 consideration. Zwaardemaker and" Quix 10 made an extensive study 

 of the problem between pitches 32 and 12228, using tuning forks for 

 the lower register and organ pipes for the upper. The methods em- 

 ployed give only the relative sensitiveness, and are open to serious 

 criticisms which have been ably discussed by Wien. 11 



The results of all of these investigations are inconsistent, a fact 

 which is not surprising when the sources of error are considered. The 

 experiments in the first group were carried on in the daytime, out-of- 

 doors, and all of the observers speak of the annoyance caused by wind, 

 birds and other extraneous sources of sound. As anyone will appre- 

 ciate who has tried such experiments, the slightest disturbing sound is 

 sufficient to preclude accuracy in measurements, the sound being so 

 faint near the threshold value that it is entirely obliterated unless 

 absolute quiet prevails. Further criticism may be made of the use of 

 the inverse square law in the computations. It is not conceivable 

 that this law should hold, even in an open field, as the absorbing power 

 of the surface of the ground alone is sufficient to render it invalid. 

 Beyond these obvious difficulties, which amply account for incon- 

 sistencies in the results, note should be made of the fact that, contrary 

 to assumption, all of the energy consumed by a sounding body does 

 not reappear as sound, thus furnishing another source of error in many 

 of the cases mentioned. 



Some interesting work has been done by Wien. The early investi- 



4 Sill. Jour., 3rd Series, 1883, Vol. 26, p. 177; and 1891, Vol. 41, p. 232. 



5 Phil. Mae;., 1894, Vol. 38, p. 36.5. 



6 Phil. Mag., 1894, Vol. 38, p. 285. 



7 Elektrotech. Zeit,, 1890, Vol. 11, p. 288; and Jour. Inst, Elec. Engrs., 

 Vol. 19, p. 754. 



8 Proc. Rov. Soc, Series A, 1905, Vol. 76, p. 360. 



9 Comptes Rendus, 1907, Vol. 144. p. 1099. 



10 Arch. f. Anat. und Phvsiol., Suppl., 1902, p. 367. 



ii Pnueger's Arch. f. Gesammte Phvsiol., 1903, Vol. 97, p. 48. 



