1874] WRITINGS OF JOSEPH HENRY. 379 



M. Powell, inspector, and Mr. Lederle, acting engineer of the 

 third district. 



The principal object was to compare the sound of bells, of 

 steam-whistles, and other instruments, and the effect of 

 reflectors, and also the operation of different hot-air engines. 

 For this purpose the committee was furnished with two small 

 sailing vessels. As these were very imperfectly applicable, 

 since they could not be moved without wind, the writer of 

 the report devised an instrument denominated an " artificial 

 ear," by which the relative penetrating power of different 

 sounding bodies could be determined and expressed in 

 numbers by the removal of the observer to a comparatively 

 short distance from the point of origin of the sound. This 

 instrument consisted of a conical horn made of ordinary 

 tinned sheet iron, the axis of which was about 4 feet in 

 length, the diameter of the larger end 9 inches, and tapering 

 gradually to If of an inch at the smaller end. The axis of 

 this horn was bent at the smaller end in a gentle curve, 

 until the plane of the section of the smaller end was at right 

 angles to the transverse section of the larger end, so that 

 when the axis of the trumpet was held horizontally and the 

 larger section vertically, then the section of the smaller end 

 would be horizontal. Across the smaller end a thin mem- 

 brane of gold-beater's skin was slightly stretched and secured 

 by a thread. On this membrane fine sand was strewn. To 

 protect the latter from disturbance by the wind it was sur- 

 rounded by a cylinder of glass cut from a lamp-chimney, 

 the upper end of which was covered with a plate of glass; 

 and in the improved condition of the instrument, with a 

 magnifying lens with which to observe more minutely the 

 motions of the sand. To use this instrument in comparing 

 the relative penetrating power of sound from different sources, 

 as for example from two bells, the axis being held horizontal, 

 the mouth was turned toward one of the bells, and the effect 

 causing agitation of the sand was noted. The instrument 

 was then removed to a station a little farther from the bell, 

 and the effect again noted, the distance being increased 

 step by step until no motion in the sand could be observed 



