464 RESEARCHES IN SOUND. 



tion of the instrument, with a magnifying lens, with which to observe 

 more minutely the motions of the sand. To use this instrument in com- 

 XJaring the relative penetrating power of sound from diiierent sources, 

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

 was turned toward one of the bells, and the efiect causing agitation of 

 the sand, was noted. The instrument was then removed to a station a 

 little further from the bell, and the effect again noted, the distance being- 

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

 through the lens. This distance, being measured in feet or yards, gave 

 the number indicating the i)enetrating power of the instrument under 

 trial. The same experiment was immediately repeated, under the same 

 conditions of temperature, air, wind, &c., with the other sounding appa- 

 ratus, and the relative number of yards indicating the distance, taken 

 as the penetrating powers of the two instruments. It should be observed, 

 in the use of this instrument, that it is intended merely to concentrate 

 the rays of sound, and not to act as a resounding cavity; since in that 

 case the sound, in unison with the resounding note, would produce effect 

 at a greater distance than one in discord. 



The indications of this instrument were compared with the results ob- 

 tained by the ear in the use of the two vessels, and in all cases were in 

 exact accordance; and it was accordingly used in the following investi- 

 gations, and has been found of great service in all subsequent experi- 

 ments on the penetration of sound. 



The only precaution in using it is that the membrane shall not be of 

 such tension as to vibrate in unison with a single sound or its octaves ; 

 or, in other words, that the instrument must be so adjusted by varying 

 the length of the axis or the tension of i he membrane that it shall be in 

 discordance with the sounds to be measured, and only act as a condenser 

 of the sonorous waves. 



The first experiments made were with regard to the influence of re- 

 flectors. For this purpose a concave wooden reflector had been prepared, 

 consisting of the segment of a sphere of 16 feet radius, and covered with 

 plaster, exposing a surface of 64 square feet. In the focus of this, by 

 means of a temporary railway, a bell or whistle could be readily intro- 

 duced or withdrawn. The center of the mouth of the bell was placed in 

 the horizontal axis of the reflector. This arrangement being completed, 

 the sound of the bell, with and without the reflector behind it, was alter- 

 nately observed. Within the distance of about 500 yards the efiect was 

 evidently increased, as indicated by the motion of the sand on the mem- 

 brane, but beyond this the difference was less and less perceptible, and 

 at the limit of audibility the addition of the reflector appeared to us en- 

 tirely imperceptible. This result was corroborated by subsequent ex- 

 periments in which a whistle was heard nearly as well in the rear of a 

 reflector as before it. It would appear from these results that while 

 feeble sounds, at small distances, are reflected as rays of light are, waves 



