380 WRITINGS OF JOSEPH HENRY. [1874 



through the lens. This distance being measured in feet or 

 yards gave the number indicating the penetrating power of 

 the instrument under trial. The same experiment was 

 immediately repeated under the same conditions of tem- 

 perature, air, wind, &c., with the other sounding apparatus, 

 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 an effect at 

 a greater distance than one in discord. 



The indications of this instrument were compared with 

 the results obtained by the ear in the use of the two vessels, 

 and in all cases were in exact accordance; and it was accord- 

 ingly used in the following investigations, and has been 

 found of great service in all subsequent experiments 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 instru- 

 ment must be so adjusted by varying the length of the axis 

 or the tension of the membrane that it shall be in discord- 

 ance with the sounds to be measured, and only act as a con- 

 denser of the sonorous waves. 



The first experiments made were with regard to the influ- 

 ence of reflectors. 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 

 introduced or withdrawn. The centre 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 alternately observed. 

 Within the distance of about 500 yards the eff'ect was evi- 

 dently increased, as indicated b}'' the motion of the sand on 

 the membrane, but beyond this, the difference was less and 



