to Transmit and to Reflect Sonorous Vibrations. 97 



sounding body, we can measure the powers of various substances 

 to transmit and to reflect sonorous vibrations. 



To accomplish this I place one of the sounding bodies in the 

 focus of a parabolic reflector, and bring the two resonators to 

 such distances from their sounding bodies that the intensities of 

 the pulses traversing their respective tubes are equal. We then 

 place in front of, but not too near, the mouth of the resonator 

 in front of the reflector the plane surface of the substance whose 

 transmitting and reflecting powers we would determine. Serra- 

 tions now appear in the flame, because part of the force of the 

 pulses which previously sounded the resonator are now reflected. 

 The resonator which has not the reflecting surface in front of it 

 is now gradually drawn away from its sounding body; and at each 

 successive point of remove the pulses propagated through the 

 resonator-tubes are brought to opposition of phase on reaching 

 the membrane by means of the glass telescoping tube. Equality 

 of impulses having been obtained, we measure the distance of 

 the resonator from its sounding body ; and this measure, together 

 with the previously known distance of this resonator when 

 equality was attained before the interposition of the reflecting 

 surface, gives the data for the computation of the intensity of 

 the transmitted vibrations. This number, subtracted from the 

 measure of the intensity when the substance was not before the 

 resonator, taken as unity, gives the reflecting -power of the sub- 

 stance. 



It is very important in such measures to be sure that a plane- 

 wave surface is reflected from the mirror. This character of wave 

 can be approximately obtained by placing the mouth of a closed 

 organ-pipe at or very near the principal focus of the mirror, and 

 testing, by the method we have described above, the equality of 

 intensity of the vibrating air in front of the mirror as we recede 

 along its axis. We thus by trial at last succeed in obtaining a 

 sufficiently plane-wave surface. Care must also be taken that 

 the surface of the reflecting substance we experiment on is so 

 large that no inflected vibrations can act on the resonator. 



I have made several measures of Intensity and of Transmitting 

 and Reflecting powers ; but as the experiments were made in a 

 room whose walls, ceiling, and floor gave reflected sonorous 

 waves, I will not present measures until I have arranged suitable 

 apartments for their accurate determination. 



November 13, 1872. 



the centre and focus of a concave mirror ; he then receded along the axis 

 of the diverging sonorous beam with a hearing-trumpet. Paper and flannel 

 were now stretched between the watch and the mirror ; and the intensity 

 of the sound was found to be diminished by the reflecting and absorbing 

 powers of these substances. 



Phil. Mag. S. 4. Vol. 45. No. 298. Feb. 1873. H 



