of the Relative Intensities of Sounds. 91 



The method I here present will, I hope, open the way to the 

 complete solution of this difficult and important problem ; and 

 1 trust that the success I have met with will encourage others 

 more learned and patient to attack with superior acumen a sub- 

 ject which must necessarily become of fundamental importance 

 in the future progress of acoustic research. 



1 . The determination of the Relative Intensities of Sounds of the 

 same Pitch. 



If two sonorous impulses meet in traversing an elastic medium, 

 and at their place of meeting the molecules of the medium 

 remain at rest, it is evident that at this place of quiescence the 

 two impulses must have opposite phases of vibration and be of 

 equal intensity. 



I have in the following manner experimentally applied this 

 principle to the accurate determination of the relative intensities 

 of vibrations giving the same note and propagated from their 

 sources of origin in spherical waves. 



Clothe two contiguous rooms with a material which does not 

 reflect sound, and place in each room one of the sounding bodies, 

 and maintain these sounds of a constant intensity ; or the two 

 sources of sound may be placed in the open air and separated 

 from each other by a non-reflecting partition. Fix at a certain 

 distance from each sounding body a resonator responding to its 

 note; attach to each resonator the same length of firm gum 

 tubing, and lead these tubes to a forked pipe so that the impulses 

 from the two resonators meet at the confluence of the two 

 branches of the forked tube, and connect the branch of the 

 forked tube, in which the sounds meet, with one of Konig's ma- 

 nometric capsules. Now sound continuously one of the bodies, 

 and the maaometric flame, when viewed in a revolving mirror, 

 will present its well-known serrated appearance. On sounding 

 the second body, impulses from it will meet those from the first 

 body ; and if the phases of vibration of the impulses on the ma- 

 nometric membrane are opposed and of equal intensities, the 

 membrane will remain at rest, and the flame will now appear in 

 the mirror as a band of light with a rectilinear upper border. 

 But although the intensities of the pulses can easily be rendered 

 equal by altering the distance of one of the resonators from its 

 sounding body, yet this change of position will alter the relation 

 of the phases reaching the membrane — so that if by mere chance 

 we get them opposed in the first position of the resonator, they 

 will no longer be so after its change of position. But on stop- 

 ping the vibrations of one of the bodies, and setting it in vibra- 

 tion at intervals, we may finally succeed in causing the impulses 

 on reaching the membrane to have opposite phases of vibration. 



