of Vibration in the Air surrounding a Sounding Body. 323 



fraction of half wave-length at which the sympathetic fork is 

 placed on the line X. 



It is now readily seen that if we could place the fork B at 

 two successive points (as B^ and B"*) on the line O X so that 

 exact concordance of flexures should be seen at each of these 

 points, then evidently the fork would be at two positions removed 

 from each other by a wave-length ; for at these points the air 

 would have, at the same instants, the same phases of vibration. 

 Thus we should have measured a wave-length. Furthermore, 

 if by any means we could move the fork B around A so that 

 during this motion it always preserved, in reference to A, the 

 same relation of vibratory phase, we should determine the 

 form of the w^ave-surface produced by the propagation of A^s 

 vibration. 



The above is an exposition of the thoughts that have occu- 

 pied my mind for several months ; and they ultimately led to the 

 following method, by which all I have mentioned can be accom- 

 plished without any difficulty — thanks to the inventive genius 

 of M. Konig, to whose skilful aid so many physicists are con- 

 tinually indebted. 



The membranes of M. Konig-'s raanometric capsules furnish 

 us with surfaces which vibrate in perfect accordance with the air 

 which touches them, and we can lead the impulses of these mem- 

 branes through gum tubes to gas-jets, placed at any desired 

 point, where the vibrations of their flames can be compared. 

 Thus they are far superior to tuning-forks, which require the 

 relations of delicate adjustments to be maintained during each 

 change of position, and therefore could only with difficulty 

 be made to serve in the measure of a wave-length, and could 

 not be employed at all to trace out a wave-surface, on account 

 of the impossibility of a continuous comparison of their vibra- 

 tions, which latter condition the manometric flames admirably 

 fulfil. 



The Experiments. 



Let us now proceed to experiment. I placed on the acoustic 

 bellows an open UTg organ-pipe, and from its ventral manometric 

 capsule I led a tube to a gas-jet placed in front of a cubical re- 

 volving mirror. I took an xiTg Helmholtz resonator, and 

 adapted to its beak a gum tube, with an interior diameter of 

 1 centimetre, and a length of over 4 metres. This tube led to a 

 firmly supported manometric capsule, whose flame was placed 

 quite close to and directly in front of the organ-pipe flame; 

 which latter had about twice the height of the resonator-flame. 

 On sounding the organ-pipe and holding the resonator quite near 

 it, the two flames, by a slight adjustment, were made to appear 



Y2 



