State of Experimental Acoustics. 295 



what is the motion thereby imparted to the surrounding air ? 

 There is a question how far the solutions of this problem, 

 which have been given, conform to the facts. The experi- 

 mental settlement of this question is quite possible ; and it 

 goes to the very foundation of a certain portion of the mathe- 

 matical equations upon which the modern theory is built. 



Again, the equivalence of sound with mechanical energy is 

 at present in the position of a mathematical speculation. The 

 establishment of this equivalence, with a systematic mode of 

 measurement, and the determination of the various laws on 

 which it depends, is a work which alone would constitute an 

 important research. I shall return to this later. 



There is another important question in aerial mechanics, 

 which calls for mention among the very first. The whole 

 theory as now developed, neglects intentionally the viscosity 

 of the air. Now the effects of viscosity are more easily dealt 

 with by experiment than by theory. It is pretty certain that 

 the effects of viscosity are not really negligible ; for if they 

 were, vortex rings could not be produced in air, nor could 

 they be extinguished when once produced ; and in fact, as we 

 all know, they can be produced, and are rapidly extinguished. 



The actual composition of the notes produced by the various 

 methods which aim at the production of simple tones is also 

 a matter of primary importance, which no attempt is being 

 made to settle. It is certain that the existing explanation of 

 even such a simple matter as the overtones of organ-pipes is 

 insufficient ; for speaking resonators, whose overtones should 

 by theory be inharmonious, give true harmonics, of small in- 

 tensity, indeed, but with a general effect not very dissimilar to 

 that heretofore supposed to be peculiar to stopped organ-pipes. 

 It is possible that the organ-pipe mouth is responsible for 

 much more than has been supposed. 



I have noticed these points only as specimens of aerial me- 

 chanics. The oscillating disk first mentioned will be easily 

 constructed, and serve for numerous experiments of this class. 

 On the whole, experimental aerial mechanics must be regarded 

 as, in the future, probably the most important part of experi- 

 mental acoustics. 



Vibration Numbers. 



The accurate determination of the vibration numbers or fre- 

 quencies of notes is at present a matter of great difficulty. 

 Where many such determinations are to be made, the employ- 

 ment of a small engine possessing a uniform rate of speed, 

 controlled by a contrivance to be described later, will place 

 this part of the subject on a new footing. The siren, the re- 

 volving stopcock (described later and already constructed), 



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