Lord Rayleigh : Acoustical Sotes. 325 



near the upper ends (A, B) symmetrically in the median 

 plane (fig. 3). Frequently a better silence may be reached 

 by moving round a little, and even then it is not absolute. 

 By use of the finger to give a finishing touch to the tuning, 

 the most silent position mnj be driven to the median plaue, 

 but even so the residual tone may not be quite extinguished. 

 It is evident that the ideal condition is easily disturbed a 

 little by slight failures of symmetry, probably connected with 

 the flames. 



As so far described, the disappearance of theprincipal tone, 

 sometimes very nearly realized, leaves a considerable amount 

 of octave outstanding. A remedy may be applied by the 

 insertion, at the middles of the tubes, of rectangular blocks of 

 wood, about two diameters long and forming a loose fit. They 

 are held in their places by springs. In this way the out- 

 standing octave may be very much reduced. 



Longitudinal Balance of Tuning-Forks. 



The vibrations of a well-constructed tuning-fork are ap- 

 proximately isolated and are conveyed to the stalk in only a 

 limited degree. When, as in the ordinary use of small forks, 

 the stalk is pressed against a sounding-board, the principal 

 tone is attended by a considerable accompaniment of octave, 

 especially at first when the vibrations are vigorous. The sub- 

 stitution of a suitably tuned resonance-box for the sounding- 

 board may easily render the octave sound preponderant *. 

 The experiments now to be recorded were an attempt to 

 ascertain how far it was possible to carry the isolation of the 

 principal tone. It should be remembered that however com- 

 plete may be the isolation as regards the stalk, there is 

 necessarily a certain small amount of direct communication 

 from the vibrating prongs to the surrounding air. For our 

 present purpose this is to be disregarded. 



At first sight it may appear that the desired state o£ things 

 must be very approximately attained in the usual construction 

 where the prongs are parallel. Something will depend upon 

 the manner in which the transition takes place between prongs 

 and stalk. In what follows I have more particularly in view 

 a construction in which the prongs form a U of tolerably 

 uniform thickness, to which a cylindrical stalk is attached 

 without much excess of metal at the junction. As a rough 

 approximation we may suppose that the inertia of the fork is 



* Phil. Mag. iii. p. 456 (1877); Scientific Papers, i. p. 318. Even 

 when the box is tuned to the fundamental note, the octave and twelfth 

 are often easily audible. I have observed this effect with three different 

 256-forks when mounted upon a particular resonance-box. There was 



