Lord Ravleioli : Acoustical Xotes. 329 



loads as before (^ inch) was sufficient to canse a perceptible 

 augmentation of the residual fundamental tone. 



As to whether these results can be turned to practical 

 account in the construction of forks, we must remember that 

 if a fork is to be used in conjunction with a sounding- 

 board or resonance-box a too complete isolation of the funda- 

 mental tone would defeat the intention. On the other hand 

 if, as in Helmholtz's vowel experiments, a fork is to be 

 employed to excite an air resonator placed near the ends 

 of its prongs, a suitable turning inwards of these prongs 

 and consequent quiescence of the stalk would be of 

 advantage. 



In conclusion attention may be drawn to the circumstance 

 that a symmetrical bell with stalk attached would not need 

 any particular adjustment in order to ensure the isolation 

 of the vibrations of the first order. If the stalk tend to 

 move outwards when contraction occurs along one particular 

 diameter of the circumference, the same tendency must 

 repeat itself half a period later when the contraction is 

 transferred to the diameter at right angles to the first. 

 A similar remark would apply to a symmetrical compound 

 fork, such as we may imagine to be produced by cutting 

 <away all the material of the bell, except in the neighbourhood 

 of two perpendicular meridians. 



A Tuning-Fork Siren and its Maintenance. 



When in 1901 1 was experimenting upon the work absorbed 

 in various cases of the production of sound * I had at my 

 disposal a Trinity House "Manual" Fog-horn. In this 

 instrument the wind is generated by cylinders and pistons, 

 and a much higher pressure is available than is usual in 

 laboratory apparatus. Among the experiments then tried 

 was the substitution of what I called a tuning-fork siren for 

 the natural reed and conical horn of the instrument. Fitted 

 to a wind-chest was a metal plate, carefully faced internally 

 and carrying a rectangular aperture about 10 mm. broad. 

 This aperture could be nearly closed by a plate 3 mm. wider, 

 which vibrated laterally in front of it. The vibrating plate 

 was mounted upon the side of one prong of a fork making 

 128 vibrations per second. When the fork was at rest, the 

 aperture was obstructed and the fit of the moving and fixed 

 plates was so good that the leakage of wind was not seriouSc 



* Phil. Mag. vi. p. 292 (1903). 



