﻿Lord Rayleigh on ^fiolian Tones. 441 



certain point, and I have experimented with rods 5 mm. and 

 o-J mm. in diameter. With a 2 mm. rod no vibration could 

 be observed. The final results were thus tabulated : — 



Diameter 

 Ratio 



8'5 mm. 

 8-35 



5'0 mm. 



7*5 



3*5 mm, 



7-8 



from which it would appear that the disturbance is not very 

 serious. The difference between the ratios for the 5*0 mm. 

 and 3'5 mm. rods is hardly outside the limits of error ; and 

 the prospect of reducing the ratio much below 7 seemed 

 remote. 



The instinct of an experimenter is to try to get rid of a 

 disturbance, even though only partially ; but it is often 

 equally instructive to increase it. The observations of 

 Dec. 21 were made with this object in view ; besides those 

 already given they included others in which the disturbance 

 due to the vibrating pendulum was augmented by the addi- 

 tion of a similar rod (8^ mm.) immersed to the same depth 

 and situated symmetrically on the same diameter of the 

 basin. The anomalous effect would thus be doubled. The 

 record was as follows: — 



2 spokes to 3 beats gave little or no vibration, 



4 ...... . fair 



. 5 . , large 



6 ...... . less 



7 little or no 



As the result of this and another day's similar observations 

 it was concluded that the 5 beats with additional obstruction 

 corresponded with 6 beats without it. An approximate cor- 

 rection for the disturbance due to improper action of the 

 pendulum may thus be arrived at by decreasing the calculated 

 ratio in the proportion of 6 : 5 ; thus 



f(8-35)= 7-0 



is the ratio to be expected in a uniform stream. It would 

 seem that this cannot be far from the mark, as representing 

 the travel at a distance from the pendulum in an otherwise 

 uniform stream during the time of one complete vibration 

 of the latter. Since the correction for the other diameters 

 will be decidedly less, the above number may be considered 

 to apply to all three diameters experimented on. 



In order to compare with results obtained from air, we 

 must know the value of i//VD. For water at 15° 0. 

 v = /Jb = '0115 C.G.s. ; and for the 8*5 mm. pendulum 

 i,/VD = -0011. Thus from (6) it appears that ND/V should 



