﻿440 Lord Rayleigh on ^Eolian Tones. 



the direction of the stream,, agreeably to what had been found 

 for the seolian harp. In what follows the vibrations, if any, 

 are radial, that is transverse to the stream. 



In conducting a set of observations it was found con- 

 venient to begin with the highest speed, passing after a 

 sufficient time to the next lower, and so on, with the minimum 

 of intermission. I will take an example relating to the main 

 rod, whose diameter (D) is 8^ mm., t= 60/106 sec., beats 

 of metronome 62 in 30 sec. The speed is recorded by 

 the number of beats corresponding to the passage of two 

 spokes, and the vibration of the pendulum (after the lapse 

 of a sufficient time) is described as small, fair, »ood,, and so 

 on. Thus on Dec. 21, 1914 : 



2 spokes to 4 beats gave fair vibration, 



5 good 



6 rather more .... 



7 good 



8 fair 



from which we may conclude 'that the maximum effect cor- 

 responds to 6 beats, or to a time (T) of revolution of the 

 turn-table equal to 2x 6x 30/62 sec. The distance (r) of 

 the rod from the axis of rotation was 116 mm., and the 

 speed of the water, supposed to move with the basin, is 

 2irr/T. The result of the observations may intelligibly be 

 expressed by the ratio of the distance travelled by the water 

 during one complete vibration of the pendulum to the 

 diameter of the latter, viz. 



t.2tt?-/T _ 2ttx 116x62 _ 



D ~ 8-5x6x106 ~ b ' 6b ' 



Concordant numbers were obtained on other occasions. 



In the above calculation the speed of the water is taken 

 as if it were rigidly connected with the basin, and must be 

 an over estimate. When the pendulum is away, the water 

 may be observed to move as a solid body after the rotation 

 has been continued for two or three minutes. For this 

 purpose the otherwise clean surface may be lightly dusted 

 over with sulphur. But when the pendulum is immersed, 

 the rotation is evidently hindered, and that not merely in 

 the neighbourhood of the pendulum itself. The difficulty 

 thence arising has already been referred to in connexion 

 with Strouhal's experiments and it cannot easily be met in 

 its entirety. It may be mitigated by increasing r, or by 

 diminishing D. The latter remedy is easily applied up to a 



