﻿Lord Rayleigh on JEolian Tones. 443 



to do this by a pendulum arrangement designed to determine 

 the wind by its action upon an elongated piece of mirror 

 (10'1 cm. x 1*6 cm.) held perpendicularly and just in front 

 of the string. The pendulum is supported on two points — 

 in this respect like the one used for the water experiments ; 

 the mirror is above, and there is a counter-weight below. 

 An arm projects horizontally forward on which a rider can 

 be placed. In commencing observations the wind is cut off 

 by a large card inserted across the aperture and just behind 

 the string. The pendulum then assumes a sighted position, 

 determined in the usual way by reflexion. When the wind 

 operates the mirror is carried with it, but is brought back to 

 the sighted position by use of a rider of mass equal to 

 •485 gm. 



Observations have been taken on several occasions, but it 

 will suffice to record one set whose result is about equal to 

 the average. The (horizontal) distance of the rider from 

 the axis of rotation was 62 mm. 5 and the vertical distance of 

 the centre line of the mirror from the same axis is 77 mm. 

 The force of the wind upon the mirror was thus 

 62 x *485 -r 77 gins, weight. The mean pressure P is 



62 x -485x981 = ^, 7 dynes 

 77x16-2 cm. 2 * 



The formula connecting the velocity of the wind V with the 

 pressure P may be written 



P = C P Y 2 , 



where p is the density; but there is some uncertainty as to 

 the constancy of C. It appears that for large plates 

 C = *62, but for a plate 2 inches square Stanton found 

 C = *52. Taking the latter value*, we have 



y2 _23-7 23-7 



~~ '52 p~ -52 x -00123 ' 



on introduction of the value of p appropriate to the circum- 

 stances of the experiment. Accordingly 



V = 192 cm. /sec. 



The frequency of vibration (t _1 ) was nearly enough 256 ; 

 so that 



It-- 192 — 7 . 9 



D ~~ 256 x -095" 



In comparing this with Stroulial, we must introduce the 1 



* But I confess that I feel doubts as to the diminution of C with the 

 linear dimension. 



