﻿444 Lord Rayleigh on AZolian Tones. 



appropriate value o£ VD, that is 19, into (5). Thus 



Whether judged from the experiments with water or from 

 those just detailed upon air, this (Strouhal's) number would 

 seem to be too low ; but the uncertainty in the value of C 

 above referred to precludes any very confident conclusion. It 

 is highly desirable that Strouhal's number should be further 

 checked by some method justifying complete confidence. 



When a wire or string exposed to wind does not itself 

 enter into vibration, the sound produced is uncertain and 

 difficult to estimate. No doubt the wind is often different 

 at different parts of the string, and even at the same part it 

 may fluctuate rapidly. A remedy for the first named cause 

 of unsteadiness fs to listen through a tube, whose open end 

 is brought pretty close to the obstacle. This method is 

 specially advantageous if we take advantage of our know- 

 ledge respecting the mode of action, by using a tube drawn 

 out to a narrow bore (say 1 or 2 mm.) and placed so as to 

 face the processions of vortices behind the wire. In con- 

 nexion with the fire-place arrangement the drawn out glass 

 tube is conveniently bent round through 180° and continued 

 to the ear. by a rubber prolongation. In the wake of the 

 obstacle the sound is well heard, even at some distance 

 (50 mm.) behind; but little or nothing reaches the ear when 

 the aperture is in front or at the side, even though quite 

 close up, unless the wire is itself vibrating, But the special 

 arrangement for a draught, where the observer is on the high 

 pressure side, is not necessary ; in a few minutes any one 

 may prepare a little apparatus competent to show the effect. 

 Fig. 3 almost explains itself. A is the drawn out glass tube; 



Fi£. 3. 



B the loop of iron or brass wire (say 1 mm. in diameter), 

 attached to the tube with the aid of a cork C. The rubber 

 prolongation is not shown. Held in the crack of a slightly 

 opened door or window, the arrangement yields a sound 

 which is often pure and fairly steady. 



