Lord Rayleigh's Acoustical Observations. 161 



the form of the shell is nearly hemispherical. During the ex- 

 periments the pitch of the principal tone was about 120 vi- 

 brations per second. The vibrations were excited by a small 

 wooden hammer,, such as is used for harmonicons, the head 

 being covered with cotton-wool tied on with string. For 

 the graver tones the thickness of the cotton wool may with 

 advantage be greater than for higher tones. 



I am not in a position to decide the question as to the 

 function of the shell ; but I think it at least doubtful whether 

 it introduces any really advantageous modification into the re- 

 lations of the component tones. It is possible that its ad- 

 vantage lies rather in obstructing the flow that would other- 

 wise take place round the edge of the membrane. It must 

 be remembered that the sounds due to the various parts of 

 a vibrating membrane interfere greatly. In the case of a 

 membrane simply stretched upon a hoop, and vibrating away 

 from all obstacles, no sound at all would be heard at points in 

 the prolongation of its plane. And even when there is a shell, 

 no sound would be heard at points on the axis of symmetry, at 

 least if the symmetrical vibrations may be left out of account. 



The ^Eolian Harp. 



So far as I am aware, it has always been assumed by writers 

 who refer to this subject that the vibrations of the string are 

 in the plane parallel to the direction of the wind ; and, indeed, 

 the action of the wind in maintaining the motion is usually 

 explained as the result of friction, and as analogous to the 

 action of a violin-bow. It is more than a year since I made 

 some experiments with the view of testing a suspicion of the 

 incorrectness of this view ; and I then arrived at the conclusion 

 that the vibrations are in fact executed in the plane perpen- 

 dicular to the direction of the wind. I suppose for simplicity 

 that the length of the string is perpendicular to the direction 

 of the wind, as is usually the case in practice. Recently I 

 have repeated these experiments in an improved form, and 

 with confirmatory results. 



The best draught is that obtained from a chimney. In my 

 later experiments a fireplace was fitted with a structure of 

 wood and paper, which could prevent all access of air to the 

 chimney, except through an elongated horizontal aperture in 

 the front (vertical) wall. The length of the aperture was 

 26 inches, and the width 4 inches; and along its middle a 

 gut string was stretched over bridges. The strength of the 

 draught could be regulated by slightly withdrawing the 

 framework from the fireplace, so as to allow the passage of 

 air to the chimney otherwise than through the slit. 



