82 Prof. Carl Barus 



on 



normally to the axis of a given wire, when the note made by 

 the whistling wind is located, at a given temperature, in pitch. 

 Other data *, such as the material, or the tension, or the length 

 of the wire, &c, are without marked effect, except as regards 

 the intensity of the sound produced. 



Strouhal further found that whenever the air-tone n, vary- 

 ing continuously with speed v, approaches the fundamental 

 or any of the overtones of the transversely vibrating wire, the 

 sound bursts forth with accentuated intensity. In virtue of 

 this discovery, Strouhal was able to give a degree of pre- 

 cision to his results which for the case of the unassisted air- 

 tone would have been unattainable ; for it was merely neces- 

 sary to work out such speeds as kept the wire in a state of 

 permanent resonance. Indeed it was now possible to obtain 

 sounds from the wire even after the actuating air-tone alone 

 had ceased to be audible. 



Commenting on the application of his results, Strouhal 

 notes its immediate bearing on anemometry. The device is 

 peculiarly adapted to the measurement of variable gusts and 

 high winds, and is thus supplementary to the ordinary 

 anemometer. 



2. It is from this point of view, i. e. in relation to what 

 may be called micro-anemometry, that the filamentary ane- 

 mometer interested me during my connexion with meteoro- 

 logical research |. Here is an instrument virtually without 

 mass, which therefore does not state the case summarily, but 

 represents the wind as it actually is. In consideration of the 

 actual complexity of aerodynamic phenomena, the simplicity 

 of Strouhal's law is an ulterior advantage. Whoever believes 

 that much is to be learned from a more searching investiga- 

 tion of the nature and origin of gusts of wind { will be in 

 sympathy with the development of what is in many respects 

 an ideal instrument of research. 



There is another important problem which lies within 

 the scope of the filar anemometer. I refer to the class of 

 researches recently accentuated by the paper of S. P. Langley§ 

 on the work of the wind. 



* The law applies more accurately in proportion as thickness increases, 

 and may be considered exact for diameters above Ol cm. 



t Cf. my letter to Prof. M. W. Harrington, in Rep. Am. Assoc, of 

 State Weather Services, Oct. 1892; Bull. U. S. Weather Bureau, No. 7, 

 pp. 44, 45. 



X Barus, American Meteor. Journal, March 1893, pp. 488-489; cf. 

 Bulletin U.S. Weather Bureau, No. 12, 1895. 



§ "The Internal Work of the Wind," Smithsonian Contributions, 

 No. 884, Washing-ton, 1893. 



