[ 179 ] 



XII. On the Resistance experienced by Small Plates exposed 

 to a Stream of Fluid, By Lord Rayleigh, 031., F.B.S* 



IN a recent paper on iEolian Tones f I had occasion to 

 determine the velocity of wind from its action upon a 

 narrow strip of mirror (10*1 cm. x 1*6 cm.), the incidence 

 being normal. But there was some doubt as to the coefficient 

 to be employed in deducing the velocity from the density of 

 the air and the force per unit area. Observations both by 

 Eiffel and by Stanton had indicated that the resultant pressure 

 (force reckoned per unit area) is ]ess on small plane areas 

 than on larger ones ; and although I used provisionally a 

 diminished value of C in the equation ¥ = CpY 2 in view of 

 the narrowness of the strip, it was not without hesitation J. 

 I had in fact already commenced experiments which appeared 

 to show that no variation in C was to be detected. Sub- 

 sequently the matter was carried a little further ; and I 

 think it worth while to describe briefly the method employed. 

 In any case I could hardly hope to attain finality, which 

 would almost certainly require the aid of a proper wind 

 channel, but this is now of less consequence as 1 learn that 

 the matter is engaoino- attention at the National Physical 

 Laboratory. 



According to the principle of similitude a departure from 

 the simple law would be most apparent when the kinematic 

 viscosity is large and the stream velocity small. Tim-, if 

 the delicacy can be made adequate, the use of air resistance 

 and such low speeds as can be reached by walking through 

 a still atmosphere should be favourable. The principle of 

 the method consist- in balancing the two areas to be com- 

 pared by mounting them upon a vertical axis, situated in 

 their common plane, and capable of turning with tin 4 mini- 

 mum of friction. If the areas are equnl, their centres must 

 be at the same distance (on opposite sides) from the axis. 

 AVlum the apparatus is carried forward through the air, 

 equality of mean pressures is witnessed by the plane of the 

 obstacles assuming a position of perpendicularity to the line 

 of motion. Ii in this position the mean pressure on one 

 side is somewhat deficient, the plane on that side advances 

 against the relative stream, until a stable balance is attained 

 in an oblique position, in virtue of the displacement 

 (forwards) of the centres of pressure from the centres of 

 figure. 



* Communicated bv the Author. 



t Phil. Mag. vol. xxix. p. 442 (1915). 



X See footnote on p. 443. 

 N 2 



