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[Philosophical Magazine, Vol. XXX. pp. 179181, 1915.]
IN a recent paper on ^Eolian Tones* I had occasion to determine the velocity of wind from its action upon a narrow strip of mirror (lO'l cm. x TO 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 less on small plane areas than on larger ones; and although I used provisionally a diminished value of 0 in the equation P = OpV2 in view of the narrowness of the strip, it was not without hesitation f. I had in fact already commenced experiments which appeared to show that no variation in G was to be detected. Subsequently 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 I learn that the matter is engaging 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. Thus, 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 consists in Balancing the two areas to be compared by mounting them upon a vertical axis, situated in their common plane, and capable of turning with the minimum of friction. If the areas are equal, their centres must be at the same distance (on opposite sides) from the axis. When 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. If in
* Phil. Mag. Vol. xxix. p. 442 (1915).   [Art. 394.] t See footnote on p. [324]. close up, unless the wire is itself vibrating. But the special arrangement for