394 AEOLIAN TONES. [Philosophical Magazine, Vol. xxix. pp. 433—444, 195, 1915.] IN what has long been known as the /Eolian Harp, a stretched string, such as a pianoforte wire or a violin string, is caused to vibrate in one of its •possible modes by the impact of wind; and it was usually supposed that the action was analogous to that of a violin bow, so that the vibrations were executed in the plane containing the direction of the wind. A closer examination showed, however, that this opinion was erroneous and that in fact the vibrations are transverse to the wind*. It is not essential to the production, of sound that the string should take part in the vibration, and the general phenomenon, exemplified in the whistling of wind among trees, has been investigated by Strouhalf under the name of Reibungstone. In Strouhal's experiments a vertical wire or rod attached to a suitable frame was caused to revolve with uniform velocity about a parallel axis. The pitch of the seolian tone generated by the relative motion of the wire and of the air was found to be independent of the length and of the tension of the • wire, but to vary with the diameter (D) and with the speed (F) of the motion. Within certain limits the relation between the frequency of vibration (N) and these data was expressible by the centimetre and the second being units. When the speed is such that the seolian tone coincides with one of the proper tones of the wire, supported so as to be capable of free independent vibration, the sound is greatly reinforced, and with this advantage Strouhal found it possible to extend the range of his observations. Under the more extreme conditions .then practicable the observed pitch deviated considerably * Phil. Mag. Vol. via. p. 149 (1879); Scientific Papers, Vol. i. p. 413. t Wied. Ann. Vol. v. p. 216 (1878). ' J In (1) F is the velocity of the wire relatively ,to,the walls of the laboratory.it serves equally for the equation in a'.os if + £ sin t', a' = A' cos if. + J3' sin t', with &' = -aa', &=£(a'2-a2), c' = 0, c = 0.