58 JOURNAL OF THE WASHINGTON ACADEMY OF SCIENCES VOL. 13, NO. 4 
“Wind of the cross! rushing and mighty! 
Heavy the blow of thy wings sweeping past! 
Wild wailing wind of misfortune and sorrow, 
Wizards of Finland ride by on the blast.” 
—Popular Esthonian song. 
The explanation of these curious sounds evidently goes back, not 
to wizards and witches as might formerly have been supposed, but 
to the action on the wind by a single twig or branch, for the differ- 
ences between the whispering of the tree, the murmuring of the 
forest, and the roaring of the mountain, are essentially differences in 
degree and not of kind. 
Now, it has long been known from the work of Strouhal? that wind 
normal to a eylinder, such as a stretched wire, produces aeolian tones 
even when the cylinder itself takes no part in the vibration; that 
the pitch of the note thus produced, independent alike of the material, 
length, and tension of the wire, varies directly as the speed, u, of the 
wind and inversely as the diameter, d, of the obstructing rod; and 
that the number, n, of such vibrations per second is given, approxi- 
mately, by the equation 
R= Wisp t/a 
the units being the centimeter and second. 
An excellent example of such sounds is the familiar singing or hum- 
ming of telegraph and telephone wires. 
Whenever the tone produced as above described coincides with 
one of the proper tones (fundamental or a harmonic) of the wire, 
the wire itself, if suitably supported, then vigorously vibrates, normal 
to the direction of the wind, and thereby increases the loudness and 
produces other interesting results. These, however, will not be 
considered further since the twigs and branches of trees, whose aeolian 
effects alone are here under discussion, have no free periods of impor- 
tance in this connection. : 
The sounds in question, that is, the tree and forest sounds, therefore, 
are not due to the elasticity of the twigs and branches, but, as in the 
case of the singing telegraph wires, to the instability of the 
vortex sheets their obstruction introduces into the air as it rushes by 
them. This obvious and, indeed, unavoidable deduction from 
Strouhal’s experiments, just referred to, has been abundantly con- 
firmed by cinema photographs of water eddies, due to flow past a 
cylinder. Vortex whirls develop in the flowing water at regular 
2 Ann. d. Phys. 5: 216. 1878; see also Lord Rayleigh, Phil. Mag. 29: 433. 1915. 
