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324                 '                               AEOLIAN  TONES                                                   C394
The formula connecting the velocity of the wind V with the pressure P may be written
where p is the density; but there is some uncertainty as to the constancy of G. It appears that for large plates 0 = "62, but for a plate 2 inches square Stanton found G = '52. Taking the latter value*, we have
237 __        23-7 = 7527 ~ -52 x -00123'
on introduction of the value of p appropriate to the circumstances of the
experiment.    Accordingly
V= 192 cm./sec.
The frequency of vibration (T-I) was nearly enough 256 ; so that
VT         1)2
VT        1J^      =7-9
D     256 x -095
In comparing this with Strouhal, we must introduce the appropriate value of YD, that is 19, into (5). Thus
_F _?T NJD ~~ D ~
Whether judged from the experiments with water or from those just detailed upon air, this (Strouhal's) number would seem to be too low ; but the uncertainty in the value of G above referred to precludes any very confident conclusion. It is highly desirable that Strouhal's number should be further checked by some method justifying complete confidence.
When a wire or string exposed to wind does not itself enter into vibration, the sound produced is uncertain and difficult to estimate. No doubt the wind is often different at different parts of the string, and even at the same part it may fluctuate rapidly. A remedy for the first named cause of unsteadiness is to listen through a tube, whose open end is brought pretty close to the obstacle. This method is specially advantageous if we take advantage of our knowledge respecting the mode of action, by using a tube drawn out to a narrow bore (say 1 or 2mm.) and placed so as to face the processions of vortices behind the wire. In connexion with the fire-place arrangement the drawn out glass tube is conveniently bent round through 180 and continued to the ear by a rubber prolongation. In the wake of the obstacle the sound is well heard, even at some distance (50 mm.) behind; but little or nothing reaches the ear when the aperture is in front or at the side, even though quite close up, unless the wire is itself vibrating. But the special arrangement for
* But I confess that I feel doubts as to the diminution of G with the linear dimension.. [1917. See next paper.] ^sition, determined in the usual way by reflexion. When the wind operates the mirror is carried with it, but is brought back to the sighted position by use of a rider of mass equal to '485 gm.