ON THE ATTENUATION OF SOUND IN THE ATMOSPHERE
from over the sea for at least 12 seconds after the syren had ceased sou The sky was clear and there were no waves to speak of. Reflection narrower sense (which does not include so called total reflection!) n irregularities in the medium whose outlines are somewhat sharply d the linear standard being the wave-length of the vibration; but this r< ment is probably satisfied by ascending streams of heated air.
In considering the effect of eddies on maintained sounds of given Major Taylor does not include either dissipation (in the narrower sei reflection. I do not understand how, under such conditions, there can ' general attenuation of plane waves. What is lost in one position in f the phase-disturbing obstacles, must be gained at another. The circums are perhaps more familiar in Optics. Consider the passage of light of wave-length through a grating devoid of absorbing and reflecting power whole of the incident light is then to be found distributed between the c image and the lateral spectra. At a sufficient distance behind the g supposed to be of limited width, the spectra are separated, and as I • stand it the calculation refers to what would be found in the beam gc form the central image. But close behind the grating, or at any di behind if the width be unlimited, there is no separation, and the a intensity is the same as before incidence. The latter appears to be tt with which we are now concerned. The problem of the grating is trea Theory of Sound, 2nd edition, § 272 a.
Of course, the more important anomalies, such as the usual fail sound up wind, are to be explained after Stokes and Reynolds by a refr which is approximately regular.
In connexion with eddies it may be worth while to mention the i case afforded by a vortex in two dimensions whose axis is parallel to the of the sound waves. The circumferential velocity at any point is propo: to 1/r, where ?- is the distance from the axis. By integration, or more diately by considering what Kelvin called the "circulation," it is c prove that the whole of the wave which passes on one side of the i uniformly advanced by a certain amount and the whole on the othc retarded by an. equal amount. A. fault is thus introduced into the oth plane character of the wave.
[1918. Major Taylor sends me the following observations:
NOTE ON THE DISPERSION OF SOUND.
Observations have shown that sound is apparently dissipated at a greater rate than the inverse square law both up and down wind. The of turbulence on a plane wave front is to cause it to deviate locally fr the velocity of sound; but here the motion is not limited to one dimension, as is shown by the swellings between the disks. Indeed the propagation of any wave at all is inconsistent with uniformity of pressure within the jet.l as of the particles of fluid with each other, is more directly shewn by an experiment on the continuance of a column of mercury, in the tube of a barometer, at a height considerably greater than that at which it usually stands, on account oi the pressure of the atmosphere. If the mercury has been well boiled in the tube, it may be made to remain in contact with the closed end, at the height of 70 inches or more " (Young's Lectures, p. 626,1807). If the mercury be wet, boiling may be dispensed with and negative pressures of two atmospheres are easily demonstrated.