FEB. 19, 1923 HUMPHREYS: THE MURMUR OF THE FOREST 59 
intervals, alternately on the one side.and then on the other, of the 
interfering cylinder, while the eddy mass vibrates from side to side 
in the same period. 
The complete mathematical analysis of these and similar vortices, 
giving the deduction of Strouhal’s rule, and many others, would be 
both interesting and valuable, but it appears that this important 
problem has not yet been fully solved. Clearly, though, if all twigs 
and branches had streamline shapes—shapes over and along which 
a current of air will flow without vortex agitation—and were properly 
oriented to the wind there would never be a whisper from a tree nor 
1B: 
Fig. 5. Phase circle 
murmur from a forest. But they are not so shaped; trees do have 
voices, and voices that are even characteristic of the species. The 
muffled plaint of the oak at the wintry blast, for instance, has but 
little in common with the sibilant sigh of the pine. And the reason 
is obvious. The twigs and branches of the one, because relatively 
large and of many sizes, produce a multitude of low tones, while the 
innumerable fine needles of the other give a smaller range of high- 
pitched notes. 
It remains now to determine how a multitude of sounds, such as 
twig-produced aeolian notes, blend together—how the pitch and 
loudness of the resultant are related to the like properties of the 
elementary constituents. 
