FEB. 19, 1923 HUMPHREYS: THE MURMUR OF THE FOREST 53 
fore, to a close approximation, the sum of the volume energy and the 
velocity energy is constant. In symbols, 
Pit: + FmuU, = Pads + mu, 
or 
Vi(pr + § prey) = V2(p2 + 2 pats,) 
in which p is the density. Hence, per unit volume, 
p + 3pu? = aconstant. 
Now, observation shows that pu? is greatest near the crest of the 
mountain, as it is along the edge of any obstruction to a moving fluid. 
Clearly, then, this is also the place of least pressure within the tube of 
flow. Therefore, air must rush into the general current at this place 
(the assumed steady state cannot exist) and thus induce a correspond- 
ing wind up the lee side of the mountain near the crest. 
Fig. 3. Opposing winds near top of mountain 
The above explanation embodies a form of the well known Bernoulli 
Theorem, hence it belongs to that finality class that precludes quib- 
bling. Nevertheless, it is not quite so final as it seems, for viscosity 
was omitted, although, as everyone knows, contiguous portions of 
the atmosphere are so knit together by the thermal motions of their 
molecules that every blast of air more or less drags along the imme- 
diately adjacent air. This drag in turn leads to boundary turbulence 
and momentum interchange. Hence the upper portions of an under 
layer of air are caught up by any swifter wind above, and increasingly 
so with increase of velocity. In this manner also, in the case under 
consideration, the pressure in the surface air just beyond the top 
of the mountain on the lee side is decreased, and a wind up that slope 
is established and maintained. 
When the upper wind crosses the valley at mountain height, as 
often happens, and as here assumed, the return current rapidly becomes 
