ManrEN.—On Anemometry. 308 
will be seen that the lighter the cups the smaller their comparative 
velocities at higher speeds, the lightest of all —that with very thin cups of 
tin—indicating little more than half the velocity registered by the large Kew 
instrument at the highest speed compared. It is curious that Mr. Fenwick 
Stowe did not see the conclusion to which this result of his experiments 
pointed, but it must be remembered that his object was not that of the 
present paper, but simply a comparison between large and small anemo- 
meters, with the view of ascertaining which were the more trustworthy 
instruments at different velocities, and in these tests the large Kew anemo- 
meter always was adopted as the standard of comparison, without any 
attempt to ascertain whether it might not be, under certain conditions, the 
less trustworthy of the two. The probability of an excessive amount of 
atmospherie horizontal movement being registered (by momentum) in ease 
of calms succeeding gusts is noted, but not the other aspect of the case— 
the possibility of this occurring also with sudden increases of wind force. 
91. One explanation has been suggested of the extraordinary pressure 
exerted by sudden gusts, as indicated by an Osler (pressure) anemometer, 
but not by the continuous self-registering Robinson (velocity) instrument. 
It is that such abnormal force may be exerted only in narrow columns of 
air—in gusts perpaps only a few yards, feet, or even inches in breadth, as is 
seen sometimes in the case of tropical hurricanes. That such non-unifor- 
mity in the wind pressure does exist is unquestionable. Evidence of this 
may be seen in every gale that blows across our harbour, whose waters 
frequently are “streaked,” as it were, with narrow gusts of excessive 
violence, which seem actually to tear up the water as they rush over it, yet 
are often only a few feet in breadth, or evenless. Such gusts possibly might 
strike the concave cups of the anemometer, while the convex cups, less than 
two feet distant, might be in a comparative calm. Such an occurrence 
would account for unduly high anemometrical readings, but the probability 
is rather a remote one. 
32. This non-uniformity of pressure—this **streakiness," if I may use 
the expression—of the wind it is important to consider from another point of 
view, the one only alluded to in passing in an earlier part of this paper, 
viz., the possibility of the accepted ratio between anemometric velocity and 
pressure not being so strictly accurate as is generally supposed to be the 
case. Of course were the atmosphere a solid body of uniform density, 
moving with uniform velocity, the dynamical force exerted by its impact on 
a vertical plane surface of given area always must be in proportion to that 
velocity. But this is not the case; indeed, the reality is almost the reverse 
of this supposition. The wind rushes forward in a number of irregular 
darts or tongues, ofien curling about in curves and eddies, seldom if ever 
