MantzN.—On Anemometry, 301 
to be revolving at a speed of 20 miles, being propelled by a 60-mile breeze, 
and that, in a sudden gust, the actual velocity of the wind was increased 
to 90 miles: would the speed of the cups be augmented only by one-third 
of the added velocity,—i.c., 10 miles per hour? Clearly, if the cups 
moved at the same rate as the wind, no possible increase of wind-velocity, 
superadded to any degree of initial velocity and momentum, could drive 
them faster than the propelling power. But, as they only move at one- 
third the speed of the latter, is it not conceivable that successive augmen- 
tations of the propelling power, superadded successively to the initial 
velocity and its attendant momentum, may raise the actual speed of the 
cups at least to a rate considerably more than one-third of that of the wind ? 
26. My theory may be illustrated thus: Suppose the wind to be travelling 
at the rate of 60 miles per hour, and suddenly to fall calm, or, which would 
amount to the same thing, to be shut off suddenly from the anemometer. 
In such a case, it would be found that, notwithstanding (1) the resistance 
of the atmosphere, which at first would ‘be equal to an opposing wind of 
nearly 20 miles an hour force, (2) the resistance of friction, and (8) the 
absence of any propelling power, the cups would revolve by momentum at a 
gradually decreasing velocity for upwards of a minute, in which period they 
would have travelled perhaps half a mile without the aid of any propelling 
power but momentum. On the other hand, although, if a dead calm were 
followed suddenly by a 60 miles-an-hour gust, the cups would not start 
instantly into full velocity, being retarded by vis inertia, yet this retardation 
would be almost inappreciable. Repeated experiments proved to me that 
a well made anemometer will take considerably less than a second to attain 
its full velocity, and in faet the cups eatch the wind's force so quickly that 
I found it impossible to arrive at any definite appreciable allowance to be 
made for vis inertia. As, too, the best made instrument is that in which 
the resistance of friction is reduced to the minimum, it follows that the 
anemometer which is the quickest in attaining full velocity will be the 
slowest in parting with that velocity. Thus a 9-inch Kew standard instru- 
ment or a 4-inch Negretti and Zambra, owing to their delicate construction 
and admirable balance, wil be found—once set in motion—to retain 
that motion for a much longer period of time than would be imagined by any 
one who had not actually tried the experiment. 
27. Now let us take another case. Suppose the wind to be strong but 
squally and unsteady—blowing generally at a speed of 80 miles per hour, 
and consequently propelling the cups at the rate of ten miles per hour. 
Next suppose a sudden gust of five seconds' duration to blow at exactly 
double that strength, or 60 miles per hour, what velocity would be indicated 
by the anemometer? The theory hitherto has been that the increase in 
