ON ANEMOMETRY. 341 
Pressure. Many such instruments have been invented, the most famous 
_ being M. Osler’s anemometer, first erected at Birmingham. 
But if the pressure (P) be received on an instrumental contrivance, which 
(its inertia being overcome) is set into a continuous motion (as the various 
sorts of windmills), the rate of this motion goes on increasing till the resist- 
ance which the motion generates balances the wind-pressure on the sails. 
This resistance consists of two parts, one caused by the displacement of the 
air in the path of revolution of the sails, and consequently proportioned to 
the square of the velocity of revolution (or to v'?); the other caused by the 
ordinary friction of machinery, which being a uniformly retarding force (db), 
destroys the power P exerted on the machine a quantity proportioned 
to the space moved over*, and consequently to v'. P then is prop. to 
av'+bv', the coefficients a and b requiring to be separately determined for 
each instrument. 
If we conceive friction to be very small, so that the second term almost 
vanishes, the velocity of revolution becomes nearly proportioned to V, the 
velocity of the wind; but if friction be very large, the velocity of revolution 
i of the machine becomes nearly proportioned to P or V%. The former is the 
case on a windmill in heavy wind-pressures ; the latter of the same machine 
when the wind-pressure is light t. 
Hence all machines of this kind have a rate of revolution proportional to 
the movement of the air, retarded by quantities which are proportioned to 
something else. The smaller we can make this retardation, the nearer to 
perfection is the instrument. Such an instrument is Whewell’s anemometer. 
Whewell’s Anemometer.— Assuming in respect of this instrument that its 
general action is like that of a windmill, we see with low velocities of wind, 
the term 6 v! is not only greater in proportion to av’? than with high veloci- 
ties, but may acquire a higher numerical value than it. 
Coulomb found with a windmill (the load being constant), the following 
proportions of wind’s velocity and revolutions of sail :— 
Wind Vel. | Revolutions. 
Vi. v. 
7:0 3:0 
125 75 
20:0 13°0 
28:0 22°0 
We may with these data compare the following calculation :— 
Wind Vel. | Revolutions. || ________ Calculation, Vv : 
Vv. v. av?+ bv = | Sum.— v®. \calculatea.| PHference- 
; 7-0 3-0 9-04 44:3 53:3 730 | +030 
12°5 75 55°3+100°6 165°9 12-88 +0°38 
20-0 13°0 169:0+191°8 360°8 18-99 —1:01 
28-0 22:0 484:04+-324°6 808°6 28°43 +0°43 
_ * This is not strictly the case with varying pressures of wind, if these act unequally on 
the bearings of the axles. 
+ Mr. Harris’s experiments with Whewell’s anemometer (Reports of the British Asso- 
ciation, 1844, p. 245) confirm this view. He had previously observed (Reports of the 
Association, 1842, p. 33) in one limited set of experiments with low velocities of wind, the 
_ space described by the pencil to be proportional to the square of the velocity of the wind. 
_ But in a larger series of trials, in strong and steady breezes, the spaces passed over by the 
pencil came nearer the simple ratio of the wind’s velocity. In strong winds the ratio be- 
tween the revolutions of the fly and the velocity of the wind is [nearly] constant. 
