ON ANEMOMETRY. : 343 
__ Anemometers on the principle of Dr. Whewell’s, almost perfectly fulfil the 
intention of the inventor at high velocities, and also give results capable of 
satisfactory interpretation at all velocities down to a certain low rate of 
wind movement, but below that the instrument ceases to be sensibly affected. 
Its sensibility to light winds might be perhaps augmented by a change of 
_ construction—especially by substituting wheel-work for the screw, and the 
sails should probably be set to the angle of maximum effect*. This instru- 
ment appears applicable tu a variety of problems in which the air-movement 
enters as an element. Among the results which have been already obtained 
for meteorology by the diligent use of it, Mr. Harris’s demonstration of the 
_ path of the air over Plymouth is very conspicuous. Were such records con- 
_temporaneously kept at only three selected stations in the British Islands, for 
a few years, or even one year, and accurately discussed and compared, how 
great and how valuable would be the accessions to our knowledge of the 
winds ! 
Osler’s Anemometer.—The pressure of the wind on Osler’s anemometer is 
resisted by the equable force of a spring, and by the friction of the machinery 
_ for registration. The pressure of the wind is well-known to be subject to fre- 
_ quent and great pulsations, all of which (except the very quickest and feeblest ) 
the instrument registers. By enlarging the scale, every the minutest variation 
of the wind’s force has been traced during the flow of the minutes and seconds, 
_ so that in one minute twelve conspicuous (besides many smaller) undulations 
of the wind’s direction, and twice as many notable risings and fallings of its pres- 
_ sure have been graphically recorded by the pencils of the anemometert. 
Thus the machinery is kept in continual and sometimes quick motion, and the 
friction which it generates is considerable. The pressure of the wind then is 
balanced by two forces, one of which is proportional to itself, the other to the 
_ frequency and extent of the fluctuations of strength and direction of the wind. 
Each of these fluctuations is a function of the pressure ; they are not necessa- 
ily similar functions; but from Osler’s experiments in November and De- 
-cember 1846, they seem to be proportionate to one another. 
_ It is of importance to the theory of this instrument to obtain a correct ex- 
pression for these functions, since if they determine movements whose extent 
is simply proportioned to the pressure, the instrument, properly set to an 
adapted scale, would register exactly for all winds the continually varying 
pressure of the atmosphere down to some certain small pressure, below which 
there will be no record. But if the movements in the machinery caused by 
these pulsations and fluctuations are in extent not simply proportional to the 
-wind’s pressure, but to something else, as for instance to the wind’s velocity, 
the forces balancing the wind’s pressure would then be of two kinds, or 
P=aP'+6¥P’; a similar form of expression to that for Dr. Whewell’s 
anemometer ; and this instrument, like Dr. Whewell’s, would require a com- 
‘puted correction, and in small wind-pressures this might become important. 
_ Mr. Osler’s latest inventions have so greatly increased the sensibility of 
his apparatus to light winds, that it may be seen working freely, when 
_the wind-pressure is no more between half a pound per foot and0. This on 
tube, of the deviation of a falling body from a vertical line, or the recession of a fine spring. 
The simple velocity of wind may be measured by the transference of clouds, the shadows 
of clouds, or the movement of light bodies near the surface of the earth. 
__ * Since these remarks were written, the Association has received from the Rev. Dr. 
Robinson a notice of the construction of an anemometer with a different mechanism, which 
appears to satisfy the much-desired object of a direct registration of the wind’s velocity. 
_ ¥ Report of the Philosophical Society of Birmingham. - 
