ON ANEMOMETRY. 347 
be made the basis of wind-registration, there will be no mechanism required 
_ for recording the measures of air-movement or pressure, and the scale of re- 
_ sults will increase in accuracy as the wind-force grows less, and may be in this 
_ direction more and more trusted as we approach to zero; a circumstance which 
would confer upon this mode of anemometry a peculiar value, and render it 
_ almost a necessary complement to the mechanical processes now in use, for 
_ these take but little notice of very light winds, which yet it is of much im- 
portance to record. 
One of these joint effects of the molecular constitution of the air, com- 
bined with its rate of movement, is seen in the rapidity with which objects 
exposed to wind acquire the temperature of the atmosphere. 
If a thermometer whose temperature differs from that of the atmosphere 
_ by m degrees be exposed in the open air, it instantly begins to undergo 
change of temperature, and loses or gains heat continually until it has be- 
come sensibly of the same temperature as the surrounding medium. This 
_ effect, a mixed result of radiation and conduction of heat, is in the open air 
very nearly in simple proportion to m; but in closed vessels, where conduc- 
tion is impeded and radiation influential, the changing temperature of the 
surrounding bodies complicates the experiment, and the wet thermometer 
- does not in this case lose or gain heat in the same simple proportion to m. 
Exposure of the thermometer to a current of wind accelerates the process 
by which its temperature is made sensibly equal to that of the surrounding 
medium, and thus by careful experiments the momentary velocity of the 
current may be estimated, as Sir John Leslie has proposed*. 
_ If we maintain a continual moisture on the bulb of the thermometer, a 
new element of change of temperature is introduced, the force of vaporiza- 
_ tion, the effect of which is finally to reduce the thermometer to the tempera- 
_ ture of evaporation, where it remains. 
_ If we commence the experiment with the temperature of the wet bulb 
raised by the quantity m above that of the surrounding atmosphere (¢), it will 
_ sink under the operation of two forces, the cooling power of air, proportioned 
to m, and the force of vaporization. (The movement of the air is not now 
considered.) By the swm of these forces it approximates to ¢, at which point 
_m being =0, the cooling power of the air ceases. By the force of vaporiza- 
tion it is depressed below this point to ¢’, but between ¢ and @' the air exerts 
a heating power proportioned to m. The rate of cooling in air of a wet-bulb 
thermometer is thus found to be complicated with two quite distinct functions, 
at every point but one, viz. at the temperature ¢ of the atmosphere; at this 
‘point it depends solely on the force of vaporization, modified by the move- 
‘ment of the air. If then we perform a series of experiments on the rate of 
cooling of a wet-bulb thermometer exposed in the open air, from ¢+4° to 
t—+°, under the influence of winds of very unequal velocity, and under the 
influences of very unequal degrees of dampness in the air, we shall be able 
to distinguish the effects of these influences, and assign to each its proper 
functional expression, But if one of these can be determined theoretically, 
fewer experiments will be required. It appears that the cooling influence of 
evaporation at different temperatures and in different hygrometrical states of 
the atmosphere can be thus determined. 
The rate of evaporation of water in the atmosphere depends upon the force 
of aqueous vapour at that temperature, diminished by the force of the aqueous 
vapour actually present in the atmosphere. Thus if f’ represent the force of 
vapour at the temperature of evaporation, and f" the force of vapour at the 
temperature of the dew-point, f’—/" is the unbalanced and active force 
* Professor Forbes in Reports of the British Association for 1832. 
2a2 
