On Hygrometry. 
191 
mum quantities of evaporation, and thence of the extreme hygromctric states of the 
air in any interval of time. 
I shall now show how the constituent temperature of vapour or the dew point 
can be determined from the indications of the moistened-bulb thermometer. Water 
is known to evaporate at every temperature. The rapidity of evaporation, it is also 
known, depends amongst other things upon its temperature ; the dependanee being 
such that it is proportional to the force of vapour due to that temperature. These 
propositions relate to a dry atmosphere. 
If the moisture in the air ire at its constituent temperature, i e. that below which it 
will not bear being cooled without condensation, then evaporation must be altogether 
suspended; because as the temperature is still the same and the vapour is at its uaxi- 
mum tension, it cannot receive any accession. We may he certain, then, hat if 
evaporation be entirely suspended, the constituent temperature of the vapour in the 
atmosphere will be the same as the temperature of the atmosphere it elf. 
In the case then in which the atmosphere contains a maximum of aqueous vapour, 
evaporation stops altogether. In the case in which there is no aqueous vapour 
whatever in the air, evaporation is as the force of vapour belonging to the temper- 
ature of the evaporating surface. In the intermediate case, that is, when a less quan- 
titv than the maximum is contained in the atmosphere, how is evaporation affect- 
ed? In this case it is found to be proportional to the tension of vapour due to 
the temperature of the evaporating surface, minus the tension of the vapour in the air. 
But now determine the rate of evaporation ? One of the most striking pheno- 
mena of evaporation is the cold produced by it ; the consequence of the absorption 
of heat attending the conversion of water into vapour. This depression of temper- 
ature must evidently be as the evaporation ; or rather the momentary depression 
will be in proportion to the rapidity of the evaporation. The momentary depression 
is equal to the momentary increment of heat which would take place, were the 
cooling power of evaporation suspended and the moistened bulb thermometer allow- 
ed to "assume the temperature of the air. This is known by experiment to be as 
the 1,233 power of the total depression. The evaporation will then be as the 
1,233 power of the depression. Hut the evaporation is as the tension of the evapo- 
rating surface minus the tension of the vapour in the air. Then, finally, the tension 
of the evaporating surface minus the tension of the vapour actually existing, is as 
the 1,233 power of the depression. As the temperature of the evaporating surface 
is given us by experiment, we have only to compare it with the temperature of the 
air to obtain the depression. From these two data the third or unknown quantity, 
the tension of the moisture in the air, is easily found. 
Trusting that this rather lengthy detail has made the rationale of the subject a 
little clearer, I proceed to the consideration of the formula with which I terminat- 
ed my last paper. 
w' . .«/ he (F — 0 30 ... 
This was (f> n -,-+/« 77“ (A) 
S 5 -D 
There is an oversight in this formula, for which the original of my paper I fear 
may be answerable. It should have been 
/ u/\ , / w' \ — L e (Y'—f) 30 
0D( «--,)+//> J C») 
that when the experiment is properly performed it may be reduced to 0. We 
lerefore, to simplify the consideration of the subject, reject it altogether ; and 
B 
D being the depression. 
The 2d member of the left side of the equation is so small on all ordinary occa- 
sions, 
will therefore 
putting A = the constant n~ and dividing by e, we shall have 
L (F'-f) 30 
. _ B 
The value of the function (f) as given by MM. Dulong and Petit being substi- 
tuted, and the constants brought to the left side, the equation will become 
ni 233 A B 
-c-ri= F - f < D > 
O d )t- 
• The nse of the character f, on one side of the equation, to express a quantity t 
on the other, a function ; is objectionable : it was occasioned by a want of the pro- 
per character. It has been attempted to guard against the obscurity by enclosing 
it in brackets thus (f). 
