50 
On Hygrometry. 
moistened thermometer might be reduced below that of deposition, which is obvl* 
ou.sly impossible. Thus let us suppose, as in one of the experiments, air having a 
temperature of 67-2, the moistened thermometer to be 52°. and the point of depo- 
sition 35°-7, the barometer being 29-75, air of this condition being enclosed in 
a receiver and rarefied to a barometer of 14.88, the point of deposition being kept 
stationary, the moistened thermometer ought to be depressed to 37“. A very little 
more rarefaction would be sufficient to reduce it below 35° 7, and were the gauge 
lowered to 7- i 1 inches, the thermometer ought to indicate 7°2. 
The truth is, that the only general result deducible from this expe’ imcnt is, that 
the KvA co ‘ati on is inversely proportional to the density; a result in itself suffici- 
ently probable*. That this is a very different conclusion from the one adopted, will lie 
made clearer by considering, that the depression of temperature is not measured by 
the absolute evaporation, but by the proportion which the quantity evaporated bears 
to the mass to be cooled. Thus, suppose a mass of water to be cooled welching 1000 
grains, and a second, the weight of which is only 500. Let the rate ot evaporation 
in the first case, be double what it is in the second ; the depressions will still be equal. 
For if we assume the latent heat of water to he 1000°, the evaporation of one grain 
will in the first case produce a fall of 1°. But during the time i n which one grain is 
convened into vapour, in the first supposed state of things, half a grain is lost in the 
second. Now this is precisely the quantity required to he evaporated, to produce a 
depression of temperature of 1°. It is to an ignorance of, or inadvertence to, this very 
obvious principle, that most of the errors in this and the other papers of the author 
are attributable, 
But tire rate of evaporation even, as before observed, is not correctly estimated. 
Dalton had shown by his experiments, that it was very nearly as the difference of 
tensions due to the temperature of the air, and that of the dew point, unless, he adds, 
when the temperature of the evaporating surface is different from that o/ the air. Ill 
that case, the tension due to the former must be taken instead of that due to the 
latter. The author of this article however takes F f as the function which influ. 
ences it, although the difference of temperature may amount in some cases to 30°, or 
even more. It is, I think, a question whether even Dalton's theorem would lead to 
correct results, when the difference of temperature of the air and evaporatiug surface 
is great, even though the diminished tension due to the latter should be the same. 
But however this bef, any possible error which might spring f rom mir neglecting 
tins modification of the expression, is a mere nothing compared with that resulting 
from the above evident mis-statement ; which would, indeed, in some cases, repre- 
sent the ratio of evaporation as three or four times too great. 
The author of this article is one of the latest writers on the subject of hvgro- 
metry. His paper m the Edinburgh Journal, on the application of the preceding 
formula to the correction of heights measured by the barometer, is I believe the 
very last publication connected with this subject. Mr. Daniel-; book is chiefly 
“ ,n 5™^ ew a an,) givee «*» information on the 
. i . , uouuuu I, ui i ioiessor LpsIip s solution Tt is 
certainly ingenious, though as 1 before remarked, disfigured bv heiiur conveved in 
the language of a hypothecs in the support of which Mr. Leslie standi I Where 
alone in the scientific world. Another fault it has in common with aUofDieProfol 
sor s writings which I have perused, that of being drawn n,. . , e , 
and laboured, as to be very often obscure, if not unintelligible * 6 S ° involved 
His solution of the problem, as far as I understand it, is this. Air will take ut. 
A , - V 1,1 m0 ’ St '! ri! w !‘icli 1S proportional to what it requires for full saturation^ 
A thermometer with moistened hull, must then, lose moisture in , tl : ' 
and consequently will have its temperature depressed hf a™ e'mal rat^^Tbh 
solution is any thing but empirical, and yet it represents toler !blv Jmwi, P' 
in a great majority ot the experiments with which I have compared ; r 1 ^ ! 
it cannot give correct results in every case is obvious. It involves Jv** But that 
Tr " “ iu ,| ” 
be 2f" SS,,le haS s,at<;l1 that the being 1 : .66, the evaporation will 
be negjectml i'because'um temne C °" sid ? r ? tion ’ «? is ““^cation of the process mav 
sion which lias ^ a evident thafttae depres- 
■whatever its value. ’ ls alwa > s tht! same of the diminished tension 
