47 
On Hygrometry . 
similarly coated, but dry, observed. Here the apparatus is portable enough, eminent- 
ly cheap, and requiring no auxiliary substances except a little water. So far it has 
every claim to our notice as superior to every method yet proposed. But unfortu- 
nately, when we come to determine from this result, so easily and cheaply obtained, 
the actual moisture in the air, we find ourselves beset with difficulties, “ puzzled 
with mazes and perplexed with errors.” 
The process, as I said before, has been adopted by Professor Leslie By apply- 
ing it to bis differential thermometer, he has, to those not conversant with the 
subject, in some measure complicated and obscured it. In particular, he loses the 
great advantage of expressing the depression of the thermometer, in the ordinary 
scale of that instrument. There are some other objections to the use of his hygro- 
meter, which it is the less necessary to dilate on, as it is not probable that any one 
will ever prefer it to the more convenient, available, and economical arrangement 
of the two thermometers. 
Professor Leslie has given a formula for converting the depression f f the moist 
thermometer, in to an expression ofthe moisture in the air.H.s solution of the problem 
is, however, disfigured by the peculiarity of his views regarding the solubility of 
■water in air : nor indeed, does it appear very intelligible even with this allowance. 
It is further grounded on an incorrect assumption of the value which the capacity 
of air for heat hears, when compared with that of water. Taking, however, his 
meaning to be such as I understand it, and correcting the constants, I have found 
it agree tolerably well with a great majority of the experiments, with which I have 
had an opportunity of comparing it. In this respect it is indisputably superior to 
the only other solution of the problem, with which I am acquainted, — that given 
in the article Hyghometky in Brewster's Encyclopaedia. 
This remark is the more called for, as the author of the article in question pro- 
nounces with the greatest confidence against the Professor’s formula in favour of 
his own. At p. 587* comparing the result of his calculation with that given by the 
Professor, he says, “ This result would correspond by the table to 6C° nearly, being 
7° higher than the point of deposition found by Mr Leslie.” Now, it is not a little 
singular, that in this particular example the dew point must have been extremely near 
59°, the Professor’s determination. In fact, in a great majority of instances, the for- 
mula of the latter, though not universally true, will give very tolerable approxima- 
tions, while that of the former leads in almost every case to erroneous results, in 
many to impossible. The comparative excellence of the two formulas may lie easily 
appreciated by any one, who will take the trouble to calculate, according to each, 
the results of ji valuable set of experiments on this subject made by M. Gay-Lussac, 
and inserted in the 15th vol. of the Journal of the Institution. 
The errors in the article we have just noticed, are truly surprising to those who 
consider the general character of this work, and the attainments of the editor who 
conducts it. It was not, indeed, for a long time that I could satisfy myself, that 
the fault was in the author rather than in his render ; notin fact until repeated 
consideration had familiarized me with the subject, and enabled me to detect the 
mistakes and paralogisms with which it abounds. It is a wish to save others from 
that loss of time which lhave suffered, that induces me now to notice it ; and it is 
the more necessary, because the author has followed up the subject in the Edinburgh 
Journal, vols. xii and xiii, where two papers are given, iu which every error of the 
above article is repeated. Nor have I yet seen any attempt to detect these errors. 
Mr Daniel in his work simply notices the article, and supposes the method itself 
uncertain, because the solutions of the theoretical question are no two alike It will 
not then, I trust, be without its use to enter a little into particulars, and show the 
grounds on which I object to the solution given in that article. 
In § 54, p. 583, the author thus announces the principle upon which his investiga- 
tion is founded. “ Since the quantity of water evaporated in (under) the same cir- 
cumstances is proportional to F — / it follows, that the cold produced by evaporation 
must be a function of the same quantity. Therefore if D represent the reduction 
of temperature by evaporation, from the moistened bulb of a thermometer covered 
with some bibulous substance, F the entire force of vapour for the temperature of 
the air *, and /' the force of the vapour actually existing in the atmosphere ; we may 
evidently have an equation of the form (Y — f) in which m is a constant to be 
determined by experiment and afterwards modified , if necessary, by a correction de- 
pending on the diminished temperature of the evaporating surface It would be a 
fiTtun ate thing for hygrometry, if the relation between the quantities could be 
expressed by so simple an equation as this ; but that it is a much more complex rela- 
