49 
On Hygrometry. 
tailed, and in these even the numerical operations are full of errors ; errors too 
which it is evident, are not attributable to the press. But in fact, the labour of these 
experiments might have been spared, and the inapplicability of the formula clearly 
established, by considering the results to which, in particular cases, it will lead. Let 
us suppose, for instance, two cases, in one of whic » the temperature of the air is 100° 
and that of the dew point 91°»V : in the other 04° and 110° In each case F — -/—• 4in. 
of mercury nearly, and therefore, according to the principle on which this formula is 
founded, B should be equalto B' ; whereas their ratio is that of 1 to 4 nearly, as 
anyone may satisfy himself by experiment; nor will t e arbitrary change which 
the original expression is made to undergo, help the argument. In this case ( F — /) 
D D' 
being constant B : IF : : p — — ; p-— — Now p and r being constants, if we sup* 
r r D' B 
pose B 7 to be greater than D, then will p~— — be less than p whereas by the 
r r 
hyp^hesis it is greater, which is absurd. It is truly surprising, how such an obvious 
conclusion could have escaped any one who had paid common attention to the 
subject. 
Tne hypothetical case I have adduced, though it places the error of the formula 
in so clear a light to those who are at all familiar with the subject, may yet for want 
of the experiments alluded to, fail of carrying conviction to those who have not the 
means of performing them. It may not, therefore, be without its use to adduce a 
practical instance of the curious results an observer may be led to, by the employ- 
ment of this formula. 
The full expression in which a correction for the barometer is included, is 
h B » 
/'=F 
180—4 B 
To apply this: in April I observed the temperature of the air^to be 03 5 and 
that of a moist bulb thermometer 02° ; the barometer being in. 27- /• Substituting 
these numbers in the above formula, we get 83°. 4 as the point of deposition. 
This result is l a .4 above the temperature of the moist bulb thermometer, and is 
too absurd to require u single remark. 
But I have not yet done with the mistakes of this article. In the preceding 
formula, the depression is assumed to be inversely as the barometer, other things 
being alike This conclusion, it is said, is derived from an experiment performed 
with an air pump, the particulars of which are fortunately given. Dry air being 
enclosed in the receiver, and kept in that condition by means ol concentrated sul- 
phuric acid, a Leslie's hygrometer was exposed to its action. The gauge, which 
at the commencement of the experiment indicated the ordinary density of 29. 6, 
was successively depressed to 23. 6, 17- 6,11* 0, and 5. 6 inches, and the corresponding 
indications of the hygrometer were found to be 27, 34, 44, 6*2, 91. These numbers 
are pronounced to be inversely as the first set. The first three certainly are, nor 
is the fourth very far removed ; but the fifth obviously bids defiance to any such 
law. Supposing tie indications of the hygrometer to bear this relation to the den- 
sities, they would he 27, 34, 45, 69, and 143 ; the last number differing by 52 
degrees from the observed result. 
It is, however, not improbable that in the progress of the experiment, the mois- 
tened cover of the bulb had become nearly dry, and this supposition, as the great 
discrepancy occurs only in the last experiment, might furnish a sufficient answer 
to the objection. It might, even be said, (and with jus ice,) that, the hypothesis 
which will bring out true results for an atmosphere so rarefied as only to support 
a column of 11. 6 inches of mercury, possesses all the accuracy which is sought 
for, or required in a practical form til a. Unfortunately, however, the law is not ac- 
cordant with facts, even in this limited application, accepting in a case scarcely 
ever likely to happen in nature, that in which the experiment was performed, — the 
case of perfectly dry air. In extending it to air of every possible variety of hygrome- 
trical condition the author was obviously blind to the very erroneous conclusions 
to which his too hasty generalisation will in most cases lead. In fact, as the de- 
gree to which air can be rarefied may be considered to have no limit, except that 
of a vacuum, so the depression might become, were this the true law, infinite ; a 
result which though, perhaps, involving no absurdity in the case of dry air, can never 
be applicable to an atmosphere holding any vapour in suspension. In air of this con- 
dition there must be a limit to the depression; otherwise, the temperature of the 
