45 
Silt, 
On Hygrometry. 
(Dt'itjinal ccommumcatione. 
I . — On Hygrometry. 
To the Editor of Gleanings in Science. 
Tlie subject of hygrometry is one of such curiosity and interest, that I hope 
you may be able to give an early insertion to the accompanying paper. Its prin- 
cipal object is to elicit discussion, by attracting the attention of observers to a de- 
partment of meteorology in which, notwithstanding the progress latterly made, 
much yet remains to be dom , before the scientific edifice can be considered com- 
plete. And if the remarks I have ventured to make on the errors which disfigure 
a work by many considered to be of authority, should serve but to teach distrust 
to those who take every thing for truth that has the Imprimatur attached, 
they will have done some service. Many valuable facts might he collected in this 
country, by those who perhaps have neither the time nor the inclination to enter 
into all the niceties of the theoretical question, or wade through the useless heap 
of algebraic notation, with which it is the fashion of many writers to perplex plain 
men, and obscure a simple subje t. Such observers are content to take the result, 
without any examination of the steps by which it was obtained ; and to them it 
must be of consequence that such result be correct. 
V I am. Sir, 
Yours obediently, 
D. 
Hygrometry has been attracting ; n Europe more attention latterly, than it had 
heretofore obtained, chiefly, it appears to me, from the happy experiments and deduc- 
tions of Mr. Dalton, who I think must be considered the father of scientific hygro- 
metry. Before his time, to the generality of philosophers this subject proved an 
impenetrable mystery. For although tbe exp riment of the Florentine Acade- 
micians was made some centuries ago, and though Le Roy had pointed out a sim- 
ple method of observing what is now called the temperature of deposition, yet no 
use seems to have been made of these valuable hints. till Mr. Dalton, by his inge- 
nious discoveries, ■‘tamped a certainty and a value oil these researches which they 
before wanted That I do not overrate what we owe to this ce’ebrated philoso- 
pher, will be evident to any one who considers the numberless unsuccessful at- 
tempts made to construct an accurate and universally comparable hygrometer, 
while as yet no standard of comparison or means of verification existed. \\ e see, 
even in following tbe course of De Saussure's labours in bis attem, ts to investigate 
the theory and value of his own invention, ! the only instrument that lias stood 
the test of subsequent examination.) wh ft difficulties belaboured under, from igno- 
rance of those valuable facts which the ingenuity and perseverance of Dalton first 
brought to light. And in particular, we cannot but admire the fact of its most 
beautiful property having been detected very recently , by one who had the advan- 
tages which De Saussure wanted*. 
Of the many hygrometers that have been proposed, depending on the expansion, 
and contraction of organic substances, if must be confessed, that this of De Saus- 
sure s alone deserves the attention of the scientific observer. For, besides tbe 
beautiful property above alluded to, it has the great merit of always speaking the 
same language; that is to say, that under the same circumstances its indications 
will be the same. It has still another merit, that o£ being extremely sensible, so 
that its indications may be used with the greatest confidence to determine the actual 
quantity of moisture in the air at any particular moment, at least within two or 
three per cent, of the truth. But notwithstanding these advimtages, there are 
still, I conceive, objections which apply to this instrument, that will militate against 
its general adoption. Those objections have a reference to the delicacy of the 
instrument, and its consequent unfitness for transport or hard usage, to say nothing 
of those which arise on the score of economy. For the ordinary observer, there 
can be no doubt that the method of dew points, as founded on Dalton’s general 
law, is preferable. Even to the possessor of this instrument, some such means 
* The relation which its indications bear to the hyperliola, as discovered by 
Gay-Lussac. From the experiments of Mr. J. Prinsep however, which form the 
subject of a paper in the 43d No. of the Journal of Science, it appears that the 
figure is a parabola. 
