REPORT ON METEOROLOGY. 
239 
by MM. Parrot and Engelhart, of the depression of the Cas¬ 
pian and Lake Aral below the Mediterranean *, and the not 
less extraordinary extension of this anomalous fact by MM. 
Humboldt, Rose, and Hoffmann, to an immense territory about 
18,000 square leagues in surface f. These conclusions, most im¬ 
portant for physical geography, might never have been attained 
but for the barometer; and at the suggestion of Baron Hum¬ 
boldt the Academy of St. Petersburg have undertaken to pro¬ 
secute the inquiry with the same instrument, to institute “baro¬ 
metrical soundings,” as they have been aptly termed, over this 
vast crater-like depression, and establish the lines of equal alti¬ 
tude. 
A most important synopsis of what has been done in Europe in 
this department will be found in the Orographie de VEurope ,—a 
collection of above 7000 heights, formed by the industry of M. 
Brugiere J to whom the scientific world is most deeply indebted, 
and whose work has been deservedly approved and rewarded by 
the Geographical Society of France. A vast proportion of the de¬ 
terminations of heights in this volume are due to the barometer. 
Humidity. 
Hygrometry, scientifically considered, has only had justice 
done to it within a very short period. Till Mr. Dalton esta¬ 
blished the true views of the connexion of temperature and the 
tension of vapour, meteorologists had vague ideas of the true 
expression of degrees of moisture. The labours of Saussure, 
though most meritorious, were destined to be superseded by a 
more elaborate analysis of the subject; indeed his views of hy¬ 
grometry were in some respects so very imperfect, that he was 
not aware of the fact, that the coolness produced by the evapo¬ 
ration of water from porous bodies, was independent of the 
rate at which the moisture was carried off by currents of air, 
—a want of knowledge which gave him much trouble. 
On the general principles of Hygrometry I have no intention 
of dwelling; I shall chiefly confine myself to a notice of the latest 
additions to the subject. It will be necessary however to premise 
one or two observations on the general state of the question. 
If the views of Mr. Dalton, noticed in an early part of this Re¬ 
port, be true, with regard to the condition in which vapour exists 
in the atmosphere,—views, which are now universally admitted, 
* The depression of the Caspian is 334 English feet below the Mediterranean, 
and it lias recently been ascertained by Captains Duhamel and Anjon that Lake 
Aral is 117 feet above the Caspian. 
f Humboldt, Fragments Asiatiques, tom. i. pp. 9. 91. 136. 
| Memoires de la Societe de Geographie, tom. iii. Paris 1830. 
