SUPPLEMENTARY REPORT ON METEOROLOGY. 89 
The maximum appears then to be about the 45th degree, and it 
diminishes on either hand. 
146. The equatorial depression, and the maximum near the 
40th degree of latitude, is indicated not only by the fived annual 
observations given in Schouw’s paper, but also by progressive 
observations made on board ship by Capt. Beechey*, Sir J. 
Herschel}, Sir E. Ryant, and Mr. MacHardy+; the latter are 
important, as showing that the same distribution prevails in the 
southern hemisphere. Mr. MacHardy’s observations in southern 
latitudes give 
Between 0° and 5°S. . . . 29°821 Eng. inches. 
SER he ou Yaya eh OROM # 
ines HO a ay Mey OED » 
pres Meme oe? 30°085 ts 
147. Prof. Poggendorff, of Berlin, has very justly remarked ft, 
that the question of the actual pressure of the air at any point of 
the earth’s surface, supposes that that pressure is measured upon 
a constant scale of force ; but owing to the variation of the force 
of gravity, the weight of a given length of the mercurial column 
is not constant, and the effect of attending to this correction is 
to exaggerate the depression at the equator, and diminish some- 
what (but not annihilate) that in the arctic regions. Such a 
corrected table, deduced from Schouw’s, will be found in the 
Comptes Rendus and in Poggendorff’s Journal. The propriety 
of the correction will be evident (as M. Poggendorff observes), 
if we recollect that the elasticity of air, and the boiling point 
of water, may be used, as well as the counterpoise of mercury, 
for indicating the atmospheric pressure. 
148. The height of the barometer, at least in temperate regions, 
varies with the season of the year. At Paris and Strasburg it 
appears to attain one maximum in summer and another in 
winter. M. Kamtz attributes the summer maximum, with great 
probability, to the pressure of vapour§: when this is allowed 
for, we have amaximum in February and a minimum in August 
or September. The prevalence of particular winds (as we shall 
see) causes temporary elevations of the barometer in particular 
parts of the earth’s surface, which may lead, and have led, to 
very erroneous conclusions. 
_ 149. There seems, however, on the whole, no reason to doubt 
the existence of such atmospheric valleys as were adverted to in 
the former report||. 
* Comptes Rendus, ii. 572. 
+ Second Report of the Meteorological Committee of the South African 
Institution, p. 2 (for which I am indebted to Sir J. Herschel). 
t Annalen der Physik, xxxvii. 468. 
§ Lehrbuch, ii. 297. || P. 228. 
