228 
SECOND REPORT— 1832 . 
siderable accuracy. The action of this instrument I have ex¬ 
amined with great labour and in great detail, being convinced 
that with some ameliorations it may be made yet more valuable 
to science, and capable of general adoption, especially by geolo¬ 
gists ; for it must be admitted, that at present, without the 
strictest care nay most seriously mislead the observer. Its 
portability L ^ceeds that of any other barometric instru¬ 
ment *. 
One of the last donations of Dr. Wollaston to science was 
a Differential Barometer for measuring minute differences of 
pressure ; but, not being intended for purposes of Meteorology, 
need not here detain us. 
Considering the great attention which is required in conduct¬ 
ing continued series of barometrical observations, and the care 
that is absolutely requisite in having accurate and comparable 
results by means of really good instruments, we cannot wonder 
that in but very few places is the mean pressure of the atmo¬ 
sphere accurately known. Much less can we pronounce upon 
the question whether the general mean pressure over the globe is 
the same f. The mean height at Paris for fourteen years, being 
one of the best-determined points, is exactly 756 millimetres. 
When Meteorology shall have taken its due place among the 
sciences, and observations are assiduously carried on at several 
points in connexion in fixed observatories, we shall have some 
data for determining likewise whether the pressure remains the 
same from age to age,—a point upon which at present we are 
wholly in the dark. Professor Schon, from observations at Wurtz- 
burg, thinks that the pressure has increased during the last fifty 
years J. But such generalization is quite premature. Some curi¬ 
ous anomalies however with regard to mean pressure seem pretty 
well established, and demand accurate observation. Von Buch 
observed that the mean pressure on the shores of the Baltic 
was less than in France, and imputed the difference to what he 
calls a vallee atmosplierique . A similar fact, not less extraordi¬ 
nary, is established by the observations of M. Erman in the 
East of Siberia §. His barometrical observations would place 
Jakuzk below the level of the sea of Ockozk, yet the river 
Lena flows down from Jakuzk to the North Sea, which must be 
almost if not precisely on a level with the sea of Ockozk. It is 
well to observe that the mean temperatures of this part of Asia 
differ very abruptly at short distances, which are probably inti¬ 
mately connected with the phenomenon. M. Erman allows for 
* My papers have been published in the Edinburgh Journal of Science, 
x. 334; N.S. iv. 91. 329. 
f See Humboldt, Relation Historique, 4to edit. tom. iii. 
X Kastner, Archiv, viii. 475. § Poggendorff’s Annalen, Oct. 1829 
