SUPPLEMENTARY REPORT ON METEOROLOGY. 93 
in correction or addition to those contained in my former 
report. 
158. M. Bessel has given an Essay on the Theory of Baro- 
metrical Measurements*. 
159. The important question of determining levels by obser- 
vations of distant barometers, is materially affected by the now- 
admitted difference of mean barometric pressures at different 
localities ; and it also appears that, unless these observations 
are steadily continued for considerable periods, they must be 
liable to serious errors. Thus M. Galle (the assistant astrono- 
mer at Berlin) has pointed out, in two interesting communica- 
tions}, that in certain situations enormous errors of barometric 
measurements arise at certain seasons, which he ascribes to the 
influence of local winds. It is very plain, that if, from any 
cause, the monthly variation of mean pressure of which we have 
already spoken, follow one course at one station and another at 
a second, the height deduced from any barometric formula will 
also depend upon the season. This M. Galle has found to be 
most remarkably the case between Katherinenburg and St. 
Petersburg. The calculated difference of height varies with the 
season, having a maximum in summer and minimum in winter; 
it depends, in fact, on the Difference of Temperature of the two 
stations. The height is greatest when the difference of tempe- 
rature is least ; and when the difference of temperature changes 
its usual sign, the height becomes greatly exaggerated. Thus 
we have the following analogies :— 
Diff. Temp. Katherinenburg above 
Reaum. St. Petersburg, in Toises. 
from — 2° to 0° 141 
0 ae 103 
2 4 93 
6 7 82 
The differences of temperature again depend immediately on the 
prevailing wind, and therefore on the season. Such an anomaly 
is not observed between Kasan and Katharinenburg tf. 
160. This anomaly is both important as a physical fact and in 
its consequences. The vast continental regions of Russia sustain 
aerial columns, which do not make hydrostatic equilibrium with 
one another. It affords a fresh reason for re-investigating the 
much-agitated question of the level of the Caspian Sea§, which 
may now probably be considered as set at rest by the results of 
* Schumacher’s Astr. Nachr., No. 279. Poggendorff, xxxvi. 187. 
+ Poggendorff, xlviii. 58. 379. 
{ Humboldt, Ehrenberg, and Rose’s Reise, i. 277, quoted by Galle. 
§ See an elaborate paper by Lenz on this subject, Poggendorff, xxvi. 353. 
