of the Black Sea and Caspian Sea. 365 
by a barometrical survey the relative height of the Black Sea 
and the Caspian Sea, and to measure the height of the most re- 
markable points of the chain of the Caucasus. The work in 
which they have published the fruits of their researches appeared 
at Berlin in 1815. 
The relative height of the two seas was ascertained twice ; viz. 
Ist, in going from the Black Sea to the Caspian Sea; and 2dly, 
in returning from the Caspian Sea to the point of departure. Our 
travellers alpempted to make corresponding observations at the 
level of the two seas; but this mode of verification succeeded 
yery imperfectly. 
The distance levelled from the mouth of the Kuban in the 
Black Sea, to the mouth of the Terek on the shores of the Cas- 
pian Sea, pursuing the sinuosities of the post road which the 
two travellers traversed, is 990 wersts*. If a straight line, this 
distance would be merely 813 wersts, corresponding to about 
91° of difference of longitude: the two extreme points are almost: 
under the same parallel. 
All the details of this vast operation are given at full length, 
and seem to deserve great confidence. Cistern barometers were 
used; but the correction of the level was made by calculation, 
which is very easy, the imterior diameter of the tube being 
known. 
A thermometer fixed into the mounting of the instrument gave 
the temperature of the mercury: another thermometer at liberty 
made known the temperature of the air at the moment of the 
observation; the scales had been rectified according to the mea- 
surement known by astronomers under the name of fathom of 
Peru; a stand made for the purpose with a leaden wire ad- 
mitted of the tubes being placed in a vertical position; an 
. anemometer gave the direction and strength of the wind, and 
consequently the measurement of the degree of confidence which 
every partial levelling seemed to deserve. The indications of 
the barometer are always expressed in 100dth parts of a line, 
and great care was taken to compare the instruments, before, 
after, and during the operation: in order to avoid all errors 
which might arise from derangements of this kind, the observers 
met every two days. They remained long enough at every sta- 
tion to take four distinct barometrical heights, and at intervals 
of 15’ at least: the epochs of these observations always corre- 
sponded perfectly. Every hour of the day is not equally favour- 
able in this kind of measurement; but it did not depend on the 
travellers to select the hour of twelve at noon, which is generally 
tliat which answers best. Their observations, however, were 
® 404°3 wersts make 1° of the meridian, 
always 
