370 Relative Heights of the Levels of the Black and Caspian Seas. 
rometers for a geological level, thinks that in order completely 
to do away the doubts which the question may still present re- 
specting the relative height of the two oceans, we must have 
recourse to instruments mere precise, and exposed to shorter 
journeys by land; while we keep an account at the same time 
of the unequal heights of the tides, of the different hours of the 
observations in the harbours on the opposite shores of America, 
and of the horary variations of the barometer, which, although 
very regular as to the hours at which they happen, are not as 
completely so as has been supposed as to the quantities which 
measure them. However this may be, the observations which 
we have given already prove, that if there does exist a difference 
of level between the Ocean and the Great Pacific, it must be 
very small. 
The little velocity and constancy which mariners have re- 
marked in the currents of the Straits of Gibraltar, show that in 
these places the Mediterranean and the Ocean have nearly the 
same level. It may nevertheless appear curious to compare un- 
der this point of view two very distant points, since, contrary 
to all idea, the levelling of the isthmus of Suez, the results of 
which have been given above, has proved that two seas which 
communicate with each other may, nevertheless, have very dif- 
ferent levels. Now the measurement of the meridian of France 
presents an uninterrupted chain of triangles, which extend from 
Dunkirk to Barcelona; the relative elevations of the various 
summits may be deduced from the reciprocal observations of 
distances from the zenith; the absolute height of one single 
station will therefore serve to find the absolute height of all the 
rest; and this will be the case, whether we set out from the 
Mediterranean to approach the Ocean, or follow a contrary di- 
rection. 
It is according to this method that M. Delambre has calcu- 
lated the elevation of Rhodes over the Mediterranean and the 
Ocean, setting out in the first place from Mont Juy to Barce- 
lona, of which Stephani had directly found the height; and 
secondly, by the help of a signal at Dunkirk which was only 
66 metres from the level of. low water: those two determinations 
agreeing within a fraction of a metre, we may conclude, if not 
that the level of the Ocean at Dunkirk is exactly the same with 
the level of the Mediterranean at Barcelona, at least that the 
inequality of height, if it exists, ought to be insensible, 
LXXIV. On 
