opinion that, so far as
observations have gone, there are no exact proofs of any such difference
of level. It was inferred from the measurements of M. Lepare, that the
level of the Mediterranean, near Alexandria, was lower by 26 feet 6
inches, than the Red Sea near Suez at low water, and about 30 feet lower
than the Red Sea at the same place at high water,[386] but Mr. Robert
Stevenson affirms, as the result of a more recent survey, that there is
no difference of level between the two seas.[387]

It was formerly imagined that there was an equal, if not greater,
diversity in the relative levels of the Atlantic and Pacific, on the
opposite sides of the Isthmus of Panama. But the levellings carried
across that isthmus by Capt. Lloyd, in 1828, to ascertain the relative
height of the Pacific Ocean at Panama, and of the Atlantic at the mouth
of the river Chagres, have shown, that the difference of mean level
between those oceans is not considerable, and, contrary to expectation,
the difference which does exist is in favor of the greater height of the
Pacific. According to this survey, the mean height of the Pacific is
three feet and a half, or 3.52 above the Atlantic, if we assume the mean
level of a sea to coincide with the mean between the extremes of the
elevation and depression of the tides; for between the extreme levels of
the greatest tides in the Pacific, at Panama, there is a difference of
27.44 feet; and at the usual spring tides 21.22 feet; whereas at Chagres
this difference is only 1.16 feet, and is the same at all seasons of the
year.

The tides, in short, in the Caribbean Sea are scarcely perceptible, not
equalling those in some parts of the Mediterranean, whereas the rise is
very high in the Bay of Panama; so that the Pacific is at high tide
lifted up several feet above the surface of the Gulf of Mexico, and then
at low water let down as far below it.[388] But astronomers are agreed
that, on mathematical principles, the rise of the tidal wave above the
mean level of a particular sea must be greater than the fall below it;
and although the difference has been hitherto supposed insufficient to
cause an appreciable error, it is, nevertheless, worthy of observation,
that the error, such as it may be, would tend to reduce the small
difference, now inferred, from the observations of Mr. Lloyd, to exist
between the levels of the two oceans.

There is still another way in which heat and cold must occasion great
move