bably as a result of the prevailing westerly winds; this mean
difference is exceeded with strong westerly winds, and disappears or is
reversed with easterly winds. The waves of the Baltic are usually short and
irregular, often dangerous to navigation. Destructive waves, probably
caused by distant earthquakes, called _Seebaren_ (cf. English "bores") have
been recorded.

The range of the tides is about one foot at Copenhagen; within the Baltic
proper ordinary tides are scarcely perceptible. There is, however, a
distinctly marked annual rise and fall due to meteorological influences
having a mean range of about 11.4 cm. (0.37 ft.), at Travemuende, and 13.9
cm. (0.46 ft.) at Swinemuende, the maximum occurring at the end of the
summer rainy period in August.

[Sidenote: Circulation.]

The circulation of water in the Baltic proper must be considered apart from
the circulation in the channels connecting it with the North Sea; and in
this relation the plateau connecting the islands Falster and Moen with the
coast of Mecklenburg and Ruegen must be taken as the dividing line. In the
great basins and hollows from Ruegen to the Gulfs of Bothnia and Finland
the upper layers of water, from 30 to 70 metres (16 to 38 fathoms) in
thickness, have almost the same salinity throughout. In these waters a
vertical circulation is kept up by convection currents. Beneath these
layers are masses of salter water, through which a thermal wave of small
amplitude is slowly propagated to the bottom by conduction. These strata
are practically stagnant, deficient in oxygen and surcharged with carbonic
acid. Their salter waters must have been originally derived from outside,
and must therefore have passed over the plateau between Falster and
Mecklenburg, but their horizontal extension is checked by the ridges
separating the deep hollows in the Baltic from each other. The inflow to
the deep basins is intermittent, probably with a long period of flux and
reflux.

The circulation in the channels connecting the Baltic proper with the North
Sea is of a complex character. It is necessary in the first place to
distinguish clearly between outflowing and inflowing waters; in practice
this is easily done, as the outflowing water always contains less than 30
parts _pro mille_ of salt, and the inflowing water more than 32 _pro
mille_. The Baltic receives much more water by rainfall, discharge of
rivers, &c., than it loses by evaporation; hence a surplus must be g