inity sometimes reaching 35.6 pro mille
(parts of salt per thousand by weight); the other consists of a
southerly current from the Irish Channel, and is colder and has a
salinity of 35.0 to 35.2 pro mille. As the water passes eastwards it
mixes with the fresher coastal water, so that the salinities generally
rise from the shore to the central line, and from east to west, though
south of Scilly Islands there is often a fall due to the influence of
the Irish Channel. The mean annual salinity decreases from between 35.4
and 35.5 pro mille in the western entrance to 35.2 pro mille at the
Strait of Dover on the central axis, and to about 34.7 pro mille under
the Isle of Wight and off the Bay of the Seine. The English Channel may
be divided into two areas by a line drawn from Start Point to Guernsey
and the Gulf of St Malo. In the eastern area the water is thoroughly
mixed owing to the action of the strong tidal currents and its
comparatively small depth, and salinities and temperatures are
therefore generally the same from surface to bottom; while westward of
this line there is often a strongly marked division into layers of
different salinity and temperature, especially in summer and autumn,
when the fresher water of the Irish Channel is found overlying the salt
water of the Bay of Biscay. The salinity of the English Channel
undergoes an annual change, being highest in winter and spring and
lowest in summer, and this change is better marked in the eastern area,
where the mean deviation from the annual mean reaches 0.3 pro mille,
than it is farther west with a mean deviation of 0.1 pro mille. There is
also reason to believe that there is a regular change with a two-year
period, years of high maximum and low minimum alternating with years of
low maximum and high minimum. Variations of long period or unperiodic
also occur, which are probably, and in one case (1905) almost certainly,
due to changes taking place some months earlier far out in the Atlantic
Ocean.

The mean annual _surface_ temperature increases from between 11 deg. C.
and 11.5 deg. C. at the Strait of Dover to over 12 deg. C. at the
western entrance.[1] The yearly range in the eastern area is
considerable, reaching 11 deg. C. off the Isle of Wight and 10 deg. C.
in the Strait of Dover; westward it gradually decreases to 5 deg. C. a
short distance north-west of Ushant. The mean maximum temperature, over
16 deg. C., is found under the English coast from Start Point