Since April, 1921, observations of temperature and salinity have been made at depths from top to bottom of the sea at Station El, 20 miles to the south-west of Plymouth, where the depth is 70 metres. These observations already throw some light on the causes of different sea temperatures during the same month in different years. As these temperature differences from year to year play an all-important part in the life of cold-blooded marine animals (1), it is of particular interest not only to follow their seasonal changes, but to arrive as far as possible at the causes which govern them.

Changes in salinity and the temperature conditions during 1921 showed a marked inflow of warm highly saline Atlantic water into the English Channel, and a very marked inflow into the El area during September, October and November of that year (2). From that time until the end of 1923 there has been an irregular fall in salinity of the water at El.