36 LIFE IN THE SEA [OH. 



water proceeding to the north-east from the Gulf 

 Stream eddy in the subtropical Atlantic. When the 

 eddy has expanded farthest to the north the drift 

 is strongest, and when it has retracted furthest to 

 the south the drift is weakest. Now the water which 

 comes up to the north-east, to the British Islands and 

 coasts of Scandinavia, is always warmer and salter than 

 the water in the North Atlantic would be normally 

 if there were no Gulf Stream drift. Thus there is 

 an annual change of salinity in our seas which depends 

 mainly upon the strength of the Gulf Stream flow. 

 The water is saltest when the drift is strongest, in the 

 months of February to June, and it is less salt when 

 the drift is weakest, in the months of November to 

 February. The further north we go the later in the 

 year does the Gulf Stream drift culminate. The 

 amount of change that can be detected in the salt- 

 ness of the sea and which is due to these causes is 

 not much the limits are about 34'5 to 35'5 per 

 thousand. Salinity is defined in this way as the 

 total amount of salts weighed in grams and present 

 in 1000 cubic centimetres of sea- water at a tempera- 

 ture of 4 C. 



Thus there is an annual period in the change of 

 salinity of the sea and superposed on this are smaller 

 periods which are due to various causes. But there 

 are also longer periods than the annual one. There 

 is a two-yearly period, for the sea is salter in the 



