PHYSIOGRAPHY OF THE SEA-SHORE 23 



mass involved, and it is this surface water that comes in 

 contact with the tidal zone during the rise and fall of the 

 tide. The effect of such changes in salinity upon animal 

 distribution may be important, as we shall have occasion to 

 see later on. The average percentage of salinity for the 

 open sea varies from 3-3 to 4-1 ; the latter in the Red Sea. 

 The Atlantic average is about 3*5. 



Density as recorded by the hydrometer shows seasonal 

 variation largely in accordance v/ith prevailing winds. It 



SALINITY 



f^ainfa// - Inches 



jAfv r£B. nAR Apr. /^ay June July Auq. 3ept ocr /vov. sec. 



— — = Salinily at Surfacti. — — = 5alinity 3/" 1 Fditnom ■■ 'Rainfall 



Fig. 2. — ^The relation betvv'een salinity and rainfall in Ballynakill 

 Harbour (Southern, 1915)- 



varies also with the state of tide. In November, 1913, in the 

 Dovey estuary in West Wales one of us found that the 

 density varied between 1-0265 (temperature ir8 C.) with a 

 high wind, and 1-0048 (temperature 9-1 C.) at low water. 



The relation between salinity and rainfall is well shown 

 in the above graph (Fig. 2) compiled by Southern (1915) 

 from data obtained in Ballynakill Harbour, Ireland, in which 

 " the salinity curve responds with fidelity to that of rain- 

 fall." Discussing these results Southern points out that 



