n] RHYTHMICAL CHANGE IN THE SEA 33 



different parts of the ocean of the earth : it is saltest 

 in the Red Sea and freshest in some parts of the 

 Baltic. But if we used delicate hydrometers, or still 

 better chemical methods of estimating the proportion 

 of salts, we should find that the salinity varies at 

 the same place throughout the year. Now there 

 are many irregularities in the changes of salinity 

 which we cannot predict ; heavy rainfalls on the 

 surface of the sea, or floods in the rivers opening 

 into it, will produce local changes of salinity by 

 diluting the sea-water. Nevertheless there are 

 regularities as well. In some places we can detect 

 a fortnightly variation which is due to the tides, 

 where these run to and from the land. Thus, opposite 

 the mouth of an estuary the sea will be, on the 

 average, fresher some time shortly after the end of 

 the ebb tide, for the water coming down from 

 the river will have been slightly fresher than that 

 which would have been present in the sea had there 

 been no tidal streams, and this effect will be stronger 

 at the period of spring tides, when the streams run 

 with greater force. 



There are also annual changes in the salinity, and 

 to understand these we must consider the general 

 scheme of oceanic circulation. The intense solar 

 radiation in the tropics heats the surface of the 

 sea, and the water becomes warmer and expands 

 so that it stands at a higher level. It therefore flows 



j. 3 



