150 RHYTHM 



new moon; for about half-way between these two phases the 

 tide rises more slowly and to a lower height. And again, just 

 as there is a half-daily and a half-monthly rhythm, so we 

 have a half-yearly rhythm in the vernal and autumn equi- 

 noxes. So regular are these rhythms that the tide is calculated 

 years in advance for all parts of the world, and navigators 

 rely trustfully on these calculations, which are not found 

 wanting. 



Then, again, we have a rhythmical change of temperature, 

 which is fairly constant for given places in the sea. About 

 February and March the sea in the northern hemisphere is 

 at its coldest, but it gradually warms up until in August it 

 attains its highest normal temperature. Of course, in all these 

 rhythms there are many disturbing features, such as the 

 weather. But these can fairly easily be discounted. Just as 

 we have the annual rise and fall of temperature, so do we 

 have a daily one, the temperature being at its lowest about 

 sunrise and gradually rising till about the middle of the 

 afternoon. And again, there is a fortnightly rhythm, inasmuch 

 as near the land the sea is warmer in the summer just after 

 the time of new or full moon, and colder at the same periods 

 during the winter. 



Other rhythms might be pointed out, such as those de- 

 pendent on the intensity of sun-light, and on the degree of 

 salinity, which in turn depends to a very large extent on the 

 water circulation of the sea. The pulsing-up of the Gulf Stream 

 is the direct result of this circulation and affects not only the 

 warmth but the salinity of the waters on our western shores. 

 " The water is salt est when the drift is strongest, in the months 

 of February to June, and is less salt when the drift is weakest, 

 in the months of November to February." All these features 

 have a profound influence on the life of both plants and 

 animals in the ocean. 



There is no part of the sea which is free from animal life; 

 but it wells up and dies down periodically at intervals during 

 the twelve months. On the surface of the sea there are 

 numerous animals which are found there and only there, such 

 as the jellyfish, numerous small Crustacea, floating molluscs, 

 worms and others. But many marine animals pass only part 



