n] RHYTHMICAL CHANGE IN THE SEA 31 



year a wave of temperature change sweeps across 

 the sea. Sometime in our latitudes about February 

 or March the sea is coldest and then, day by day, 

 the temperature rises until it attains a maximum 

 in August, and then it falls until the time of the 

 next minimum. Just as surely as the earth revolves 

 round the sun does the sea experience this annual 

 temperature change ; but if we study the variation 

 day by day with a delicate thermometer we find 

 that the rhythm is not a simple one. There are, as 

 in the case of the tides, irregularities which are due 

 to storms and other unpredictable occurrences, but 

 there is also a daily rhythm. Just about sunrise the 

 sea is coldest and then it gradually rises in tempera- 

 ture until about 4 p.m., the time of the daily 

 maximum. Also there are rises and falls of tem- 

 perature synchronous with the tidal periods : a few 

 days after the time of full moon, or new moon, the 

 sea near the land is warmer during the summer than 

 it would be if there were no tidal streams ; and 

 during the winter it is a little colder. Thus there 

 is a daily temperature period superposed upon a 

 fortnightly one ; and this again is superposed upon 

 an annual period. 



There are corresponding variations in the intensity 

 of the sunlight that falls upon the surface of the sea. 

 At about the end of the winter solstice this is at 

 a minimum, for the sun's height is then lowest and 



