32 LIFE IN THE SEA [CH. 



its rays fall obliquely upon the surface of the sea. 

 But day by day the sun rises earlier and sets later, 

 and it also rises to a greater height, until the date 

 of the summer solstice when the amount of light 

 radiated on to the surface of the sea reaches a 

 maximum. The intensity of sunlight passes through 

 the same cycle of changes on the sea as on the land, 

 and the times of maximum and minimum are the 

 same in either case ; but this is not the case with 

 the temperature change, for the minimum and 

 maximum occur a little later in the sea than on the 

 land. There is also a fortnightly period of light 

 intensity in the sea, for once a month the moon 

 passes through all her phases, and for a certain 

 number of days reflected sunlight falls on the surface 

 of the sea during the night. The amount of energy 

 contained in the moonlight is not very great, but we 

 shall see that only a very small proportion of the 

 direct rays of the sun are used in the absorption 

 of energy by plants, and it is probably the case that 

 the light of the moon is strong enough to affect 

 the rate of growth of planktonic organisms. Thus 

 in the case of the intensity of sunlight there are 

 daily, fortnightly and annual periods. 



Even if we depended on ordinary means of 

 observation, such as tasting the water, or observing 

 the height of the waterlines of ships, we should be 

 able to see that the salinity of the sea varied in 



