TIDES, WAVES 103 



It will be seen that sea-water at 77° F. is 

 only half as viscous as the same water at 32° F., 

 that is to say, the same body would sink twice 

 as rapidly in water at a temperature of 77° 

 as in water at 32°. Sea- water at a high tem- 

 perature may be said to be relatively " thin " 

 as compared with the same water at a low 

 temperature. The bearing of this on the 

 development of suspension-organs by floating 

 organisms in cold and warm waters will be 

 referred to later. 



Penetration of Light. — When the sun's rays 

 strike the surface of the sea, some of them are 

 reflected and the others penetrate into the 

 water, being gradually absorbed at different 

 depths according to the wave-length of the 

 ray and the clearness of the water. The dark 

 heat rays are absorbed most quickly — in the 

 uppermost layers — whilst the light rays pene- 

 trate deeper, the blue rays penetrating the 

 deepest of all. 



Many observations have been made to deter- 

 mine the intensity of light at different depths 

 by means of photographic plates. In the 

 Mediterranean the effect of light has been 

 traced down to depths of 260 fathoms off the 

 Riviera and 300 fathoms off Capri. The latest 

 observations, made during bright sunshine 

 in June, 1910, in the Sargasso Sea by Helland- 

 Hansen during the " Michael Sars " Expedi- 



