104 THE OCEAN 



tion, showed that at 550 fathoms there was 

 still sufficient light to affect a sensitive plate 

 after exposure for 80 minutes. Another plate 

 was exposed at a depth of about 900 fathoms 

 for two hours, but showed no effect whatever. 

 It is thus seen that the limit at which 

 light-penetration in the open ocean can 

 be detected is somewhere between 600 and 

 900 fathoms, that is, considerably deeper 

 than was previously supposed. Further ob- 

 servations with filters for rays of different 

 colour showed that at a depth of 275 fathoms 

 many blue rays were present but hardly any 

 red ones, whilst at 55 fathoms all the light- 

 rays were present, though there were fewer of 

 the red than of the others. In the seas of 

 higher latitudes and nearer land the light- 

 rays do not penetrate so deep, largely owing 

 to the suspended particles in the water. 



Waves. — Though having an apparent for- 

 ward movement, wave motion in the open sea 

 does not involve a massive movement of the 

 water, the wave-form only moving on, while 

 the water-particles describe curves and are 

 supposed to come to rest approximately at the 

 points whence they started. A strong wind 

 may blow forward the top of a wave, making 

 it break and giving it a motion independent of 

 the true wave motion, and even should the 

 wave not break the surface water slips along 



