86 THE WONDER OF LIFE 



equatorial movement of bottom- water rich in oxygen must 

 be of considerable biological importance for the animals of 

 the Deep Sea. 



(4) Darkness. — There is but httle penetration of Hght 

 beyond 250 fathoms, so that the world of the Deep Sea is 

 in utter darkness, save only in so far as that is reheved by 

 gleams of ' phosphorescent ' light. In some places where 

 there is much of this luminescence, it may be that the 

 scene is hke the ill-Ughted suburbs of a town on a very dark 

 night, or hke a moorland with no light save from the stars. 

 In his 1911 cruise on the Michael Sars, the late Sir John 

 Murray found that the light hmit had been under-esti- 

 mated. By using more delicate apparatus, notably the 

 Helland-Hansen photometer, he was able to show that 

 there is a clear effect at 300 fathoms, and some effect at 

 500 fathoms, which is about half a mile down. At 900 

 fathoms no effect of hght was detectable. These were 

 very sensitive tests, however, and for practical purpose 

 we may still say that there is very little hght below 250 

 fathoms. 



(5) Calm and Silent. — Another physical feature is the 

 pervading cahn, for the severest storms are shallow in 

 their grip, and though the cold polar water is ever creeping 

 along the bottom towards the equator, this is a relatively 

 slow movement. Only in a few places is there evidence of 

 what may be called a current. If there were rapid move- 

 ment the deep ooze which covers vast areas of the sea-floor 

 would be raised in whirling clouds. Thus we must think 

 of the deep sea as extraordinarily still and qiiiet, for there 

 can be no noise to break the abiding silence of the abysses. 



(6) Monotony. — There is some variety in the composi- 

 tion of the sea-floor, for the remains of calcareous organisms 



