42 THE OCEANOGRAPHY OF THE XORTH-WESTERX OCEAN [PART I 



eye perceives it, is entirely absorbed by the overlying layers of sea. 

 We may conclude then that at an average depth of about 200 

 metres profound darkness reigns. But this does not exclude the 

 possibility of some degree of perception, by abyssal animals, of 

 radiations from the atmosphere, although this may be of such 

 a nature that our eyes would not be affected by it. 



At the bottom of a deep sea uniform conditions obtain. The 

 bottom is a flat plain with few inequalities, for those indicated by 

 the sounding machines are slight compared with those we know 

 on the land, and though precipitous declivities must occur these 

 are very exceptional. The sea bottom is composed of soft semi- 

 fluid oozes into which objects must easily sink. A uniform 

 temperature which is that of the freezing point of fresh water, or 

 only a degree or two above this, obtains. Profound or absolute 

 darkness, broken only by the light of some phosphorescent creature, 

 is there. Daily or seasonal changes never occur and almost 

 absolute uniformity of conditions reigns. Add to these the 

 enormous pressure of the overlying water, which is about one 

 atmosphere for every ten metres of depth, and we have conditions 

 in which it is almost incredible that life as we know it can exist. 



The water circulation. To complete this sketch of the oceano- 

 graphy of the North -Western Ocean we have now to consider the 

 circulation of the sea water. We have seen that temperature 

 observations give indications of considerable movements of the 

 waters of both the Atlantic and Norwegian oceans. It is indeed 

 by indirect methods that these have been traced. Direct obser- 

 vations of the movements of the water both at the surface and at 

 the bottom have been made, and many ingenious forms of apparatus 

 have been devised for this purpose. But the main facts of the 

 circulation of the North-Atlantic Ocean have been made out by 

 practically simultaneous observations of the temperature and 

 salinity of the sea currents over wide areas. A knowledge of these 

 two important characters of sea water enables us to determine with 

 some degree of certainty from what area the water has been 

 derived. The salinity is the total amount of dissolved salts per 

 litre of water : it is a number which varies from 37 in the 

 subtropical Atlantic to eight in the Baltic : that is to say, water 



