CH. Il] THE OCEANOGRAPHY OF THE NORTH-WESTERN OCEAN 41 



any other part of the ocean bed of the world with the exception of 

 a place in Bransfield Sound in the Antarctic where a temperature 

 of — 1*6' C. has been observed. 



Colour and transparency. Over this area another physical 

 difference is that of the varying distribution of light both at the 

 surface and in the deeper layers of the sea. If we attach a white 

 disc, such as an enamelled iron plate, to the end of a sounding 

 line and lower this into the sea we find it disappears at a variable 

 depth, which depends for the most part on the amount of mud or 

 fine inorganic particles in suspension in the water. Where these 

 are not present differences in the transparency of the sea depend 

 on the colour of the water and on the amount of the plank tonic 

 organisms in it. In the turbid waters off the coast of Lancashire 

 such a disc may disappear at a distance of a few feet if we make 

 the experiment during the time of strong spring tides, or at 

 a time when strong winds are prevalent. Even in the most 

 favourable circumstances, that is during fine weather, in brilliant 

 sunlight and at some considerable distance from the land, it is 

 seldom that a white object can be seen at a greater depth than 

 20 metres (10 fathoms). In the Lake of Geneva a lighted electric 

 lamj) can be seen at a depth of 38 metres, in the Pacific Ocean 

 a white disc may be seen at 40 metres. The most transparent 

 waters are those of the Mediterranean, where the maximum 

 translucency of the water is estimated at 45 metres. With these 

 differences in transparency there are also differences in colour. 

 The Baltic is dirty yellow, the North Sea is greenish and the open 

 seas are green to blue. 



Fol and Sarasin investigated the penetration by light of the 

 water of the Lake of Geneva by means of photographic plates. 

 These were enclosed in shutters and lowered into the water to 

 variable depths, and the shutter was opened by means of a 

 " messenger " or some similar contrivance. It was thus found that 

 at a distance from the surface of 170 metres the plates were 

 affected, but it was estimated that the intensity of the light could 

 not be greater than that of a dark moonless night. Similar 

 experiments carried out in the Mediterranean gave a depth of about 

 400 metres as the limiting one : below this light, as the human 



