PHYSICAL OCEANOGRAPHY 259 



siderable quantities, i litre of water at a temperature of 10° C. and 

 with a salinity of 35 per thousand, for instance, containing when 

 saturated 12 c.c, of nitrogen. It is possible that marine bacteria 

 partly dissociate nitric compounds so as to liberate nitrogen, 

 and partly bind free nitrogen in various salts. These variations 

 are always small, and not easily demonstrable. As a rule, 

 though not without exception, the surface-water is saturated 

 with nitrogen from the air, and when the water leaves the 

 surface it carries down with it practically the same amount of 

 nitrogen. 



A vessel running a certain course at a speed measured by Currents ii 

 the log often proves to have arrived at another point than that ^^^ ^^^' 

 which would be expected from the reckonings. This will be 

 the case when there is a strong wind, but even in a calm a dis- 

 placement is frequently experienced, which is then caused by a 

 current, and when the calculated position is compared with that 

 actually arrived at, the difference will indicate the effect of the 

 current on the ship. In sailing across the Gulf Stream off the* 

 east coast of North America, for instance, the ship is carried 

 north or north-east of its latitude according to the compass and 

 the log. The deviation is then an expression of the direction 

 and velocity of the current, and much information with regard 

 to the set of the currents has been obtained in this way. But 

 the method is not trustworthy when there is a wind acting on 

 the ship. The drift of various objects floating on the sea. Drift of 

 wreckage for example, has also been studied. When wreckage ^'^eckage. 

 belonging to the *' Jeanette," which foundered in the Arctic Sea, 

 was found in the North Atlantic, Nansen concluded that a 

 current must run from the polar basin between Greenland and 

 Spitzbergen into the Atlantic Ocean, and on this supposition 

 he planned the " Fram " Expedition. In the Atlantic Ocean 

 wrecks are often encountered drifting about with wind and 

 current. These are reported, and from such reports one can 

 follow the movements of wrecks for a long time. Fig. 175 shows 

 some such wreck-courses ; many of the wrecks have drifted 

 from North America towards Europe, thus showing the effect 

 of the Gulf Stream ; others have been carried eastward in the 

 direction of the Azores, then south, and finally west back towards 

 America again. But in these cases the wind always plays an 

 important part, so that it is difficult to form a correct idea of the 

 movements of the water. In the far north and far south we Floating 

 can follow the drift of the icebergs ; one, for instance, breaking ''^^^^''g^- 



