before 

 turning 



while it 

 turns 



This Nansen water-sampling bottle is 

 essentially a metal tube with valves at 

 each end. It is fitted with reversing 

 thermometers (see p. 257). As many as 20 

 bottles in a series are lowered on a wire. 

 A "messenger" weight released from the 

 ship travels down the wire and trips the 

 first bottle. As it reverses, valves at 

 each end close and trap a water sample; 

 another messenger is released which trips 

 the second bottle, and so on down the line. 



The temperature chart on page 209 is taken 

 from the atlas of the Meteor Expedition to 

 the Atlantic in 1925-27. The chart shows 

 temperatures at a depth of 4000 meters 

 (2187 fathoms) in the deep basins east and 

 west of the Mid-Atlantic Ridge, which rises 

 well above this level. The cold Antarctic 

 Bottom Water from the Weddell Sea spreads 

 northward, particularly in the western 

 basins. In the far north Is the deepest 

 part of the cold North Atlantic Deep Current 

 which flows southward. The isotherms 

 shown are lines joining places of equal 

 temperature (In degrees centigrade) . 



Banks of Newfoundland. At the same time, dense fog is liable to 

 be formed in the same area when the warm winds from the Sargasso 

 Sea meet the cold water of the Labrador Current. After the Titanic 

 disaster in 191 1, the International Ice Patrol was set up to keep 

 watch on icebergs and warn ships of their movements. Observation 

 of the water masses and the currents of the Labrador Sea has made 

 it possible to predict ice movements with some success, a good 

 example of the practical use of the study of currents. 



As well as influencing climate, currents have a direct effect on 

 the fertility of the oceans. Where there is a steady upwelling of 

 water from below, the water brought to the surface may be colder 

 than usual for a given latitude. This happens most noticeably near 

 the coasts of southwest Africa and western South America, off Peru. 

 In these regions the prevailing winds, which are all offshore, tend 

 to drive the coastal surface water out to sea, and colder water wells 

 up from depths of a few hundred feet to replace the water driven 

 away. This deep water is rich in nutrients needed for the growth 

 of plankton, and when it rises to the sunlit upper layers there is a 

 tremendous outburst of growth of both marine plants and animals. 

 Regions of upwelling are, therefore, particularly fertile and may 

 support large populations of fish. In turn, this means there is food 

 for large numbers of sea birds living along the coasts. As a result, 

 thriving fishing and guano industries have grown up, especially on 

 the Peruvian coast. In certain years, though, the winds (and there- 

 fore the currents) may shift so that the upwelUng is weaker, or the 

 area may be invaded by less fertile water. Whenever this happens 

 there are disastrous mortalities of fish and sea birds ; for miles the 

 beaches have been covered thickly with dead fish. 



Less violent, but none the less serious for fisheries, are the longer 

 term fluctuations of water movements. The decay of the herring 

 fishery in the EngUsh Channel and the fall in catches of sardines off 

 the coast of California have both been ascribed to gradual changes 

 in the water circulation. These changes have resulted in less nutrient 

 material being brought into the areas than is needed to support an 

 economically worthwhile population of fish. 



So far only the surface currents have been mentioned. Over 

 most of the deep ocean basins they do not extend downward more 

 than a few hundred feet. The total depth of water in the oceans, 

 though, is usually between two and three miles. There is move- 

 ment even in these deeper layers. Here, though, the average 

 currents are much slower than those on the surface. In the shallow 

 seas, where much of the world's fishing is done, a knowledge of the 

 bottom currents and the subsurface water movements can be useful 

 to fishermen. Sometimes these bottom currents may run quite 

 differently from those on the surface, although in shallow water the 

 tidal streams predominate. One example of a contrary undercurrent 

 in the shallow seas is found in the Bosporus. Here the surface 

 water of the Black Sea flows out into the Sea of Marmara, but 

 underneath there is an opposing flow of saltier water (from the 

 Mediterranean) into the Black Sea. At times this undercurrent is so 

 strong that if fishing nets were lowered into it, a boat would be 

 towed against the surface current. 



In most parts of the oceans, there is no obvious "profit" to be 

 gained from a study of the deep currents ; we cannot, for instance, 

 enable ships to make a faster passage. Knowledge of the deep- 



