NATURE OF THE STREAM 139 



attached to the same cable. When returned to the surface the 

 water samples are analyzed to determine their saltness. Though 

 simple in plan this kind of sampling has its difficulties — as 

 for instance, when the steel bottle is by mischance sent down 

 to deep water closed instead of open. This invariably results 

 in the metal being crushed flat by the tremendous pressure, 

 which may in the deepest waters exert a force of over 1,000 

 tons on an eighteen-inch bottle. 



From the thousands of samples analyzed since H.M.S. 

 Challenger first began her voyage has come the knowledge 

 that the warm northward surface drift across the equator is 

 compensated for by means of a deep cold southward flow 

 that originates between Iceland and Greenland, where colder 

 waters of the East Greenland Current meet with part of the 

 North Atlantic Drift. The mixed waters are further cooled 

 in winter, so that they become very dense and sink to the 

 bottom at an average rate of 2,000,000 tons per second, and 

 here they begin a slow journey which takes them south of the 

 equator as far as latitude 60° S. An equal amount of cold 

 water sinks in the Labrador Sea. During the several years this 

 massive flow takes to travel from Greenland to the latitude of 

 Argentina it is every second added to by another 2,000,000 

 tons of water flowing out of the Mediterranean, where the 

 surface is constantly evaporating and becoming saltier and 

 therefore heavier. This dense water pours over the shallow 

 rim of the sea along the bottom of the Straits of Gibraltar 

 and joins the Atlantic Deep Drift, while in return lighter 

 Atlantic surface water flows into the Mediterranean at a rate 

 of over two knots. The entire deep drift of Atlantic water 

 continues south beyond the equator. Thus the South Atlantic 

 water is repaid, and in like manner other currents throughout 

 the oceans are nicely poised and balanced in a continuous 

 circulation. 



While the slower submarine rivers of the ocean were giving 



