water movements has come about only with the scientific study of 

 the oceans. 



Nearly everything we know about the deep water we have found 

 out by taking samples of the water, then by measuring its proper- 

 ties — temperature, salinity, the dissolved oxygen content, and so on. 

 To collect water from great depths, many kinds of samplers 

 have been designed, the Nansen bottle being typical. Usually, 

 samples are taken at chosen depths at each of a number of selected 

 positions along a line or spread over an area. Each position is called 

 a "station". At a station where the water is more than ten thousand 

 feet deep, it may take as long as three or four hours to collect 

 samples from twenty or more different levels. After the samples 

 have been analyzed, charts showing the distribution of the different 

 properties of the water can be prepared. From the way these pro- 

 perties vary from one place to another, we can infer something 

 about the way the deep water is moving. 



Even though we cannot see directly how fast the deep currents 

 are moving, we can learn something about them by studying the 

 distribution of water density. Density, which is the mass of water 

 per unit volume, can be calculated from the temperature and salt 

 content - the colder water being heavier than the warm, and the 

 saltier water being heavier than the less salty. Once the density 

 distribution is known, then the pressure distribution (force per tmit 

 area) can be found. Then by taking into account the effect of 

 the Earth's rotation we can calculate the currents that should be there 

 to balance the observed pressure gradients. The meteorologist 

 follows a very similar process to find the speed of the winds from 

 the spacing of the isobars on a weather map. But in the sea the 

 height of the surface cannot be measured accurately, as it can be on 

 land. This means that there is no convenient reference level from 

 which to measure the pressure, and so only differences between 

 currents at different depths can be calculated in this way. To obtain 

 an over-all picture of the currents at all depths we must know, or 

 assume, something about the current at one particular depth. 



Despite all these uncertainties, oceanographers have built up a 

 fairly consistent picture of the deep-water circulation, mainly from 

 extensive systematic surveys made during the 1920s and 1930s. 

 Although temperature soundings had been taken more than a 

 hundred years earlier, and the Challenger expedition and others of 

 the late nineteenth century had brought back information about 

 temperature and chemical composition of sea water, it was not 

 until deliberate surveys were made of particular areas that charts 

 and profiles showing in detail the distribution of the various pro- 

 perties of sea water at all depths could be constructed. Such surveys 

 include the Meteor survey of the South Atlantic in 1925-27, and the 

 Discovery investigations in the Southern Ocean from 1925-51. Since 

 then, interest in the deep water has grown and many more samples 

 of water at all depths have been collected. 



During the International Geophysical Year (1957-58) there was 

 a great deal of activity in deep-water investigation. In the Atlantic, 

 for instance, east-west lines of stations were worked at every eight 

 degrees of latitude, providing information about temperature, 

 salinity, and dissolved oxygen content at varying depths ; in addi- 

 tion, intensive studies of more limited areas were made. Similar in- 

 vestigations were carried out in the Pacific and Southern oceans. 



A scientist aboard a researcii siiip stands 

 on tlie hydrograpliic piatform to remove 

 a reversing water bottie from tiie wire. 



211 



