In the Atlantic Ocean it rises to 37.5 parts per thousand in the 

 northern subtropical region, where descending dry air leads to 

 high evaporation, and is lowest in the subpolar regions where the 

 rainfall is heavy, and in the doldrums where there is also a lot of 

 rain. The Atlantic Ocean has higher salinities than the Pacific Ocean, 

 probably because it has greater evaporation, and possibly because 

 it is more influenced by dry winds off the land, but the deep-water 

 circulation may also play a significant part. As we will see in Dr. 

 Swallow's section on currents, salinity as well as temperature deter- 

 mines the layering of ocean water. The warm subsurface water 

 found on Cook's voyage is now known to be more saline than the 

 colder water above it. Nansen found cold, relatively fresh water 

 above warmer, more saline water in the Arctic. Salinity differences 

 between one part of the ocean and another contribute to the density 

 differences which affect the circulation of the water. 



Ocean currents have always been of interest to seamen. The 

 effect of the wind on the water, always the most striking thing 

 about the sea to sailors, has been generally held responsible for 

 currents. But this has not prevented other explanations — such as 

 the early misconception that the westward flow of water in the 

 equatorial region is caused by a tendency for the water to be left 

 behind as the Earth rotates eastward underneath it. Objects at the 

 Earth's surface feel the effect of the Earth's rotation, but not in 

 such a simple way. Except right on the Equator, every bit of the 

 Earth's surface has some horizontal rotation around, say, a post 

 standing vertically on it. 



We can perform an experiment to demonstrate this. If a pen- 

 dulum bob is suspended on a very long cord and set in motion, it 

 will continue to swing in the same direction; but if the direction is 

 marked on the ground every hour or so, the mark will be seen to 

 swing slowly around, because the Earth's surface is going around 

 the opposite way underneath. At the North Pole the line would 

 perform a complete counterclockwise rotation in twenty- four hours. 

 But at the Equator the line would not rotate. At intermediate 

 latitudes the time of rotation varies between twenty-four hours at 

 the poles to infinity at the Equator. 



So every part of the Earth's surface has a different sort of rota- 

 tion.- This means that water moving from one place to another — 

 whether horizontally or vertically — is affected by this geostrophic 

 effect, or Coriohs effect as it is sometimes called. Although this 

 effect is well known, it requires someone with a good knowledge 

 of fluid mechanics to judge exactly how the motion of the ocean 

 water is affected. The application of this specialized knowledge is 

 the mainspring of the new attack on all studies of ocean circulation. 



Major James Rennell, surveyor for the East India Company, 

 was an outstanding pioneer in the study of ocean currents. His first 

 chart and Remarks on the Agulhas or South African Current appeared 

 in 1778, and his most comprehensive work, Currents of the Atlantic 

 Ocean, was published by his daughter in 1832 after his death. An- 

 other pioneer in charting currents was Benjamin Franklin, who 

 published a chart of the Gulf Stream in 1770; and twelve years 

 later Sir Charles Blagden published a useful account of the same 

 current. Described as the creator of statistical studies of ocean 

 currents, Rennell was followed by Heinrich Berghaus and Matthew 

 Maury. In 1835 Maury persuaded the United States government 



Mayor James Rennell 





This section of Rennell's chart of the North 

 Atlantic clearly shows the trend of the 

 Gulf Stream. IHeavy arrows show estimated 

 mean current flow. Thin arrows: current 

 observations. Thin dotted arrows: wind 

 observations. iHeavy figures: sea temperature. 

 Italics figures: sea soundings. 



183 



