PHYSICAL OCEANOGRAPHY 245 



in depths between 50 and 100 metres the temperature is below 

 zero, and where there are banks at these depths they are 

 covered wath ice-cold water ; hence the great difference between 

 such banks and those which lie within the region of the warm 

 currents. Fig. 95, p. 1 10, represents a section across the New- 

 foundland Banks from the Gulf Stream (Station 69) northwards 

 to a point just outside St. John's (Station 74). On the northern 

 part of the bank it is very cold, for there we are in the middle of 

 the Labrador Current; on the southern slope it is much warmer, 

 because of the vicinity of the Gulf Stream. There are accord- 

 ingly great differences in temperature and salinity in different 

 parts of the Newfoundland Banks, especially in the deeper 

 parts. 



From Fig. 170 we see that the salinity was below t,t, per 

 thousand at the surface, that it increased rapidly downwards (to 

 34.6 per thousand at 200 metres), and afterwards more slowly, but 

 it nowhere attained the salinity of the " Atlantic water," viz. more 

 than 35.0 per thousand. This is characteristic of the Arctic and 

 Antarctic regions, especially in summer. The water brought by 

 the currents from the North Polar basin is a kind of coast- 

 water. The great rivers of Siberia and of the north of America 

 empty volumes of fresh water into the Polar Sea, where it 

 mixes with the salt water, diminishing the surface salinity, 

 which is further reduced by the melting of the drifting ice in 

 summer. The low salinity at the surface renders the density 

 comparatively small, but it increases rapidly downwards, so 

 that the water at 100 metres is heavier than at any of the three 

 stations within the warm water region just mentioned. We 

 have not in any of these examples taken into consideration the 

 fact that the density is slightly increased with increase of depth 

 by the pressure due to the weight of the overlying water. 



The pressure in the sea increases by about i atmosphere The pressure 

 for every 10 metres of depth. Thus there is a pressure of '''^^^^^^■ 

 about 100 atmospheres 1000 metres below the surface, and of 

 500 atmospheres at a depth of about 5000 metres. When 

 differences in pressure occur in adjacent areas at the same level 

 below the surface, various currents arise, just as air-currents 

 are produced by differences of barometric pressure. The 

 circumstance that the water is not equally heavy everywhere is 

 one of the main causes of the ocean currents, and, the water 

 being easily moved, small differences of pressure will be sufficient 

 to produce a sensible motion. By the great pressure the water 



