34 LIFE IN THE SEA [CH. 



away to the north and south towards the poles, but 

 at about latitude 60 N. it has become cooled by 

 radiation of its heat to the atmosphere, and being 

 salter than the water normally present in the sea 

 at these latitudes it becomes heavier and sinks 

 towards the bottom. Part of the water thus sinking 

 flows back again towards the equator. Round the 

 poles the sea contains much ice, but when sea water 

 freezes most of the salt is squeezed out, and there- 

 fore when this ice melts it forms nearly fresh water. 

 The water of the circumpolar seas is therefore fresher 

 than normal sea-water and it accumulates at the 

 surface and then begins to flow towards the equator. 

 But it soon mixes with the other sea-water and 

 becomes much salter, and then being cold and heavy 

 it tends to sink to the bottom. This occurs at about 

 latitude 60 N. (considering the Atlantic alone) and 

 part of this sinking water flows along the sea-bottom 

 back again to the poles. Thus we have two main 

 systems of ocean currents, (1) warm and very salt 

 water flowing to the north and south from the 

 equator, and (2) cold and fresher water flowing also 

 to the south and north from the poles. Now these 

 drifts of water do not actually flow to the north and 

 south : they would do so if the earth were entirely 

 covered with sea and if it did not rotate ; but because 

 of the presence of the land the currents are diverted 

 in various ways ; and because of the rotation of the 



