CHAPTER V 



PHYSICAL OCEANOGRAPHY 



In the middle of last century the idea of "physical oceano- 

 graphy " did not exist, but in the course of a few decades it 

 has become a widespread branch of knowledge, with a copious 

 literature and bulky text-books. A few figures may serve to 

 show how important is the study of the sea. The waters of the 

 globe cover more than two-thirds of its surface, and their 

 volume is about 1300 millions of cubic kilometres, or thirteen 

 times that of all the land above sea-level. The mean height 

 of the land is 700 metres, while the average depth of the sea 

 is 3500 metres. Sea-water contains various salts in solution, 

 the total weight of which is nine times that of the earth's 

 atmosphere. 



The reason why the ocean, which plays such an important 

 part in the economy of the earth, has not been investigated 

 until recently is because of the special difificulties which are 

 encountered in making investigations. One great difficulty is, 

 as has been previously mentioned, that it is impossible to 

 observe directly what is going on beneath the surface, and it is 

 necessary to have a special set of apparatus that can be relied 

 upon. The methods have developed with phenomenal rapidity, 

 but the observations are still few in proportion to the extent of 

 the ocean, and consequently it is often difficult to obtain a 

 complete and true image of the actual conditions. Many of the 

 results obtained are therefore merely preliminary, and further 

 study may alter our views on various points ; for the solution of 



