A TEXTBOOK OF OCEANOGRAPHY 



CHAPTER I 



Introduction 



The seas form so large a part of the environment of mankind 

 that the reasoned description of their great area and contents 

 is a subject of the very first importance. The main features of 

 tlie seas and océans of the globe affect human Hfe and its 

 problems in endless ways. The océans exercise a profound 

 influence over climate and végétation, the possibiHties of 

 cuhivation and human settlement. Oceanic movements, both 

 tidal and non-tidal, affect the harbours of the globe and our 

 maritime intercourse, and had still more intimate bearings on 

 that intercourse in the days before the introduction of steam 

 as a means of the propulsion of vessels and the era of great 

 engineering feats such as ship-canal construction. Thèse 

 movements also affect the migrations of fish and cetacea, and 

 so influence fisheries and the growth of sea-power. 



Pettersson and others hâve described the important 

 influence of secular variation of tides and currents upon 

 changes of climate within historic times, and consequently 

 upon the rise and fall of human societies. The story of the 

 seas of the past is recorded imperfectly in the sedimentary 

 rocks ; even the few facts known throw important light upon 

 diverse scientific problems. 



At the outset it is essential to understand that the great 

 water masses are not alike in condition or constitution, nor is 

 any one of them homogeneous. Just as meteorologists are 

 certain that masses of air of différent température and water- 



j 



