INTRODUCTION 3 



changes in temperature and salinity. At the entrance to the 

 Baltic there is an upper layer of colder and fresher water- 

 below it a warmer salter layer. 



The movements of these large water masses relatively to 

 ther and to the land afford the oceanographer an 

 endless field for research. Oceanic circulation due to differ- 

 ences between the physical characters of masses of sea 

 water ,s only one phase of the problem to be studied. The 

 da movements, varying in fortnightly, half-yearly, and 

 secular cyctes of many kinds, many of them as yet imperfectly 

 known, add to the complexity and interest of the story The 

 influence of prevailing winds on ocean movements, so'far as 

 rface waters are concerned, is another aspect of oceano- 

 graph.cal research, with many difficulties that some of the 



textbooks have overlooked completely 

 Probably all the large scale movements of the waters of the 

 ocean are profoundly affected by the contour of the ocean floor 

 and the presence of land; this, again, is a subject for further 

 research. The deep abysses just off the lines of high volcanic 

 .fluence and more or less parallel with them (e.g., off T apan 

 and the South American Cordilleras); the Wyville-Thomson 

 ridge connecting the north of Scotland and Iceland, of ereat 

 interest in the evolution of the Atlantic; the Grand Banks of 

 Newfoundland, .with the problems of their physical character 

 1 suggest important aspects of study along the line of junction 

 etween geography, geology, and oceanography. The confor 

 nation of the coastal sea-floor gives an opening into questions 

 the dynam.cs of tidal and other waves, and is of direct 

 -.ical importance in connection with questions of harbour 

 construction and protection. The details of local sea and 

 =oastal currents have their bearing on the characteristics of 

 e deposits and the building and erosion of our coasts 

 All this interplay of physical influences has an endless 

 ^relation in the world of life. The herring of Cardigan Bay 

 nter the bay from the south, and first spawn when they feel 

 : influence of the fresh water of the Dovey estuary Again 



