TEANSACTIONS OF SECTION E. 615 



and I am very glad to find from Professor Mohn's report that this defect has now 

 been entirely overcome. 



It follows from the nature of these many and varied enterprises that the 

 department of geographical science to whose progress they have most specially 

 contributed, is the physical geography of the sea ; and the special appliances with 

 which they have been provided have been principally instruments for determining 

 the temperature of the water at different depths, the depth of the sea and tbe 

 nature of the sea-bottom, and, in some few cases, the distribution of the deep-sea 

 fauna. It is of course impossible for me in so short a time even to sketch their 

 several lines of investigation, or to attempt to assign to each its share in the general 

 advance of knowledge ; I think it may be better that I should give an outline of 

 some of the conditions of tbe regions to which they refer by the light of their 

 combined results. I am aware that in taking this course I shall be forced to face 

 questions ou which there has been some controversy ; and I can only say that I 

 will avoid the controversial aspects of such questions as far as possible, and merely 

 describe as shortly as I can the condition of things as they appear to me. 



The General Ocean Circulation. — It was pointed out long ago by Sir Charles 

 Lyell that many of the most marked phenomena of the present physical condition 

 of the globe depend upon the fact that the surface of the world is divided into two 

 hemispheres, one of which contains nearly the whole of the dry land of this world, 

 while the other is almost entirely covered by water. The centre of the land- 

 hemisphere is somewhere in Great Britain, and the centre of the water-hemisphere, 

 which includes the southern sea, the South Pacific, whatever antarctic land there 

 may be, Australia, and the southern point of South America, is in the neighbour- 

 hood of New Zealand. With a full knowledge of the absolute continuity of the 

 ocean, we have hitherto been too much in the habit of regarding it as composed of 

 several oceans, each possibly under special physical conditions. All recent obser- 

 vations have, however, shown us that the vast expanse of water which has its 

 centre in the southern hemisphere, is the one great ocean of the world, of which 

 the Atlantic, with the Arctic Sea and the North Pacific, are merely northward 

 extending gidfs ; and that any physical phenomena affecting obviously one portion 

 of its area must be regarded as one of an interdependent system of phenomena 

 affecting the ocean as a whole. 



Shallow as the stratum of water forming the ocean is— a mere film in propor- 

 tion to the radius of the earth — it is very definitely split up into two layers, which, 

 so far as all questions concerning ocean movements and the distribution of tem- 

 perature is concerned, are under very different conditions. At a depth varying 

 in different parts of the world, but averaging perhaps 500 fathoms, we arrive at a 

 layer of water at a temperature of 40° F., and this may be regarded as a kind of 

 neutral band separating the two layers. Above this band, the temperature varies 

 greatly oyer different areas, the isothermobathic lines being sometimes tolerably 

 equally distributed, and at other times crowding together towards the surface; 

 while beneath it, the temperature almost universally sinks very slowly and with in- 

 creasing slowness to a minimum at the bottom. 



The causes of natural phenomena, such as the movements of great masses of 

 water, or the existence over large areas of abnormal temperature conditions, are 

 always more or less complex, but in almost all cases one cause appears to be so 

 yery much the most efficient that in taking a general view all others may be prac- 

 tically disregarded ; and speaking in this sense it may be said that the trade-winds 

 and their modifications and counter-currents are the "cause of all movements in the 

 stratum of the ocean above the neutral layer. This system of horizontal circula- 

 tion, although so enormously important in its influences upon the distribution of 

 climate, is sufficiently simple. Disregarding minor details, tbe great equatorial 

 current driven from east to west across the northerly extensions of the ocean by 

 the trade- winds, impinges upon the eastern coasts of the continents. A branch 

 turns northwards and circles round the closed end of tbe Pacific, tending to curl 

 back to the North American coast from its excess of initial velocity ; and in the 

 Atlantic, following a corresponding course, the Gulf Stream bathes the shores of 

 northern Europe, and a branch of it forces its way into the Arctic basin, and bat- 



