TRANSACTIONS OF THE SECTIONS. 177 



ally in the face of the trade-wind— and that they are not merely local, stretching 

 as 'they do across the wide extent of the Pacific, and in the Atlantic, during the 

 summer months of our hemisphere, extending nearly across from the Guinea coast 

 to the "West-India Islands. They have, too, this significant feature, that their narrow 

 zone is confined to the northern side almie of the great west-going equatorial cun-ents ; 

 this zone is approximately hetween the parallels of 7° and 10° N., and thus cor- 

 responds with the belt of greatest atmospherical heat on the earth's surface. 



That the functions of the countercurrents in the physics of the ocean are im- 

 portant, must, I think, be conceded. They appear to act on their eastern limits as 

 feeders to the equatorial currents, and, from the seasonal expansion, which has 

 been well traced in the Atlantic, are probably more immediately associated with 

 some oscillatory morement of the waters following, though perhaps only remotely 

 connected with, the srm's moTements in declination. 



A brief summary of the thermal conditions of the oceanic basins will now enable 

 lis to review the salient features of Ocean Circulation, and the more immediate 

 scientific position the question has assumed. 



In all seas within the ton-id and temperate zones, providedany given area is not 

 cut off by submarine barriers from a supply of polar or glacial water, the sea-bed 

 is covered by a thick stratum of water 'the temperature of which is confined 

 between 32° and 35° F. In the Pacific Ocean this cold stratum must be derived 

 from antarctic sources, for the opening of Behriug Strait is too small to admitof 

 an appreciable efUux of arctic waters. In this ocean the cold stratum obtains 

 generally at depths below 9000 feet from the surface, with an almost invariable 

 isothermal line of 40° F. at from 2500 to 3000 feet from the surface. Similarly, 

 in the Indian-Ocean basin the cold stratum at the bottom is derived from 

 antarctic sources ; for the temperatm-e of 33°-5 F. underlies the hot surface-waters 

 of the Arabian Gulf 



In the South Atlantic, antarctic waters, with a bottom-temperature of 31° to 

 33°-5F., certainly cross the equator: the bed of the North- Atlantic basin then 

 warms up to 35° ;' marked diversities in both the temperatures and thickness of 

 the successive layers of water fi-om the surface downwards are found ; and in the 

 central parts of the basin it is not until the vicinity of the Faroe Islands is reached 

 that arctic waters of an equivalent temperature to those from antarctic sources 

 are experienced. 



Tm-ning now to the scientific aspect of the question : — 



The doctrine of a general Oceanic Tliermal Circulation assumes two general pro- 

 positions : — 1, the existence of a deep under-flow of glacial water from each pole to 

 the equator ; and, 2, the movement of the upper stratum of oceanic water from the 

 equatorial region towards each pole, as the necessary complement of the deep polar 

 rmder-flow— this double movement being dependent "upon the distm-bance of 

 hydrostatic equilibrium, constantly maintained by polar cold and equatorial 

 heat." 



Proposition 2, in its general application as to the movement of surface-waters, 

 is unquestionable ; but that of a deep under-flow from the poles, as a necessary 

 complement, remains open to doubt. Proposition 1, in its wide generality, must, 

 from what we know of the Pacific, be confined to the Atlantic Ocean; audit 

 appears to me that it is on the interpretation of the movement of the waters in its 

 northern basin that the hypothesis of a vertical circulation, and the potency of 

 thermal agency in bringing it about, must be judged. 



"We have followed the movements of antarctic waters in the Atlantic to the 

 40th parallel, as illustrated by the progress of icebergs ; we know that the move- 

 ment deflects the strong Agulhas current, and that the cold waters well up on the 

 western shore of the South-African continent, cooling the equatorial current near 

 its presumed source ; the thrusting power of this body of water is therefore great. 

 About the equator it rises comparatively near to tlie surface. But we now come 

 to another and distinct movement, the equatorial cm-rent ; and on this, I appre- 

 hend, the material agency of the winds cannot be denied, in forcing an enormous 

 ^^ mass of siu'face-water from east to west across the ocean. The Gulf-stream re- 

 sults ; and the comparative powers of this stream, as especially influencing the 

 climate of our own and neighbouring countries, together with the forces at work 



