396 
Proceedings of the Royal Society 
place in a horizontal direction. In order to get quit, as much as 
possible, of the effects of friction on the sides and bottom of the 
vessel, the experiment was repeated in a pond, with a like result. 
From these experiments we may conclude that, save under very 
exceptional circumstances, the wind can only give rise to a hori- 
zontal circulation of the oceanic waters, — the exceptional cases 
being, when the wind-driven current is deflected by the irregulari- 
ties in the outline of the bed of the ocean, or strikes against a 
deep and nearly vertical coast; or when the north wind drives the 
waters of the Antarctic Sea against the great barrier of ice-cliffs 
which surround the south pole. The depth to which the vertical 
currents will descend in these cases will depend much on the rela- 
tive densities of the water in the currents and of the water sur- 
rounding them. 
It may possibly be objected that we are not entitled to come to 
any conclusion, from experiments made on so small a scale, as to 
what takes place in the ocean. Such objections would be perfectly 
valid, if there was not some evidence, in the conditions we find in 
the ocean, to support these conclusions. If the return currents in 
the ocean flowed underneath the wind-driven surface currents, 
then we should be perfectly justified in expecting some evidence 
of their presence. For instance, we would expect that the water 
near the bottom, underneath the wind-driven currents near the 
equator, would be hotter than the water at corresponding depths 
at other parts of the ocean. An examination, however, of the tem- 
perature sections of the Atlantic Ocean, taken by the “ Challenger,” 
shows no evidence whatever of the return current by this route; 
we are therefore compelled to conclude that the water must return 
by the surface, and that the wind does only produce horizontal 
currents , and therefore cannot account for the presence of the cold 
water which we find all over the bottom of the ocean, from the 
poles to the equator. 
The second point I wish to refer to, is the effect of these wind- 
driven surface currents on the cold water underneath them. I; 
have said that almost the whole of the motion of the wind-driven 
currents takes place in a horizontal direction. Such was the 
general result given by the experiment. But, in addition to this, 
there is another point to which I must refer. The wind driven 
