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of Edinburgh , Session 1876 - 77 . 
horizontal currents have an influence on the water underneath 
them which, for the sake of clearness, I have reserved for separate 
consideration here. Let us draw an imaginary section across a 
wind-driven current, at a point near its source, that is, near where 
the wind begins to act on it. And let us imagine another section 
of this same current, at a point some distance farther “ down” the 
stream. As the current is acted upon by the wind between these two 
sections, it will be much deeper at the second section than at the 
first, and will also be going at a greater velocity. There will, there- 
fore, be much more water passing the second section than passed the 
first, and the water necessary to supply this growing stream must 
be supplied to it between the two imaginary sections. The result 
is, part of the necessary supply rushes in at the sides', but part of it 
rises from the still water underneath the surface stream. This 
lifting of the deeper water by the surface current was very evident 
in the experiment already referred to, so long as the surface current 
was shallow, and gradually became less, as might be expected, 
when the current deepened. 
From these considerations, we are naturally led to expect that 
the hot surface water at’ those parts of the ocean over which winds 
are constantly blowing, will be much reduced in depth, and that 
the cold bottom water will be found at a less depth underneath 
these surface currents than at any other part of the ocean. Part 
of this cold water will, in all probability, get mixed up with the 
bottom water of the surface current. And further, we would expect 
that this wind-driven hot surface water, after it passes beyond the 
windy regions, will gradually lose its motion and increase in 
depth. 
These expectations are in a remarkable manner supported by the 
evidence of the temperature sections of the Atlantic taken by the 
(c Challenger,” and given in Dr Carpenter’s paper in the u Pro- 
ceedings of the Royal Geographical Society,” vol. xviii. No. iv., 
1874. Take, for instance, the section between Tenerife and St 
Thomas. In the first part of the journey there is not much altera- 
tion in the relative position of the isotherms, but after crossing 
the Tropic of Cancer and getting into the region of the north-east 
trade winds, the hot surface isotherms gradually approach each 
other, and the isotherm of 40°, which off the coast of Tenerife was 
