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a piece of ice was wedged in between the two sides ; that re- 
presented the Polar area. At the other end we applied heat at 
the surface, not at the bottom — to imitate the exact conditions 
of the case — the heat being applied by a bar of metal which 
was laid on the surface of the water, and then carried over the 
end of the trough and heated by a spirit lamp ; that represented 
the Equatorial area. Then we put in some colouring matter, 
red at the warm end, and blue at the cold end. What hap- 
pened ? The water tinged with blue put in at the surface of 
the Polar area, being chilled by contact with the ice, immedi- 
ately fell down to the bottom ; it then crept slowly along the 
bottom of the trough, and at the Equatorial end it gradually rose 
towards the surface ; and, having done so, it gradually returned 
along the surface to the point from which it started. The red 
followed the same course as the blue, but started from a dif- 
ferent point. It crept along the surface from the Equatorial to 
the Polar end, and there fell to the bottom, just as the blue had 
done, and formed another stratum, creeping along the bottom 
and coming again to the surface. Each colour made a distinct 
circulation during the half hour in which the audience had this 
experiment in view. — Now that was a very beautiful experiment ; 
and I can myself see no flaw in the application of the argu- 
ment, that what is true on a small scale in this trough is true 
of a mass of water extending from the Equatorial to the Polar 
area. 
Lastly, let us return for a moment to the subject of Deep-sea 
Climates. You see that this vertical circulation is a great Cos- 
mical matter — not a mere local phenomenon, and not confined 
to the present time as the Gulf Stream is. It is a phenomenon 
which must have had its place in all Geological history. The 
Gulf Stream, and the superficial Arctic current which brings its 
water back again, constitute a horizontal circulation, the con- 
tinuance of which depends on the interruption of the Equa- 
torial Current by the coast-line of Central America. But 
wherever there were deep seas, and the Polar water and Equa- 
torial areas were in communication, there must have always 
been this vertical circulation. 
One very curious consequence of this vertical circulation, 
which I believe to be very important in relation to the Life of 
the ocean, is this — that by its means, if this doctrine be true, 
every drop of water in the ocean will, in its turn, be brought 
from the bottom and exposed to the surface. Now, in the 
Mediterranean there is no such circulation ; and we found in the 
great depths of the Mediterranean an extraordinary paucity of 
animal life, instead of finding the abundance which we en- 
countered in the great depths of the Atlantic. I will not say that 
this is the sole cause of the difference, but it has a good deal to 
