TEMPERATURE AND MOVEMENTS OF THE DEEP SEA. 
133 
the maintenance of a greater weight or downward pressure 
ofw ater in the Polar area ; so by these two influences — the 
lowering of the level, and the increase of the density of the 
column — we have this constant disturbance of level and dis- 
turbance of equilibrium, producing an inflow from the Equa- 
torial towards the Polar regions on the surface, and an outflow 
from the Polar towards the Equatorial area at the bottom. 
This is the doctrine of the General Oceanic Circulation to 
which I have been led. I say 44 I,” because it has happened that 
I have been the member of the Expedition to whose share this 
part of the inquiry fell, and I have applied myself to all the 
points bearing upon it. I have taken the opinion of some of 
the most eminent Mathematicians and Physicists of this coun- 
try, with regard to the validity of the principles I have ad- 
vanced ; and I am glad to say that I do not bring them forward 
merely on my own authority, but am assured that this doc- 
trine will stand the test of very rigid inquiry. A short time 
before the death of Sir John Herschel, I had the satisfaction 
of receiving a letter from him, fully accepting the doctrine I 
have propounded ; and his acceptance is the more significant, 
since he had previously repudiated the doctrine of Captain 
Maury, that an Oceanic Circulation (of which he regarded the 
Gulf Stream as a part) is maintained by the expansion pro- 
duced by Equatorial heat. 
64 Assuredly,” wrote Sir John Herschel, 44 after well consider- 
ing all you say, as well as the common sense of the matter, and 
the experience of our hot water circulation-pipes in our green- 
houses, &c., there is no refusing to admit that an Oceanic cir- 
culation of some sort must arise from mere Heat, Cold, and 
Evaporation, as verce causce ; and you have brought forward 
with singular emphasis the more powerful action of the Polar 
Cold, or rather the more intense action, as its maximum effect 
is limited to a much smaller area than that of the maximum of 
Equatorial Heat. The action of the Trade and Counter-trade 
Winds in like manner cannot be ignored ; and henceforward 
the question of Ocean-currents will have to be considered under 
a twofold point of view ” — namely, as he goes on to explain, the 
horizontal circulation produced by the action of Wind on the 
surface, and the vertical circulation dependent on opposition of 
Temperature. 
At a meeting of the Royal Geographical Society I was 
enabled, by the kindness of Dr. Odling, to exhibit an illustrative 
experiment, which was considered extremely satisfactory ; and 
I think I can explain it to you in such a manner that you will 
easily understand its value. We had a trough, with plate-glass 
sides, about six feet long and a foot deep, and the sides not 
more than one inch from each other. At one end of this trough 
