372 THE DEPTHS OF THE SEA. [cHAP. VIII. 
penter in a lecture to the Royal Geographical 
Society, in an illustration drawn from two supposed 
basins, one under equatorial conditions and the other 
under polar, connected by a strait,’ says: “ The effect 
of surface-heat upon the water of the tropical basin 
will be for the most part limited to its uppermost 
stratum, and may here be practically disregarded. 
But the effect of surface-cold upon the water of the 
polar basin will be to reduce the temperature of its 
whole mass below the freezing-point of fresh water, 
the surface stratum sinking as it is cooled, by virtue 
of its diminished bulk and increased density, and being 
replaced by water not yet cooled to the same degree. 
The warmer water will not come up from below, but 
will be drawn into the basin from the surface of the 
surrounding area; and since what is thus drawn 
away must be supplied from a yet greater distance, 
the continual cooling of the surface stratum in the 
polar basin will cause a ‘set’ of water towards it to 
be propagated backwards through the- whole inter- 
vening ocean in connection with it, until it reaches 
the tropical area.’’ And further on in the same 
address: “It is seen that the application of cold at 
the surface is precisely equivalent as a moving power 
to that application of heat at the bottom by which 
the circulation of water is sustained in every heating 
apparatus that makes use of it.” No doubt the 
application of cold to the surface of a mass of water 
previously at the same temperature throughout, would 
1 On the Gibraltar Current, the Gulf-stream, and the general 
Oceanic Circulation. By Dr. W. B. Carpenter, F.R.S. Reprinted 
from the Proceedings of the Royal Geographical Society of London, 
1870. 
