CHAP. VIII. | - THE GULF-STREAM. 373 
have the same effect as the application of heat to 
the bottom, and in either case we should have an 
instance of simple convection, the warmer under- 
water rising through a colder upper layer; but 
that is not what we have in the polar sea; for the 
temperature of the arctic sea gradually sinks from 
a few fathoms beneath the surface to a minimum 
temperature, and consequent maximum density, at 
the bottom. Therefore in this case the application 
of cold at the surface is not equivalent to the appli- 
cation of heat to the bottom in a hot-water heating 
apparatus, and Dr. Carpenter has shown that he is 
aware of this by requiring the backward propagation 
of a surface-current. 
That a certain effect in increase of specific gravity 
must be produced by the cooling of the surface film 
of the arctic ocean there seems to be little doubt; 
but the area of maximum effect is very limited, and 
during the long arctic winter the greater part of that 
area is protected by a thick layer of ice, one of the 
worst possible conductors. 
It certainly appears to me that this cause is 
totally inadequate to induce a powerful current of 
great depth, six thousand miles long and several 
thousand miles in width, the effect which Dr. Car- 
penter attributes to it. 
During the summer of 1870, and afterwards in 
1871, Dr. Carpenter made a series of observations on 
the current in the Strait of Gibraltar. The existence 
of an under-current out of the Mediterranean was 
considered to be established by these observations, 
and the conclusions arrived at as to its cause did not 
differ materially from those already very generally 
