CHAP. viii. J 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 ; hut 

 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 



