OCEANIC CIRCULATION. 237 



in virtue of its reduction in temperature and increase 

 of density, its place being taken, not by the rising up 

 of water from beneath, but by an inflow of water from 

 the neighbouring area; and since sea-water becomes 

 continually heavier in proportion to its reduction of 

 temperature, this cooling action will go on without the 

 check which is interposed in the case of fresh water. 1 

 Thus the water becoming denser and heavier will 

 descend, and there will be a continual tendency to 

 the flowing off of its deepest portion into the warmer 

 area by which the polar basin is surrounded ; producing 

 a reduction in the level of the polar area, which must 

 create a fresh indraught of surface-water from the warmer 

 area around to supply its place. This, in its turn, 

 being subjected to the same cooling action, will descend 

 and flow off at the bottom, producing a fresh reduction 

 of level and a renewed indraught at the surface. 



Dr. Carpenter illustrated this theory, or rather the 

 combined action of polar cold and equatorial heat, by 

 an experiment, the plan of which had occurred also to 

 myself, and been described by me in conversation 

 somewhat earlier. A long narrow trough having glass 

 sides was filled with water, and a piece of ice was 

 wedged in at one end between its side plates just 

 beneath the top, whilst the surface of the water at the 

 other end was warmed by a piece of metal, of which a 

 part projected beyond the trough, and was heated by a 



1 Fresh water expands with reduction of temperature, near the 

 freezing point, and hence, becoming lighter, the descending motion above 

 described is interfered with in the case of fresh water. 



