XI.] 



THE SEA AND ITS WORK. 



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in which heat is propagated through a body of liquid, and 

 the process is called coiivedion,^ to distinguish it from con- 

 duction, or the method by which heat is propagated through 

 solid bodies. In conduction, the heat is passed on from 

 particle to particle, and thus travels through the mass, while^ 

 in convection, the heated particles themselves move. Again, 

 if a piece of ice be dropped into a tumbler of slightly warm 

 water, a system of currents will also be established, as in 

 Fig. 47. From the bottom of the piece of ice a clear 





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Fig. 46. — Currents in water by heat. 



Fig. 47. — Currents in water by cold. 



stream of heavy cold liquid flows down the middle of the 

 glass, like a stream of clear oil, while the neighbouring 

 water, which is comparatively warm, flows upwards in 

 currents nearer to the sides of the vessel. 



Unequal cooling or heating of the great natural masses 

 of water will be competent to produce a circulation similar 

 to that just described. During the recent voyage of the 

 Challenger the temperature of the sea at different depths 



^ Convection, from Lat. con, and veho, I cairy • tlie heat being 

 carried by currents through the fluid mass. 



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