556 



Dr. W. B. Carpenter on the 



[June 13 3 



when this chilled surface-film has reached the bottom, instead of limiting 

 itself to the area of superficial exposure to cold, it flows off laterally in 

 every direction ; and this lateral outflow will go on so long as the disturb- 

 ance of equilibrium is maintained by the cooling of the surface-films which 

 are successively drawn into the area. 



29. That such must be the case, appears to me so self-evident that I am 

 surprised that any person conversant with the principles of Physical Science 

 should hesitate in admitting it, still more that he should explicitly deny it. 

 But since others may feel the same difficulty as Mr. Croll, it may be worth 

 while for me to present the case in a form of yet more elementary simpli- 

 city. — Let us suppose two cylindrical vessels of equal size to be filled with 

 Sea-water to the same level, but the temperature of the water in one of 

 them (W) to be 60°, while that of the other (C) is 30°; it is obvious that 

 the pressure exerted by the latter column upon the bottom and sides of 

 the cylinder it occupies must be greater than that exerted by the former, 

 in proportion to its excess of Specific Gravity. If, now, communications be 

 opened between the two by transverse pipes at the top and bottom of each 

 cylinder, an outflow of cold water will take place by the lower pipe from C 

 to W, and an inflow of warm water by the upper pipe from "W to C, until 



equilibrium is restored ; which it will be by the transfer of the lower half 

 of the cold column from C to W, and of the upper half of the warm column 

 from W to C. But if the water which flows through the upper pipe from 

 W to C be subjected, on entering the top of C, to a surface-cold which 



diminishing rate, provided the action of Cold is continuously maintained; and would 

 thus continue until the whole of the water is reduced to the temperature of its greatest 

 density, when it would come to an end by the equalization of specific gravity through- 

 out. The application of Heat at the other end of the trough only serves to maintain the 

 circulation, by keeping up the difference of temperature which is necessary for the conti- 

 nued disturbance of the equilibrium which the circulation is always tending to restore. 

 The horizontal Row of the chilled water along the bottom of the trough is clearly due to 

 the greater downward and therefore lateral pressure of the column at the cold end, re- 

 sulting from its constantly renewed excess of density. 



