Mr. J. Croll on the Physical Cause of Ocean-currents. 109 



Dr. Carpenter then refers to a point so obvious as hardly to 

 require consideration, viz. the effect which results when the sur- 

 face of the entire area of a lake or pond of water is cooled. The 

 whole of the surface-film being chilled at the same time, sinks 

 through the subjacent water, and a new film from the warmer 

 layer immediately beneath the surface rises into its place. This 

 being cooled in its turn, sinks, and so on. He next considers 

 what takes place when only a portion of the surface of the pond 

 is cooled, and shows that in this case the surface-film which 

 descends is replaced not from beneath, but by an inflow from 

 the neighbouring area. 



" That such must be the case," says Dr. Carpenter, " appears 

 to me so self-evident that I am surprised that any person con- 

 versant 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 simplicity" (§ 29). 



Then, in order to show the mode in which the general oceanic 

 circulation takes place, he supposes two cylindrical vessels, W 

 and C, of equal size to be filled with sea- water. Cylinder, W re- 

 presents the equatorial column, and the water contained in it 

 has its temperature maintained at 60°; whilst the water in the 

 other cylinder C, representing the polar column, has its tempe- 

 rature maintained at 30° by means of the constant application 

 of cold at the top. Free communication is maintained between 

 the two cylinders at top and bottom ; and the water in the cold 

 cylinder being, in virtue of its low temperature, denser than the 

 water in the warm cylinder, the two colums are therefore not in 

 static equilibrium. The cold, and hence heavier column tends 

 to produce an outflow of water from its bottom to the bottom of 

 the warm column, which outflow is replaced by an inflow from 

 the top of the warm column to the top of the cold column. In 

 fact we have just a simple repetition of what he has given over 

 and over again in his various memoirs on the subject. But why 

 so repeatedly enter into the modus operandi of the matter ? Who 

 feels any difficulty in understanding how the circulation is 

 produced ? 



Polar Cold considered by Dr. Carpenter the primum mobile. 

 — It is evident that Dr. Carpenter believes that he has found in 

 polar cold an agency the potency of which, in producing a 

 general oceanic circulation, has been overlooked by physicists ; 

 and it is with the view of developing his ideas on this subject 

 that he has entered so fully and so frequently into the exposi- 

 tion of his theory. "If I have myself done any thing," he 

 says, " to strengthen the doctrine, it has been by showing that 



