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



of surface-films from above downwards, but it will be a down- 

 ward movement of the entire mass, as if water in a tall jar were 

 being drawn off through an orifice at the bottom " (§ 29). There 

 is a downward motion of the entire column, producing an out- 

 flow of water at the bottom towards the equatorial column W, 

 which outflow is compensated by an inflow from the top of the 

 equatorial column to the top of the polar column C. But what 

 causes column C to descend ? The cause of the descent is its 

 excess of weight over that of column W. Column C descends 

 and column W ascends, for the same reason that in a balance 

 the heavy scale descends and the light scale rises. Column C 

 descends not simply because it is cold, but because it is colder 

 than column W. Column C descends not simply because in 

 consequence of being cold it is dense and therefore heavy, but 

 because in consequence of being cold it is denser and therefore 

 heavier than column W. It might be as cold as frozen mer- 

 cury and as heavy as lead ; but it would not on that account 

 descend unless it were heavier than column W. The descent 

 of column C and ascent of column W, and consequently the 

 general oceanic circulation, results, therefore, according to Dr. 

 Carpenter's explanation, from the difference in the weights of 

 the two columns ; and the difference in the weights of the two 

 columns results from their difference of density ; and the differ- 

 ence of density of the two columns in turn results from their 

 difference of temperature. But it has already been proved that 

 the difference of temperature between the polar and equatorial 

 columns depends wholly on the difference in the amount of heat 

 received by each from the sun. The equatorial column W pos- 

 sesses more heat than the polar column C, solely because it re- 

 ceives more heat from the sun than column C. Consequently 

 Dr. Carpenter's statement that the circulation is produced by 

 polar cold rather than by equatorial heat, is just as much in con- 

 tradiction to his own theory as it is to the principles of mecha- 

 nics. Again, his admission that the general oceanic circulation 

 " cannot be produced by the application of heat to the surface," 

 is virtually a giving up the whole point in debate ; for according 

 to his gravitation theory, and every form of that theory, the cir- 

 culation results from difference of temperature between equatorial 

 and polar seas ; but this difference, as we have seen, is entirely 

 owing to the difference in the amount of heat received from the 

 sun at these two places. The heat received, however, is ". surface- 

 heat;" for it is at the surface that the ocean receives all its heat 

 from the sun ; and consequently if surface-heat cannot produce 

 the effect required, nothing else can. 



M. Dubuat's experiments. — Referring to the experiments of 

 M. Dubuat adduced by me to show that water would not run 



