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



show, will produce a tide in a globe composed of a substance 

 where no currents or general flow of the materials could pos- 

 sibly take place. 



Pressure as a Cause of circulation. — We shall now briefly refer 

 to the influence of pressure (the indirect effects of gravity) in 

 the production of the circulation under consideration. That 

 which causes the polar column C to descend and the equatorial 

 column W to ascend, as has repeatedly been remarked, is the 

 difference in the weight of the two columns. The efficient cause 

 in the production of the movement is, properly speaking, gra- 

 vity ; cold at the poles and heat at the equator, or, what is the 

 same thing, the excess of heat received by the equator over that 

 received by the poles is what maintains the difference of tempe- 

 rature between the two columns, and consequently is that also 

 which maintains the difference of weight between them. In 

 other words, difference of temperature is the cause which main- 

 tains the state of disturbed equilibrium. But the efficient cause 

 of the circulation in question is gravity. Gravity, however, 

 could not act without this state of disturbed equilibrium; and 

 difference of temperature may therefore be called, in relation to 

 the circulation, a necessary condition, while gravity may be 

 termed the cause. Gravity sinks column C directly, but it raises 

 column W indirectly by means of pressure. The same holds 

 true in regard to the motion of the bottom-waters from C to 

 W, which is likewise due to pressure. The pressure of the 

 excess of the weight of column C over that of column W im- 

 pels the bottom-water equatorwards and lifts the equatorial 

 column. But on this point I need not at present dwell, as I 

 have in my last paper entered into a full discussion as to how 

 this takes place*. 



We come now to the most important part of the inquiry, viz. 

 how is the surface-water impelled from the equator to the poles ? 

 Is pressure from behind the impelling force here as in the case 

 of the bottom-water of the ocean ? It seems to me that, in 

 attempting to account for the surface-flow from the equator to 

 the poles, Dr. Carpenter's theory signally fails. The force to 

 which he appeals appears to be wholly inadequate to produce the 

 required effect. 



The experiments of M. Dubuat, as already noticed, prove that 

 any slope which can possibly result from the difference of tem- 

 perature between the equator and the poles is wholly insufficient 

 to enable gravity to move the waters ; but it does not necessarily 

 prove that the pressure resulting from the raised water at the 

 equator may not be sufficient to produce motion. This point 

 will be better understood from the following figure, where, as 

 * Phil. Mag. for October 1871. 



