Mr. J. Croll on the Physical Cause of Ocean-currents. 259 



pressed under the equatorial column. Thus, when the polar 

 column sinks a foot, the equatorial column rises by the same 

 extent. The equatorial water continuing to flow down the 

 slope, the polar column descends; a foot of water is again 

 pressed from underneath the polar column and a foot pressed 

 under the equatorial. As foot after foot is thus removed 

 from the bottom of the polar column while it sinks, foot after 

 foot is pushed under the equatorial column while it rises ; so 

 by this means the water at the surface of the ocean in polar re- 

 gions descends to the bottom, and the water at the bottom in 

 equatorial regions ascends to the surface — the effect of solar 

 heat and polar cold continuing, of course, to maintain the sur- 

 face of the ocean in equatorial regions at a higher level than at 

 the poles, and thus keeping up a constant state of disturbed 

 equilibrium. Or, to state the matter in Dr. Carpenter's own 

 words, " The cold and dense polar water, as it flows in at the 

 bottom of the equatorial column, will not directly take the place 

 of that which has been drafted off from the surface ; but this 

 place will be filled by the rising of the whole superincumbent 

 column, which, being warmer, is also lighter than the cold 

 stratum beneath. Every new arrival from the poles will take 

 its place below that which precedes it, since its temperature will 

 have been less affected by contact with the warmer water above 

 it. In this way an ascending movement will be imparted to 

 the whole equatorial column, and in due course every portion 

 of it will come under the influence of the surface-heat of the 

 sun." (Proceedings of the Royal Society, vol. xix. p. 215.) 



But the agency which raises the water of the undercurrent up 

 to the surface is the pressure of the polar column. The equa- 

 torial column cannot rise directly by means of gravity. Gra- 

 vity, instead of raising the column, exerts all its powers to pre- 

 vent its rising. Gravity here is a force acting against the 

 current. It is the descent of the polar column, as has been 

 stated, that raises the equatorial column. Consequently the 

 entire amount of w T ork performed by gravity in pulling down 

 the polar column is spent in raising the equatorial column. 

 Gravity performs exactly as much work in preventing motion 

 in the equatorial column as it performs in producing motion in 

 the polar column ; so that, so far as the vertical parts of Dr. 

 Carpenter's circulation are concerned, gravity may be said 

 neither to produce motion nor to prevent it. And this re- 

 mark, be it observed, applies not only to P and E Q, but also 

 to the parts P' P and E E' of the two columns. When a mass 

 of water EE', say, 2 feet deep, is removed off the equatorial 

 column and placed upon the polar column, the latter column 

 is then heavier than the former by the weight of 4 feet of water. 



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