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



column derive its pressure? It can only press to the extent 

 that its weight exceeds that of the equatorial column. That 

 which exerts the pressure is therefore the mass of water which 

 has flowed down the slope from the equator upon the polar 

 column. It is in this case the vertical movement that causes 

 this undercurrent. The energy which produces this current 

 must consequently be derived from the 18 foot-pounds result- 

 ing from the slope ; for the energy of the vertical movement, 

 as has already been proved, is derived from this source; or, 

 in other words, whatever power this vertical movement may 

 exert is so much deducted from the 18 foot-pounds derived 

 from the full slope. 



Let us now consider the fourth and last movement, viz. the 

 ascent of the undercurrent to the surface of the ocean at the 

 equator. When this cold undercurrent reaches the equatorial 

 regions, it ascends to the surface to where it originally started 

 on its circuit. What, then, lifts the water from the bottom of 

 the equatorial column to its top ? This cannot be done directly, 

 either by heat or by gravity. When heat, for example, is ap- 

 plied to the bottom of a vessel, the heated water at the bottom 

 expands and, becoming lighter than the water above, rises 

 through it to the surface ; but if the heat be applied to the sur- 

 face of the water instead of to the bottom, the heat will not pro- 

 duce an ascending current. It will tend rather to prevent such 

 a current than to produce one — the reason being that each 

 successive layer of water will, on account of the heat applied, 

 become hotter and consequently lighter than the layer below it, 

 and colder and consequently heavier than the layer above it. 

 It therefore cannot ascend, because it is too heavy ; nor can it 

 descend, because it is too light. But the sea in equatorial re- 

 gions is heated from above, and not from below ; consequently 

 the water at the bottom does not rise to the surface at the equa- 

 tor in consequence of the heat which it receives. A layer of 

 water can never raise the temperature of a layer below it to a 

 higher temperature than itself; and since it cannot do this, it 

 cannot make the layer under it lighter than itself. That which 

 raises the water at the equator, according to Dr. Carpenter's 

 theory, must be the downward pressure of the polar column. 

 When water flows down the slope from the equator to the pole, 

 the polar column, as we have seen, becomes too heavy and the 

 equatorial column too light ; the former then sinks and the 

 latter rises. It is the sinking of the polar column which 

 raises the equatorial one. When the polar column descends, 

 as much water is pressed underneath the equatorial column as 

 is pressed from underneath the polar column. If 1 foot of water 

 is pressed from under the polar column, a foot of water is 



