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



ture of the water at the equator is about as low as that of the 

 poles. We must therefore suppose the water at the centre of 

 our pool to diminish in temperature from the surface down- 

 wards, so that at a depth of half an inch the water is at 32°. 

 We have in this case a thin layer of warm water half an inch 

 thick at the centre, and gradually thinning off to nothing at 

 the edge of the pool. The lightest water, be it observed, is at 

 the surface, so that an ascending or a descending current is 

 impossible. The only way whereby the heat applied can have 

 any tendency to produce motion is this : — The heating of the 

 water expands it, consequently the surface of the pool must 

 stand at a little higher level at its centre than at its edge, where 

 no expansion takes place; and therefore, in order to restore 

 the level of the pool, the water at the centre will tend to flow 

 towards the sides. But what is the amount of this tendency ? 

 Is it sufficient to overcome the molecular resistance of the water 

 to go into motion ? The amount of this tendency depends upon 

 the amount of the slope. We have already seen that unless the 

 slope exceeds 1 in 1,000,000, no motion can take place; but 

 the slope in the case under consideration amounts to only 1 in 

 1,820,000 ; consequently motion is absolutely impossible. 



That the great masses of warm water found by Dr. Carpenter 

 in the North Atlantic cannot be due to currents produced by 

 difference of temperature, as he supposes, can be proved in an- 

 other way. 



According to his theory there ought to be as much warm 

 water flowing from intertropical regions towards the Antarctic 

 regions as towards the Arctic. We may therefore, in our calcu- 

 lations, consider that the heat which is received in tropical 

 regions to the south of the equator goes to warm the southern 

 hemisphere, and the heat which it receives on the north side of 

 the equator goes to warm the northern hemisphere. The warm 

 currents found in the North Atlantic in temperate regions we 

 may conclude came from the regions lying to the north of the 

 equator — or, in other words, from that part of the Atlantic lying 

 between the equator and the tropic of Cancer. At least, accord- 

 ing to Dr. Carpenter's theory, we have no reason to believe 

 that the quantity of warm water flowing from the tropical re- 

 gions to the temperate and polar in the Atlantic is greater than 

 the area between the equator and the tropic of Cancer can sup- 

 ply — because he maintains that a very large proportion of the 

 cold water found in the North Atlantic came, not from the 

 Arctic, but from the Antarctic regions. But if the North At- 

 lantic is cooled by a cold stream from the southern hemisphere, 

 the southern hemisphere in turn must be heated by a warm cur- 

 rent from the North Atlantic— unless we assume, which is very 



