Mr. J. Croll on the Physical Cause of Ocean-currents. 253 



form and direction of the great system of oceanic circulation, 

 but also the way that the circulation is caused by heat. It is 

 no doubt true that if we apply heat (say that of a spirit- 

 lamp) to the bottom of a vessel filled with water, the water at 

 the bottom of the vessel will become heated and rise to the 

 surface; and if the heat be continued an ascending current of 

 warm water will be generated ; and this, of course, will give 

 rise to a compensating under current of colder water from all 

 sides. In like manner it is also true that, if heat were applied 

 to the bottom of the ocean in equatorial regions, an ascending 

 current of hot water would be also generated, giving rise to an 

 undercurrent of cold water from the polar regions. But all 

 this is the diametrically opposite of what actually takes place in 

 nature. The heat is not applied to the bottom of the ocean, so 

 as to make the water there lighter than the water at the surface, 

 and thus to generate an ascending current ; but the heat is 

 applied to the surface of the ocean, and the effect of this is to 

 prevent an ascending current rather than to produce one, for it 

 tends to keep the water at the surface lighter than the water at 

 the bottom. In order to show how the heat of the sun pro- 

 duces currents in the ocean, Prof. Buff should have applied the 

 heat, not to the bottom of his vessel, but to the upper surface of 

 the water. But this is not all, the form of the vessel has some- 

 thing to do with the matter. The wider we make the vessel 

 in proportion to its depth, the more difficult is it to produce 

 currents by means of heat. But in order to represent what 

 takes place in nature, we ought to have the same proportion 

 between the depth and the superficial area of the water in our 

 vessel as there is between the depth and the superficial area of 

 the sea. The mean depth of the sea, according to Sir John 

 Herschel, may be taken at about four miles *. It may be 

 somewhat more, or it may be somewhat less, than this; but that 

 will not materially affect our result. The distance between 

 pole and pole we shall take in round numbers as 12,000 miles. 

 The sun may therefore be regarded as shining upon a circular 

 sea 12,000 miles in diameter and four miles deep. The depth 

 of the sea to its diameter is therefore as 1 to 3000. Suppose, 

 now, that in our experiment we make the depth of our vessel 

 1 inch, we shall require to make its diameter 3000 inches, or 

 250 feet. Let us, then, take a pool of water 250 feet in dia- 

 meter, and 1 inch deep. Suppose the water to be at 32°. Apply 

 heat to the upper surface of the pool, so as to raise the tempe- 

 rature of the surface of the water to 80° at the centre of the 

 pool, the temperature diminishing towards the edge, where it is 

 at 32°. It is. found that at a depth of two miles the tempera- 

 * Physical Geography, article 17. 



