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



" that the bulk of the water in the warm area must have come 

 thither from the south-west. The influence of the Gulf-stream 

 proper (meaning by this the body of superheated water which 

 issues through the 'Narrows' from the Gulf of Mexico), if it 

 reaches this locality at all (which is very doubtful), could only 

 affect the most superficial stratum ; and the same may be said of 

 the surface-drift caused by the prevalence of south-westerly 

 winds, to which some have attributed the phenomena usually 

 accounted for by the extension of the Gulf-stream to these 

 regions. And the presence of the body of water which lies 

 between 100 and 600 fathoms deep, and the range of whose 

 temperature is from 48° to 42°, can scarcely be accounted for 

 on any other hypothesis than that of a great general movement of 

 equatorial water towards the Polar area, of which movement the 

 Gulf-stream constitutes a peculiar case modified by local condi- 

 tions. In like manner the Arctic stream which underlies the 

 warm superficial stratum in our cold area constitutes a peculiar 

 case, modified by the local conditions to be presently explained, 

 of a great general movement of polar water towards the equatorial 

 area, which depresses the temperature of the deepest parts of 

 the great oceanic basins nearly to the freezing-point." 



In support of this theory of a general movement of water 

 between equatorial and polar regions, Dr. Carpenter adduces 

 the authority of Humboldt and of Prof. Buff*. I have been 

 unable to find any thing in the writings of either from which it 

 can be inferred that they have given this matter special con- 

 sideration. Humboldt merely alludes to the theory, and that 

 in the most casual manner ; and that Prof. Buff has not carefully 

 investigated the subject is apparent from the very illustration 

 quoted by Dr. Carpenter from the ' Physics of the Earth/ 

 " The water of the ocean at great depths," says Prof. Buff, " has 

 a temperature, even under the equator, nearly approaching to 

 the freezing-point. This low temperature cannot depend on 



any influence of the sea-bottom The fact, however, is 



explained by a continual current of cold water flowing from the 

 polar regions towards the equator. The following well-known 

 experiment clearly illustrates the manner of this movement. A 

 glass vessel is to be filled with water with which some powder 

 has been mixed, and is then to be heated at bottom. It will 

 soon be seen, from the motion of the particles of powder, that 

 currents are set up in opposite directions through the water. 

 Warm water rises from the bottom up through the middle of 

 the vessel, and spreads over the surface, while the colder and 

 therefore heavier liquid falls down at the sides of the glass." 

 This illustration is evidently intended to show not merely the 

 * Proceedings of the Royal Society, vol, xvii. p. 187, xviii. p. 4G3. 



