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



effect. For example, a polar under-current one half the size of 

 the Gulf-stream would be sufficient to keep the entire water of the 

 globe (below the stratum heated by the sun's rays) at an ice- 

 cold temperature. Internal heat would not be sufficient under 

 such circumstances to maintain the mass 1° Fahr. above the 

 temperature it possessed when it left the polar regions. 



(2) But suppose that this immense mass of cold water occu- 

 pying the great depths of the ocean were, as Dr. Carpenter 

 assumes it to be, in a state of constant motion towards the 

 equator, and that its sectional area were 900 times that of the 

 Gulf- stream, it would not therefore follow that the quantity of 

 water passing through this large sectional area must be greater 

 than that flowing through a sectional area of the Gulf-stream; 

 for the quantity of water flowing through this large sectional 

 area depends entirely on the rate of motion. 



(3) I am wholly unable to understand how it could be supposed 

 that this under-flow, according to my view, is set in motion by 

 the Gulf- stream, seeing that I have shown that the return under- 

 current is as much due to the impulse of the wind as the Gulf- 

 stream itself. 



I am also wholly unable to comprehend how Dr. Carpenter 

 should imagine, because the bottom-temperature of the South 

 Atlantic happens to be lower, and the polar water to lie nearer 

 to the surface in this ocean than in the North Atlantic, that 

 therefore this proves the truth of his theory. This condition 

 of matters is just as consistent with my theory as with his. When 

 we consider the immense quantity of warm surface-water which, 

 as has been proved, is being constantly transferred from the 

 South into the North Atlantic (a quantity which to a large ex- 

 tent is compensated by a cold current from the Antarctic regions), 

 we readily understand how the polar water comes nearer to the 

 surface in the former ocean than in the latter. In fact the whole 

 of the phenomena are just as easily explained upon the principle 

 of under-currents as upon Dr. Carpenter's theory. 



Dr. Carpenter lays considerable stress on the important fact 

 established by the ' Challenger ' expedition, that the great depths 

 of the sea in equatorial regions are occupied by ice-cold water, 

 while the portion heated by the sun's rays is simply a thin 

 stratum at the surface. It seems to me that it would be difficult 

 to find a fact more hostile to his theory than this. Were it not 

 for this upper stratum of heated water there would be no differ- 

 ence between the equatorial and polar columns, and consequently 

 nothing to produce motion. But the thinner this stratum is 

 the less is the difference, and the less there is to produce motion. 

 I have been favoured by the Hydrographer to the Admiralty 

 with a series of temperature-soundings taken along the equator ; 



