Mr. J. Croll on the Physical Cause of Ocean- currents. 273 



of the heating-power of the stream ; but it will be seen, by re- 

 ferring to a page a little further on than the one from which he 

 quotes, viz. to page 89, that, as Mr. Findlay had maintained 

 that I had doubled the actual volume*, I reduced it to one 

 half of my former estimate. My object for doing so was that I 

 might be enabled to show that, so far as my general conclu- 

 sions regarding the influence of the Gulf-stream were con- 

 cerned, it is a matter of indifference whether I adopt Mr. 

 Findlay's estimate or my own. The inference drawn in my 

 former paper regarding Dr. Carpenter's theory is therefore based 

 on the assumption that the volume of the stream is only one 

 half what he had supposed I had assumed it to be. 



He states that, in estimating the volume of the stream, I had 

 taken the velocity at the surface for the mean velocity. I am 

 unable to perceive on what grounds he was led to such a con- 

 clusion. I did not state that the Gulf-stream is anywhere fifty 

 miles broad and 1000 feet deep, and flowing at the rate of four 

 miles an hour. What I stated was that the quantity of water 

 conveyed is probably equal to that of a stream fifty miles broad, 

 1000 feet deep, and flowing of course at every point from the 

 surface to the bottom at the rate of four miles an hour; but I 

 never anticipated that any one would conclude from this that I 

 imagined the Gulf-stream actually flowed with the same velocity 

 at every point from the surface to the bottom. Such an opinion 

 as this I never held nor ever could have held. Most certainly 

 the velocity of the Gulf- stream, like that of currents in general, 

 diminishes from the surface, or from near to the surface, down- 

 wards. 



He states that I have also overestimated the mean tempera- 

 ture of the stream. I have taken the mean temperature of the 

 stream at the moment of its leaving the Gulf of Mexico at 65°; 

 and he says that this is too high an estimate. He states also 

 that what has misled me on this point is, that my esti- 

 mate seems based on the assumption that in proceeding from 

 above downwards the temperature descends uniformly between 

 the different points of observation (§ 40). And in order to 

 show that this is not the law of decrease of temperature down- 

 wards, he presents us with the Table of observations made in 

 the Mediterranean, to which reference has already been made. 

 And from this Table he concludes, and no doubt justly, that 

 the rate of decrease is greatest near to the surface of the stream 

 and diminishes as we proceed downwards. But supposing I had 

 adopted Dr. Carpenter's Table as representing the law accord- 

 ing to which the temperature of the water diminishes from the 

 surface of the stream downwards, I am still unable to perceive 



* Proceedings of the Royal Geographical Society, vol. xiii. p. 233. 

 Phil. Mag. S. 4. Vol. 42. No. 280. Oct. 1871. T 



