1 76 Mr. J. Croll on the Physical Cause of Ocean-currents. 



cates of that theory should prove that I have overestimated 

 the thermal power of the Gulf-stream. This, however, can 

 only be done by detecting some error either in my computa- 

 tion or in the data on which it is based ; yet neither Dr. Car- 

 penter nor any one else, as far as I know, has challenged 

 the accuracy of my figures. The question at issue is the cor- 

 rectness of the data ; but the only part of the data which can 

 possibly admit of being questioned is my estimate of the volume 

 and temperature of the stream. Dr. Carpenter, however, does 

 not maintain that I have overestimated the temperature of the 

 stream ; on the contrary, he affirms that I have really under- 

 estimated it. " If we assume," remarks Dr. Carpenter, " the 

 limit of the stratum above 60° as that of the real Gulf- stream 

 current, we shall find its average temperature to be somewhat 

 higher than it has been stated by Mr. Croll, who seems to have 

 taken 65° as the average of the water flowing through the 

 entire channel. The average surface-temperature of the Florida 

 channel for the whole year is 80°; and we may fairly set the 

 average of the entire outgoing stream, down to the plane of 60°, 

 at 70°, instead of 65° as estimated by Mr. Croll" (§ 141). It 

 follows, then, that every pound of water of the Gulf-stream 

 actually conveys 5 units of heat more than I have estimated it 

 to do — the amount conveyed being 30 units instead of 25 units 

 as estimated by me. Consequently, if the Gulf-stream be equal 

 to that of a current of merely 41^ miles broad and 1000 feet 

 deep, flowing at the rate of 2 miles an hour, it will still convey 

 the estimated quantity of heat. But this estimate of the volume 

 of the stream, let it be observed, scarcely exceeds one third of 

 that given by Herschel, Maury, and Colding (Phil. Mag. Oct. 

 1871, p. 272), and little more than one half that assigned to it 

 by Mr. Laughton, and but very little exceeds that given by Mr. 

 Findlay *, an author whom few will consider likely to overrate 

 either the volume or heating-power of the stream. 



The important results obtained during the ' Challenger ' ex- 

 pedition have clearly proved that I have neither overestimated 

 the temperature nor the volume of the Gulf-stream. Between 

 Bermuda and Sandy Hook the stream is 60 miles broad and 

 600 feet deep, with a maximum velocity of from 3^ to 4 miles 

 an hour. If the mean velocity of the entire section amounts to 

 2\ miles an hour, which it probably does, the volume of the 

 stream must equal that given in my estimate f. But we have no 



* Mr. Findlay considers that the daily discharge does not exceed 333 

 cubic miles (Brit. Assoc. Kep. 1869, p. 160). My estimate makes it 

 378 cubic miles. Mr. Laughton's estimate is 630 cubic miles (Paper " On 

 Ocean-currents," Journ. of Hoyal United-Service Institution, vol. xv.). 



t Dr. Carpenter states (§ 140) that 48 miles per day is the mean 



