Mr. J. Croll on the Physical Cause of Ocean-currents. 173 



mences to prove something totally different, viz. that there are 

 a great many important causes affecting climate which I entirely 

 ignore or overlook, and that those causes which I ignore or 

 overlook have a far greater influence on climate than the heat 

 of the Gulf-stream. It may be perfectly true that there are a 

 great many important causes affecting climate which I have not 

 considered ; and it may likewise be true that those causes, left 

 out of consideration, have a far more powerful influence on cli- 

 mate than the heat conveyed by the Gulf-stream j nevertheless 

 it may be true also that all my statements regarding the in- 

 fluence of the Gulf-stream on climate are perfectly correct. It 

 does not necessarily follow that a horse may not possess a cer- 

 tain amount of strength, and be able to perform a certain 

 amount of work, simply because there are other horses which 

 possess a much greater amount of strength, and can perform a 

 much greater amount of work. 



What I have endeavoured to prove in reference to the Gulf- 

 stream is : — that the amount of heat conveyed by it is so enor- 

 mous as to be equal to one fourth of all the heat received from 

 the sun by the North Atlantic in temperate regions ; and that 

 were it not for the Gulf-stream and other ocean-currents, only 

 a very small portion of the globe would be suited to the present 

 orders of sentient beings * — that London, instead of possessing 

 a mean annual temperature of nearly 50°, would have a mean 

 temperature of not over 10°. But I never argued that there 

 were not other causes to which we are far more indebted than 

 to the heat of the Gulf-stream. Were it not for those other 

 causes, the temperature of London would not be simply 40°, 

 but upwards of 500° below what it is at present. 



The bearing which my estimate has on Dr. Carpenters Theory. 



There is one point to which I wish to direct special attention, 

 viz. the bearing which my conclusions regarding the quantity 

 of heat conveyed by the Gulf-stream have on Dr. Carpenter's 

 theory of a general interchange of equatorial and polar water. 

 But, in order better to understand this matter, it will be neces- 

 sary to refer very briefly to a point which has already been dis- 

 cussed at considerable length in former papers. In my earlier 

 paper on the amount of heat conveyed by the Gulf-stream f, I 

 estimated the volume of that stream as equal to that of a current 

 50 miles broad and 1000 feet deep, flowing (from the surface 

 to the bottom) at 4 miles an hour. Of course I did not mean, 

 as Dr. Carpenter seems to suppose, that the stream at any par- 



* Phil. Mag. for Feb. 1870. 



t Trans, of Geol. Soc. of Glasgow for April 1867 J Phil. Mag. for June 

 1867 (Supplement). 



