Mr. J. Croll on the Physical Cause of Ocean-currents. 169 



discuss, it was surely that, relating to the Gibraltar current; for 

 if the objection referred to be sound, it shows the mechanical 

 impossibility of his theory. It proves that whether there be an 

 under current or not, or whether the dense water lying in the 

 deep trough of the Mediterranean be carried over the submarine 

 ridge into the Atlantic or not, the explanation offered by Dr. 

 Carpenter is one which cannot be admitted. It is incumbent on 

 him to explain either (1) how the almost infinitesimal difference 

 of density which exists between the Atlantic and Mediterranean 

 columns down to the level of the ridge caa produce the upper 

 and under currents carrying the deep and dense water of the Me- 

 diterranean over the ridge, or (2) how all this can be done by 

 means of the difference of density which exists below the level 

 of the ridge. 



We shall now pass to the consideration of Dr. Carpenter's 

 objections to my conclusions regarding the influence of the Gulf- 

 stream on climate. 



Dr. Carpenter's Objections to my estimate of the Thermal power 

 of the Gulf-stream. 



"The doctrine of the extension of the Gulf-stream proper to 

 the polar area, carrying with it a vast amount of equatorial heat, 

 has been advocated with great ability by Mr. James Croll ; who, 

 employing the modern method of computing units of heat, essays 

 to prove that the quantity of heat carried from the equatorial 

 area by the Gulf-stream is so enormous, as to be competent not 

 only to do all that Dr. Petermann attributes to it, but a great 

 deal more" (§ 99). 



" Without attempting to follow Mr. Croll through his calcu- 

 lations, I may stop to point out what appear to me to be the 

 fallacies of his method ; since if this can be proved erroneous, 

 Mr. CrolPs great array of figures is utterly valueless" (§ 100). 



Now, in order to show that my " array of figures is utterly 

 valueless/' it is necessary to prove either that I have overesti- 

 mated the amount of heat conveyed by the Gulf-stream, or the 

 effects produced by that heat. This is evident ; for if I am cor- 

 rect both as to the amount of heat conveyed and the effects 

 produced by that heat, the figures must possess all the value 

 which I claim for them. 



We shall now consider how Dr. Carpenter manages to render 

 my estimate of the heat conveyed by the stream "utterly va- 

 lueless." 



" In the first place, in Mr. CrolPs preliminary comparison of 

 the temperatures of the northern and southern hemispheres, he 

 altogether ignores the influence on the distribution of heat 

 over the globe which is exerted by the great relative preponde- 



