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



ordinary observation, which he advocates. The conclusion at 

 which I arrived was that the motive force resulting from the 

 difference of specific gravity is equal to one fourth of a grain 

 on the cubic foot (63 lbs.) ; and I also adduced evidence to 

 show that a force so infinitesimal is not only insufficient to 

 move the water at the rate of several miles an hour, as in 

 the case of the Gulf-stream and other currents, but is actu- 

 ally insufficient to produce any sensible motion whatever, — in 

 short, that Dr. Carpenter's theory of a general interchange of 

 Equatorial and Polar water resulting from difference of spe- 

 cific gravity is quite as physically impossible as Lieut. Maury's 

 theory. But as Dr. Carpenter, in the memoir referred to*, has 

 stated his theory with much fulness, and supported his positions 

 with much ingenuity of argument, it will be necessary for me to 

 enter a little more minutely into some of the points under dis- 

 cussion before considering the influence of the winds as a cause 

 of currents. This will, undoubtedly, be best effected by an ex- 

 amination of Dr. Carpenter's arguments. Until these are shown 

 to be insufficient to support the theory maintained by him, it is 

 needless to begin the consideration of the effects of the winds. 

 And as he has in his memoir done me the honour to discuss 

 at considerable length some of my objections to his theory, I 

 trust that it will not be deemed either discourteous or presump- 

 tuous in me that I should enter somewhat more fully into the 

 subject. 



There are three ways whereby it may be determined whether 

 or not the circulation of the ocean is due to difference in specific 

 gravity : viz. (1) the matter may be determined by direct expe- 

 riment; (2) it may be determined by ascertaining the absolute 

 amount of force acting on the water to produce motion, in virtue 

 of difference of specific gravity, and then comparing it with the 

 force which has been shown by experiment to be necessary to the 

 production of sensible motion ; or (3) by determining the greatest 

 possible amount of work which gravity can perform on the waters 

 in virtue of difference of specific gravity, and then ascertaining if 

 the work of gravity does or does not equal the work of the resist- 

 ances in the required motion. It is strange that Dr. Carpenter 

 nor any of the advocates of the gravitation theory have ever 

 adopted any of these methods. Dr. Carpenter seems to take for 

 granted that a circulation of the ocean similar to what he advo- 

 cates is a " physical necessity." But this is the very point at 

 issue. If the work of gravity so far exceeds the work of the re- 

 sistances in such a motion of the waters as that supposed by Dr. 

 Carpenter, then, indeed, such a motion is a physical necessity ; 



* "On the Gibraltar Current, the Gulf-stream, and the General Oceanic 

 Circulation," Proceedings of the Royal Geographical Society, vol. xv. p. 54. 



