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



density, though we have only 10 feet as the height of the 

 ocean at the equator above the place of maximum density, 

 this place is at lat. 56° instead of at the pole. Now a slope of 

 10 feet from the equator to lat. 56° is about the same in steep- 

 ness as a slope of 18 feet from the equator to the pole. It there- 

 fore follows that Herschel, in taking 39° as the temperature of 

 maximum density, did not underestimate the effect of gravity. 



Dr. Carpenter nowhere, so far as I am aware, calls in question 

 my estimate of the height at which the surface of the ocean at 

 the equator stands above that at the poles. He does not say that 

 the slope from the equator to the poles is greater than I have 

 estimated it to be, nor does he say that I have underestimated 

 the force of gravity impelling the water down this slope, in con- 

 cluding it to be only one fourth of a grain on one cubic foot 

 (63 lbs.) of water. Neither does he affirm that a force so infi- 

 nitesimal could produce the necessary circulation. On the con- 

 traryj he admits that it would not*, and says that I justly main- 

 tain that a circulation could not be sustained by this means. 

 He appears to admit my results so far as I have gone, but main- 

 tains that I have not gone far enough. I have justly estimated 

 the effects of heat, but I have, he says, entirely ignored the 

 agency of cold. "Mr. Croll/' he says, "in arguing against 

 the doctrine of a General Oceanic Circulation sustained by dif- 

 ference of temperature, and justly maintaining that such a cir- 

 culation cannot be produced by the application of heat at the 

 surface, has entirely ignored this agency of cold " *. And this 

 agency of cold which I have ignored he considers is of far greater 

 importance than the one which I advocate. The agency of cold 

 he regards as the primum mobile of the general oceanic cir- 

 culation. 



But surely Dr. Carpenter is mistaken in supposing that I 

 have entirely ignored the agency of cold. In what I have ad- 

 vanced, as much has been attributed to cold as to heat. The 

 height of the surface of the ocean at the equator above the sur- 

 face at the poles is, in my opinion, as much due to cold as to 

 heat. The slope is due to the difference of density between the 

 equatorial and polar waters. Now this difference is just as 

 much the result of the contraction of the polar water by cold as 

 of the expansion of equatorial water by heat. 



It is evident that the agency of cold referred to by Dr. Car- 

 penter, which I have ignored, must be something else than the 

 mere influence of cold in the production of the slope by the con- 

 traction of the polar waters. The cold, according to him, must 

 exercise some power in the way of producing motion of the water 

 over and above what is derived in virtue of the slope; and 

 * See footnote to § 25. 



