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



The explanation goes further : — " It is along the surface and 

 upper portion of the ocean that the equatorial waters flow towards 

 the poles, and it is along the bottom and under portion of the 

 ocean that polar waters flow towards the equator; or, in 

 other words, the warm water keeps the upper portion of the 

 ocean and the cold water the under portion/'' With this 

 explanation I to a great extent agree. It is evident that, 

 in reference to the northern hemisphere at least, the most 

 of the water which flows from intertropical to polar regions (as, 

 for example, the Gulf-stream) keeps to the surface and upper 

 portion of the ocean ; but, for reasons which I have stated in 

 my last paper*, a very large proportion of this water must return 

 in the form of under currents ; or, which is the same thing, the 

 return compensating current, whether it consist of the actual 

 water which originally came from the equator or not, must flow 

 towardsl the equator as an under current. That the cold water 

 which is found at the bottom of the Atlantic and of intertropical 

 seas must have come as under currents is perfectly obvious, be- 

 cause water which should come along the surface of the ocean 

 from the polar regions would not be cold when it reached in- 

 tertropical regions. 



The explanation hypothetical. — Here the general agreement 

 between us in a great measure terminates ; for Dr. Carpenter is 

 not satisfied with the explanation generally adopted by the ad- 

 vocates of the wind theory, viz. that the cold water found in tem- 

 perate and intertropical areas comes from polar regions as com- 

 pensating under currents, but advances a hypothetical form- of 

 circulation to account for the phenomenon. He assumes that 

 there is a general set or flow of the surface and upper portion of 

 the ocean from the equator to polar regions, and a general set or 

 flow of the bottom and under portion of the ocean from polar 

 regions to the equator. Mr. Ferrel ('Nature/ June 13, 1872) 

 speaks of that " interchanging motion of the water between the 

 equator and the pole discovered by Dr. Carpenter/' In this, 

 however, Mr. Ferrel is mistaken ; for Dr. Carpenter not only 

 makes no claim to any discovery of the kind, but distinctly ad- 

 mits that none such has yet been made. Although in some of 

 his papers he speaks of a " set of warm surface-water in the 

 southern oceans toward the Antarctic pole" as being well known 

 to navigators, yet he nowhere affirms, as far as I know, that the 

 existence of such a general oceanic circulation as he advocates 

 has ever been directly determined from observations. This 

 mode of circulation is simply inferred or assumed in order to 

 account for the facts referred to above. "At present," Dr. 

 Carpenter says, " I claim for it no higher character than that 

 * Phil. Mag. for October 1871, p. 267. 



