Mr. J. Croll on the Physical Cause of Ocean- currents. 105 



current which forms part of a general system of circulation pro- 

 duced by the impulse of the winds cannot possibly be an under 

 current ; for in the case referred to we have proof that the thing 

 is not only possible but actually exists. This point, however, 

 will be better understood after we have considered the evidence 

 in favour of a general system of oceanic currents. 



Much of the difficulty experienced in comprehending how 

 under currents can be produced by the wind, or how an impulse 

 imparted to the surface of the ocean can ever be transmitted to 

 the bottom, appears to me to result, to a considerable extent at 

 least, from a slight deception of the imagination. The thing 

 which impresses us most forcibly in regard to the ocean is its 

 profound depth. A mean depth of, say, three miles produces a 

 striking impression ; but if we could represent to the mind the 

 vast area of the ocean as correctly as we can its depth, shallow- 

 ness rather than depth would be the impression produced. If 

 in crossing a meadow we found a sheet of water one hundred 

 yards in diameter and only an inch in depth, we should not call 

 that a deep pool, we should call it a very shallow pool. The pro- 

 bability is that we should speak of it as simply a piece of ground 

 covered with a thin layer of water. Yet such a thin layer of 

 water would be a correct representation in miniature of the ocean; 

 for the ocean in relation to its superficial area is as shallow as 

 the pool of our illustration. In reference to such a pool or thin 

 film of water, we have no difficulty in conceiving how a disturb- 

 ance on its surface would be transmitted to its bottom. In fact 

 our difficulty is in conceiving how any disturbance extending 

 over its entire surface should not extend to the bottom. Now 

 if we could form as accurate a sensuous impression of the vast 

 area of the ocean as we do of such a pool, all our difficulty in 

 understanding how the impulses of the wind acting on the vast 

 area of the ocean should communicate motion down to its bot- 

 tom would disappear. 



The known condition of the ocean inconsistent with Dr. Car- 

 penter's hypothesis. — Dr. Carpenter says that he looks forward 

 with great satisfaction to the results of the inquiries which are 

 being prosecuted by the Circumnavigation Expedition, in the 

 hope that the facts brought to light may establish his theory of 

 a general oceanic circulation ; and he specifies certain of these 

 facts which, if found to be correct, will establish his theory. It 

 seems to me, however, that the facts to which he refers are just 

 as explicable on the theory of under currents as on the theory of 

 a general oceanic circulation. He begins by saying, "If the 

 views I have propounded be correct, it may be expected that 

 near the border of the great Antarctic ice-barrier a temperature 

 below 30° will be met with (as it has been by Parry, Martens, 



