Mr. J. Croll on the Physical Cause of Ocean-currents. 267 



that I have determined simply the amount of the force acting 

 on the water at the surface of the ocean and not that on the 

 water at all depths — that I have estimated the amount of work 

 which gravity can perform on a given quantity of water at the 

 surface, but not the total amount of work which gravity can 

 perform on the entire ocean. This will not do as an objection, 

 because the surface of the ocean is the place where the greatest 

 difference of temperature, and consequently of density, exists be- 

 tween the equatorial and polar waters, and therefore it is here 

 that gravity exerts its greatest force. And if gravity be unable 

 to move the water at the surface, it is much less able to do so 

 under the surface. So far as the question at issue is concerned, 

 any calculations as to the amount of force exerted by gravity 

 at various depths are needless. 



Dr. Carpenter, in proof of his theory, adduces the fact that 

 at a depth of 2000 fathoms or so the ocean, from the poles to 

 the equator, has a temperature not above 32°. He concludes, 

 and that justly, that this low temperature at the equator is evi- 

 dence of the existence of an undercurrent from the poles to the 

 equator. But he maintains, like Maury and others, that such 

 an undercurrent could not result from the action of winds on the 

 surface of the ocean, but must be due to difference of specific 

 gravity. This opinion, I find, is supported by no less authori- 

 ties than Sir William Thomson and Professor Stokes. But with 

 all due deference to these great physicists, I am, for the follow- 

 ing reasons, unable to accept such an opinion. Suppose, for 

 argument's sake, that the Gulf-stream is caused by the winds, 

 and that it flows into the Arctic seas. It is evident that the 

 water which is thus being constantly carried from the intertro- 

 pical to the Arctic regions must in some way or other find its 

 way back to the equator ; in other words, there must be a re- 

 turn-current equal in magnitude to the direct current. Now 

 the question to be determined is what path must this return- 

 current take ? It appears to me that it will take the, path of least 

 resistance, whether that path may happen to be at the surface 

 or under the surface. But that the path of least resistance will, 

 as a general rule, lie at a very considerable distance below the 

 surface is, I think, evident from the following considerations. 

 At the surface the general direction of the currents is opposite 

 to that of the return-current. The surface-motion of the water 

 in the Atlantic is from the equator to the pole ; but the return- 

 current must be from the pole to the equator. Consequently 

 the surface-currents will oppose the motion of any return-cur- 

 rent unless that current lie at a considerable depth below the 

 surface-currents. Again, the winds, as a general rule, blow in 

 an opposite direction to the course of the return- current, because, 



