Mr. J. Croll on the Physical Cause of Ocean-currents, 99 



of a good working hypothesis to be used as a guide in further 

 inquiry" (§ 16) ; and lest there should be any misapprehension 

 on this point, he closes his memoir thus: — "At present, as I 

 have already said, I claim for the doctrine of a general oceanic 

 circulation no higher a character than that of a good working 

 Jiypothesis consistent with our present knowledge of facts, and 

 therefore entitled to he provisionally adopted for the purpose of 

 stimulating and directing further inquiry." 



I am unable to agree with Dr. Carpenter on this latter point. 

 It seems to me that there is no necessity for adopting any hy- 

 pothetical mode of circulation to account for the facts, as they 

 can be quite well accounted for by means of that mode of circu- 

 lation which does actually exist. It has been determined from 

 direct observation that surface-currents flow from equatorial to 

 polar regions ; and their paths have been actually mapped out. 

 But if it is established that currents flow from equatorial to polar 

 regions, it is equally established that return currents flow from 

 polar to equatorial regions ; for if the one actually exists, the 

 other of necessity must exist. We know also on physical grounds, 

 to which I have already referred, and which fall to be considered 

 more fully in a subsequent part of this paper, that a very large 

 portion of the water flowing from polar to equatorial regions 

 must be in the form of under currents. If there are cold under 

 currents, therefore, flowing from polar to temperate and equa- 

 torial regions, this is all that we really require to account for the 

 cold water which is found to occupy the bed of the ocean in 

 those regions. It does not necessarily follow, because cold 

 water may be found at the bottom of the ocean all along the 

 equator, that there must be a direct flow from the polar regions 

 to every po'int of the equator. Water brought constantly from 

 the polar regions to various points along the equator by means 

 of under currents will necessarily accumulate, and in course of 

 time spread over the bottom of the intertropical seas. It must 

 either do this, or the currents on reaching the equator must 

 bend upwards and flow to the surface in an unbroken mass. 

 Considerable portions of some of those currents may no doubt 

 do so and join surface-currents ; but probably the greater portion 

 of the water coming from polar regions extends itself over 

 the floor of the equatorial seas. In a letter in ' Nature/ Jan. 11, 

 1872, I endeavoured to show that the surface-currents of the 

 ocean are not separate and independent of one another, but form 

 one grand system of circulation, and that the impelling cause 

 keeping up this system of circulation is not the trade-winds 

 alone, as is generally supposed, but the prevailing winds of the 

 entire globe considered also as one grand system. The evidence 

 for this opinion, however, will be considered more fully in the 

 next part of this paper. 



H2 



