of Oceanic Circulation. 289 



source from which this influx of water can derive heat, save 

 from the earth's crust, which amount is so trifling* as to pro- 

 duce no sensible effect. 



It is therefore evident that the great mass of cold water occu- 

 pying the depths of the ocean cannot be urged as an objection 

 to the wind theory. 



2. But it is asserted that the impulse of the wind on the 

 surface of the ocean cannot produce and maintain deep under- 

 currents. This is an objection which has been urged by some 

 eminent physicists; but it is based upon a misapprehension of the 

 manner in which, according to the wind theory, under-currents 

 are produced. 



It is true, as the objectors maintain, that a wind simply im- 

 pelling the water forward will not necessarily produce an under- 

 current, since compensation will more readily take place by 

 return surface-currents, as in this case the path of least resis- 

 tance will generally be at the surface. But when the general 

 surface of one half of an ocean basin is being constantly impelled 

 forward by prevailing winds in a contrary direction to that in 

 which it is being impelled in the other half, compensation can- 

 not possibly take place by means of return surface-currents. For 

 a full discussion of this point I must refer the reader to my 

 work, ' Climate and Time/ Chap. XIII. 



It is, however, needless to advance arguments a priori against 

 the possibility of such under-currents; for we have actually 

 several well-known examples of such currents, the particulars of 

 which will also be found in the work to which I refer. 



3. But supposing it could be shown that the winds cannot 

 directly produce under-currents, it can nevertheless be demon- 

 strated that they can do so indirectly. A vertical circulation 

 filling the deep recesses of the ocean under the equator with 

 polar-cold water, follows as readily and truly from the wind 

 theory as it does from the gravitation theory. It has been 

 shown that the general tendency of the system of the winds is 

 to impel the surface- water of the equatorial regions into the 

 temperate and polar regions as rapidly as it is heated. But 

 such a transference of surface-water must tend to destroy static 

 equilibrium by making the equatorial too light and the tem- 

 perate and polar columns too heavy, as truly as though the 

 transference had taken place by means of difference of tempera- 

 ture. The effect must be to produce a constant ascent of the 

 equatorial column and an inflow of cold water below equal to 

 the outflow above. In short, the wind must produce a system 

 of circulation precisely the same as that supposed to take place 

 by difference of temperature. 



Phil. Mag. S. 4. Vol. 50. No. 331. Oct. 1875. U 



