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



according to supposition, the winds blow in the direction of the 

 surface- currents. From all these causes the path of least re- 

 sistance to the return-current will, as a general rule, not be at 

 the surface, but at a very considerable depth below it. 



It is maintained also that the winds cannot produce a vertical 

 current except under some very peculiar conditions. We have 

 already seen that, according to Dr. Carpenter's theory, the ver- 

 tical motion is caused by the water flowing off the equatorial 

 column, down the slope, upon the polar column, thus destroy- 

 ing the equilibrium between the two by diminishing the weight 

 of the equatorial column and increasing the weight of the polar 

 column. In order that equilibrium may be restored, the polar 

 column sinks and the equatorial one rises. Now must not the 

 same effect occur, supposing the water to be transferred from the 

 one column to the other, by the influence of the winds instead of 

 by the influence of gravity ? The vertical descent and ascent of 

 these columns depend entirely upon the difference in their 

 weights, and not upon the nature of the agency which makes 

 this difference. So far as difference of weight is concerned, 

 2 feet of water, say, propelled down the slope from the equato- 

 rial column to the polar by the winds, will produce just the same 

 effect as though it had been propelled by gravity. If vertical 

 motion follows as a necessary consequence from a transference 

 of water from the equator to the poles by gravity, it follows 

 equally as a necessary consequence from the same transference 

 by the winds ; so that one is not at liberty to advocate a vertical 

 circulation in the one case and to deny it in the other. 



Dr. Carpenter, as well as Maury, maintains that currents pro- 

 duced by the winds cannot extend to any great depth. It is 

 certainly true that sudden commotions caused by storms do not 

 generally extend to great depths. Neither will winds of short 

 continuance produce a current extending far below the surface. 

 But prevailing winds which can produce such immense surface- 

 flow as that of the great equatorial currents of the globe and 

 the Gulf-stream, which follow definite directions, must commu- 

 nicate their motion to great depths, unless water be frictionless, 

 a thing which it is not. Suppose the upper layer of the ocean 

 to be forced on by the direct action of the winds with a constant 

 velocity of, say, four miles an hour, the layer immediately below 

 will be dragged along with a constant velocity somewhat less 

 than four miles an hour. The layer immediately below this se- 

 cond layer will in turn be also dragged along with a constant 

 velocity somewhat less than the one above it. The same will take 

 place in regard to each succeeding layer, the constant velocity of 

 each layer being somewhat less than the one immediately above it, 

 and greater than the one below it. The question to be determined 



