Mr. J. Croll on the Physical Cause of Ocean-currents. 269 



is, at what depth will all motion cease ? I presume that at present 

 we have not sufficient data for properly determining this point. 

 The depth depends upon the amount of molecular resistance 

 offered by the water to motion — in other words, on the amount 

 of the shearing- force of the one layer over the other. The 

 fact, however, that motion imparted to the surface will extend 

 to great depths can be easily shown by direct experiment. If 

 a constant motion be imparted to the surface of water, say, 

 iD a vessel, motion will ultimately be communicated to the 

 bottom, no matter how wide or how deep the vessel may be. 

 The same effect will take place whether the vessel be 5 feet 

 deep or 500 feet deep. 



But it does not follow that though a current, such as the 

 Gulf-stream, impelled by the influence of the winds may not 

 extend to any great depth, the water which it conveys may 

 not descend to the bottom of the ocean. The water of the Gulf- 

 stream is much Salter than the water of the North Atlantic. 

 Now by the time that this salt water reaches the shores of, say, 

 Iceland or the Shetlands, its temperature will be so far reduced 

 that though it may be 10° or 12° higher than the polar water 

 underneath^ still, in consequence of its greater saltness, it may be 

 actually denser, and on this account may sink to the bottom and 

 displace the polar water. In short, the difference in density 

 between the salt water of the Gulf- stream, by the time that it 

 reaches our shores, and the cold polar water is so infinitesimal 

 that it can exercise but little or no influence in determining the 

 position of the current. The fact pointed out by Professor 

 Wyville Thomson must also be borne in mind, viz. that the 

 Gulf-stream flows into an almost closed basin. 



The Gibraltar Current. 



If difference of specific gravity fails in accounting for the cur- 

 rents of the ocean in general, it certainly fails in a still more de- 

 cided manner in accounting for the Gibraltar current. The 

 existence of the submarine ridge crossing between Capes Tra- 

 falgar and Spartel affects currents resulting from difference of 

 specific gravity in a manner which does not seem to have sug- 

 gested itself to Dr. Carpenter. The pressure of water and other 

 fluids is not like that of a solid, say, the weight in the scale of 

 a balance, simply a downward pressure. Fluids press down- 

 wards like the solids, but they also press laterally. The pres- 

 sure of water is hydrostatic. If we fill a basin with water or 

 any other fluid, the fluid remains in perfect equilibrium, pro- 

 vided the sides of the basin be sufficiently strong to resist the 

 pressure. The Mediterranean and Atlantic, up to the level of 

 the submarine ridge referred to, may be regarded as huge 



