Mr. J. Croll on the Physical Cause of Ocean-currents. 237 



"It is not necessary/' he says, "to associate with oceanic 

 currents the idea that they must of necessity, as on land, run 

 from a higher to a lower level. So far from this being the case, 

 some currents of the sea actually run up hill, while others run 

 on a level. The Gulf-stream is of the first class" (§ 403). "The 

 top of the Gulf-stream runs on a level with the ocean ; therefore 

 we know it is not a descending current " (§ 18). And in § 9 he 

 says that between the Straits of Florida and Cape Hatteras 

 the waters of the Gulf-stream " are actually forced up an inclined 

 plane, whose submarine ascent is not less than 10 inches to the 

 mile." To the same effect see §§ 25, 59. 



It is perfectly true that " it is not necessary to associate with 

 ocean-currents the idea that they must of necessity, as on land, 

 run from a higher to a lower level." But the reason of this is 

 that ocean-currents do not, like the currents on land, owe their 

 motion to the force of gravitation. If ocean-currents result from 

 difference of specific gravity between the waters in tropical and 

 polar regions, as Maury maintains, then it is necessary to assume 

 that they are descending currents. Whatever be the cause which 

 may give rise to a difference of specific gravity, the motion which 

 results from this difference is due wholly to the force of gravity; 

 but gravity can produce no motion unless the water descend. 



This fact must be particularly borne in mind while we are con- 

 sidering Maury's theory that currents are the result of differ- 

 ence of specific gravity. 



Ocean- currents, then, according to Maury, owe their existence 

 to the difference of specific gravity between the waters of inter- 

 tropical and polar regions. This difference of specific gravity he 

 attributes to two causes — (1) to difference as to temperature, (2) 

 to difference as to saltness. There are one or two causes of a 

 minor nature affecting the specific gravity of the sea, to which 

 Maury alludes ; but these two determine the general result. Let 

 us begin with the consideration of the first of these two causes, 

 viz. : — 



Difference of specific gravity resulting from difference of tempe- 

 rature. — Maury explains his views on this point by means of an 

 illustration. " Let us now suppose," he says, " that all the water 

 within the tropics, to the depth of one hundred fathoms, sud- 

 denly becomes oil. The aqueous equilibrium of the planet 

 would thereby be disturbed, and a general system of currents 

 and counter currents would be immediately commenced — the oil, 

 in an unbroken sheet on the surface, running toward the poles, 

 and the water, in an undercurrent, toward the equator. The 

 oil is supposed, as it reaches the polar basin, to be recon- 

 verted into water, and the water to become oil as it crosses 

 Cancer and Capricorn, rising to the surface in intertropical 



