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



ocean-currents being the result of difference of specific gravity 

 caused by a difference of temperature and difference of saltness ? 

 Here is a distinct recognition of the fact that difference in salt- 

 ness, instead of producing currents, tends rather to prevent the 

 existence of currents, by counteracting the effects of difference 

 in temperature. And so effectually does it do this, that for 40°, 

 or nearly 3000 miles, on each side of the equator there is abso- 

 lutely no difference in the specific gravity of the ocean, and 

 consequently nothing, either as regards difference of tempera- 

 ture or difference of saltness, that can possibly give rise to a 

 current. 



But it is evident that, if between the equator and latitude 

 40° the two effects completely neutralize each other, it is not at 

 all likely that between latitude 40° and the poles they will not 

 to a very large extent do the same thing. And if so, how can 

 ocean-currents be due either to difference in temperature or to 

 difference in saltness, far less to both. If there be any differ- 

 ence of specific gravity of the ocean between latitude 40° and the 

 poles, it must be only to the extent by which the one cause has 

 failed to neutralize the other. If, for example, the waters in 

 latitude 40°, by virtue of higher temperature, are less dense 

 than the waters in the polar regions, they can be so only to the 

 extent that difference in saltness has failed to neutralize the 

 effect of difference in temperature. And if currents result, they 

 can do so only to the extent that difference in saltness has thus 

 fallen short of being able to produce complete compensation. 

 Maury, after stating his views on compensation, seems to become 

 aware of this ; but, strangely, he does not appear to perceive, or, 

 at least, he does not make any allusion to the fact, that all this 

 is fatal to the theories he had been advancing about ocean -cur- 

 rents being the combined result of differences of temperature 

 and difference of saltness. For, in opposition to all that he had 

 previously advanced regarding the difficulty of finding a cause 

 sufficiently powerful to account for such currents as the Gulf- 

 stream, and the great importance that difference in saltness 

 had in the production of currents, he now begins to maintain 

 that so great is the influence of difference in temperature in 

 causing currents that difference in saltness, and a number of 

 other compensating causes are actually necessary to prevent the 

 ocean- currents from becoming too powerful. 



" If all the intertropical heat of the sun/' he says, "were to 

 pass into the seas upon which it falls, simply raising the tem- 

 perature of their waters, it would create a thermo-dynamical 

 force in the ocean capable of transporting water scalding hot 

 from the torrid zone, and spreading it while still in the tepid 

 state around the poles .... Now, suppose there were no 



