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



force corresponding to such a slope (of l-10th of a second, 0"'l) 

 is less than one two-millionth part of gravity, we may dismiss this 

 as a cause capable of creating only a very trifling surface-drift, 

 and not worth considering, even were it in the proper direction 

 to form, by concentration, a current from east to west, which it 

 would not be, but the very reverse" — Physical Geography 3 ar- 

 ticle 57.) 



It is singular how any one, even though he regarded this 

 conclusion as but a rough approximation to the truth, could 

 entertain the idea that ocean -currents can be the result of dif- 

 ference in specific gravity. There are, however, one or two 

 reasons which may be assigned why the above has not been 

 generally received as conclusive. These calculations refer to 

 the difference of gravity resulting from difference of tempera- 

 ture ; but this is only one of the causes to which Maury ap- 

 peals, and even not the one to which he most frequently alludes. 

 Maury insists so strongly on the effects of difference of saltness, 

 that many would no doubt suppose that, although Herschel may 

 have shown that difference in specific gravity arising from dif- 

 ference of temperature could not account for the motion of 

 ocean-currents, yet nevertheless this, combined with the 

 effects resulting from difference in saltness, might account for 

 their motion. This, of course, would not be the case with those 

 who perceived the contradictory nature of Maury's two causes ; 

 but most people probably read the ' Physical Geography of the 

 Sea' without being aware that the one cause is destructive of 

 the other. Another reason is, a few very plausible-looking 

 objections have been strongly urged by Maury and others 

 against the theory that ocean-currents can be caused by the 

 impulses of the trade-winds, which have not been duly con- 

 sidered; and probably these objections appear to many as for- 

 midable against this theory as Herschel's arguments appear 

 against Maury's theories. 



There is one slight objection to Herschel's result : he takes 

 39° as the temperature of maximum density. This, however, 

 as we shall see, does not materially affect his conclusions. 



Observations on the temperature of the maximum density of 

 sea- water have been made by Erman, Despretz, Rossetti, Neu- 

 mann, Marcet, Hubbard, Horner, and others. No two of them 

 have arrived at exactly the same conclusion. This probably 

 results from the fact that the temperature of maximum density 

 depends upon the amount of salt held in solution. No two seas, 

 unless they are equal as to saltness, have the same temperature 

 of maximum density. The following Table of Despretz will 

 show how rapidly the temperature of both the freezing-point 

 and of maximum density is lowered by additional amounts of salt. 



