Mr. J. Croll on the Physical Cause of Ocean-currents. 235 



specific gravity between the waters at the equator and the poles, 

 I shall now proceed to consider at some length his views on the 

 subject, the more especially as we find in his work on the phy- 

 sical geography of the sea almost every argument that can be 

 advanced in favour of the theory which he advocates. 



Although considerable diversity of opinion has prevailed in 

 regard to the cause of ocean-currents, yet it is remarkable how 

 little is to be found of a purely scientific character bearing 

 directly on the dynamics of the subject. 



Another reason which has induced me to select Maury's work 

 is, that it not only contains a much fuller discussion on the 

 cause of the motion of ocean-currents than is to be found any- 

 where else, but also that it has probably passed through a 

 greater number of editions than any other book of a scientific 

 character in the English language in the same length of time. 



Lieut. Maury on the Cause of the Motion of Ocean-currents. 



Although Lieut. Maury has expounded his views on the 

 cause of ocean-currents at great length in the various editions of 

 his work, yet it is somewhat difficult to discover what they really 

 are. This arises chiefly from the generally confused and some- 

 times contradictory nature of his hydrodynamical conceptions. 

 After a repeated perusal of several editions of his book, the fol- 

 lowing, I trust, will be found to be a pretty accurate representa- 

 tion of his theory : — 



Ocean-currents, according to Maury, due to difference of spe- 

 cific gravity. — Although Maury alludes to a number of causes 

 which, he thinks, tend to produce currents, yet he deems their 

 influence so small that, practically, all currents may be referred 

 to difference of specific gravity. 



" If we except/'' he says, " the tides, and the partial currents 

 of the sea, such as those that may be created by the wind, we 

 may lay it down as a rule that all the currents of the ocean owe 

 their origin to the differences of specific gravity between sea- 

 water at one place and sea-water at another ; for wherever there 

 is such a difference, whether it be owing to difference of tempe- 

 rature or to difference of saltness, &c, it is a difference that 

 disturbs equilibrium, and currents are the consequence" (§ 467)*. 

 To the same effect see §§ 896, 37, 512, 520, and 537. 



Notwithstanding the fact that Maury is continually referring 

 to difference of specific gravity as the great cause of currents, it 

 is difficult to understand in what way he conceives this differ- 

 ence to act as a cause. 



Difference of specific gravity between the waters of the ocean 



* The edition from which I quote, unless stated to the contrary, is the 

 one published bv Messrs. T. Nelson and Sous, 1870. 



R2 



