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



regions, and returning as before " (§20). " Now/' lie says (§22), 

 " do not the cold waters of the north, and the warm waters of 

 the Gulf, made specifically lighter by tropical heat, and which we 

 see actually preserving such a system of countercurrents, hold, at 

 least in some degree, the relation of the supposed water and oil ? " 



In § 24 he calculates that at the Narrows of Bernini the 

 difference in weight between the volume of the Gulf-water 

 that crosses a section of the stream in one second, and an equal 

 volume of water at the ocean temperature of the latitude, sup- 

 posing the two volumes to be equally salt, is fifteen millions of 

 pounds. Consequently the force per second operating to pro- 

 pel the waters of the Gulf towards the pole would in this case, 

 he concludes, be the u equilibrating tendency due to fifteen mil- 

 lions of pounds of water in the latitude of Bernini." In §§ 511 

 and 512 he states that the effect of expanding the waters a£ 

 the torrid zone by heat, and of contracting the waters at the 

 frigid zone by cold, is to produce a set of surface-currents of 

 warm and light water from the equator towards the poles, and 

 another set of undercurrents of cooler and heavy water from the 

 poles towards the equator. See also to the same effect §§ 513, 

 514, 896. 



There can be no doubt that Maury concludes that the waters 

 in intertropical regions are expanded by heat, and those in polar 

 regions are contracted by cold, and that this tends to produce a 

 surface- current from the equator to the poles, and an undercur- 

 rent from the poles to the equator. 



We shall now consider his second great cause of ocean-cur- 

 rents, viz. : — 



Difference of specific gravity resulting from difference in degree 

 of saltness. — Maury maintains, and that correctly, that saltness 

 increases the density of water — that, other things being equal, the 

 saltest water is the densest. He suggests " that one of the pur- 

 poses which, in the grand design, it was probably intended to 

 accomplish by having the sea salt and not fresh, was to impart to 

 its waters the forces and powers necessary to make their circu- 

 lation complete " (§495). 



Now it is perfectly obvious that if difference in saltness is to 

 cooperate with difference in temperature in the production of 

 ocean-currents, the saltest waters, and consequently the densest, 

 must be in the polar regions, and the waters least salt, and con- 

 sequently lightest, must be in equatorial and intertropical re- 

 gions. Were the saltest waters at the equator, and the freshest 

 at the poles, it would tend to neutralize the effect due to heat, 

 and, instead of producing a current, would simply tend to pre- 

 vent the existence of the currents which otherwise would result 

 from difference of temperature. 



