Mr. J. Croll on the Physical Cause of Ocean- currents. 243 



gravity of this upper layer, therefore, is diminished just as much 

 as the specific gravity of the sea- water in the evaporating regions 

 was increased. And thus we have a surface- current of saltish water 

 from the poles towards the equator, and an undercurrent of water 

 Salter and heavier from the equator to the poles" (§ 522). 



" This property of saltness imparts to the waters of the ocean 

 another peculiarity, by which the sea is still better adapted for the 

 regulation of climates, and it is this : by evaporating fresh water 

 from the salt in the tropics, the surface-water becomes heavier 

 than the average of sea-water. This heavy water is also warm 

 water; it sinks, and being a good retainer, but a bad conductor 

 of heat, this water is employed in transporting through under- 

 currents heat for the mitigation of climates in far distant 

 regions" (§ 526). 



" For instance, let us suppose the waters in a certain part of 

 the torrid zone to be 90°, but; by reason of the fresh water 

 which has been taken from them in a state of vapour, and con- 

 sequently, by reason of the proportionate increase of salts, these 

 waters are heavier than waters that may be cooler, but not so 

 salt. This being the case, the tendency would be for this warm 

 but salt and heavy water to flow off as an undercurrent towards 

 the polar or some other regions of lighter water " (§ 554). 



That Maury supposes the warm water at the equator to flow to 

 the polar regions as an undercurrent is further evident from the 

 fact that he maintains that the climate of the arctic regions is 

 mitigated by a warm undercurrent, which comes from the equa- 

 torial regions, and passes up through Davis Straits. See §§ 

 534-544. 



The question now suggests itself: to which of these two an- 

 tagonistic causes does Maury really suppose ocean-currents must 

 be referred ? Whether does he suppose, difference in tempera- 

 ture or difference in saltness, to be the real cause ? I have been 

 unable to find any thing from which we can reasonably conclude 

 that he prefers the one cause to the other. It would seem that 

 he regards both as real causes, and that he has failed to perceive 

 that the one is destructive of the other. But it is difficult to 

 conceive how he could believe that the sea in equatorial regions, 

 by virtue of its higher temperature, is lighter than the sea in 

 polar regions, while at the same time it is not lighter but 

 heavier, in consequence of its greater saltness — how he could be- 

 lieve that the warm water at the equator flows to the poles as 

 an upper current, and the cold water at the poles to the equator 

 as an undercurrent, while at the same time the warm water at 

 the equator does not flow to the poles as a surface-current, nor 

 the cold water at the poles to the equator as an undercurrent, 

 but the reverse. But, unless these absolute impossibilities be 



