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



We shall examine this theory at some length, for two reasons : 

 1, because it lies at the root of a great deal of the confusion 

 and misconception which have prevailed in regard to the whole 

 subject of ocean-currents ; 2, because, if the theory is correct, 

 it militates strongly against the physical theory of secular 

 changes of climate advanced in the preceding part of this paper. 

 We have already seen that when the excentricity of the earth's 

 orbit reaches a high value, a combination of physical circum- 

 stances tends to lower the temperature of the hemisphere which 

 has its winter solstice in aphelion, and to raise the temperature 

 of the opposite hemisphere, whose winter solstice will, of course, 

 be in perihelion. The direct result of this state of things, as 

 was shown, is to strengthen the force of the trade-winds on the 

 cold hemisphere, and to weaken their strength on the warm 

 hemisphere ; and this, in turn, we also saw tends to impel the 

 warm water of the intertropical region over on the warm hemi- 

 sphere, and to prevent it, in a very ]arge degree, from passing 

 into the cold hemisphere. This deflection of the ocean-currents 

 tends to an enormous extent to increase the difference of tem- 

 perature previously existing between the two hemispheres. In 

 other words, the warm and equable condition of the one hemi- 

 sphere, and the cold and glacial condition of the other, are, to a 

 great extent, due to this deflection of ocean-currents. But if the 

 theory be correct which attributes the motion of ocean-currents 

 to a difference in density between the sea in intertropical and 

 polar regions, then it follows that these currents (other things 

 being equal) ought to be stronger on the cold hemisphere than 

 on the warm, because there is a greater difference of tempera- 

 ture and, consequently, a greater difference of density between 

 the polar seas of the cold hemisphere and the equatorial seas, 

 than between the polar seas of the warm hemisphere and the 

 equatorial seas. And this being the case, notwithstanding the 

 influence of the trade-winds of the cold hemisphere blowing 

 over upon the warm, the currents will, in all probability^ be 

 stronger on the cold hemisphere than on the warm. In other 

 words, the influence of the powerful trade-winds of the cold 

 hemisphere to impel the warm water of the equator over upon 

 the warm hemisphere will probably be more than counterba- 

 lanced by the tendency of the warm and buoyant waters of the 

 equator to flow towards the dense and cold waters around the 

 pole of the cold hemisphere. But if ocean-currents are due not 

 to difference in specific gravity, but to the influence of the trade- 

 winds, then it is evident that the waters at the equator will be 

 impelled, not into the cold hemisphere, but into the warm. 



As Lieut. Maury appears to be the acknowledged exponent of 

 the theory which attributes ocean -currents to the difference of 



