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



at the same time be both lighter and heavier than the water at 

 the poles. They may be either the one or the other, but they 

 cannot be both. Let it be observed that it is not two currents, 

 the one contrary to the other, with which we have at present to 

 do ; it is not temperature producing currents in one direction, 

 and saltness producing currents in the contrary direction. 

 We have two theories regarding the origin of currents, the 

 one diametrically opposed to the other. The tendency of the 

 one cause assigned is to prevent the action of the other cause. 

 If temperature is allowed to act, it will make the intertropical 

 waters lighter than the polar, and then, according to theory, a 

 current will result. But if we bring saltness into play (the 

 other cause) it will do the reverse : it will increase the den- 

 sity of the intertropical waters and diminish the density of the 

 polar ; and so far as it acts it will diminish the currents pro- 

 duced by temperature, because it will diminish the difference of 

 specific gravity between the intertropical and polar regions 

 which had been previously caused by temperature. And when 

 the effects of saltness areas powerful as those of temperature, the 

 difference of specific gravity produced by temperature will be 

 completely effaced, or, in other words, the waters of the equato- 

 rial and polar seas will be of the same density, and consequently 

 no current will exist. And so long as the two causes continue 

 in action, no current can arise, unless the energy of the one 

 cause should happen to exceed that of the other ; and even then 

 a current will only exist to the extent by which the strength of 

 the one exceeds that of the other. 



The contrary nature of the two theories will be better seen by 

 considering the way in which he supposes difference in saltness 

 is produced and acts as a cause. 



If there is a constant current resulting from the difference in 

 saltness between the equatorial and polar waters, then there 

 must be a cause which maintains this difference in saltness. The 

 current is simply the effort to restore the equilibrium lost by this 

 difference ; and the current would very soon do this, and then all 

 motion would cease, were there not a constantly operating cause 

 maintaining this disturbance. What, then, according to Maury, 

 is the cause of this disturbance, or, in other words, what is it 

 that keeps the equatorial waters Salter than the polar ? 



The agencies in operation which keep the waters in equatorial 

 regions Salter than the polar are stated by him to be heat, radia- 

 tion, evaporation, precipitation, and secretion of solid matter in 

 the form of shells, &c. The two most important, however, are 

 evaporation and precipitation. 



The trade-winds enter the equatorial regions as relatively dry 

 winds thirsting for vapour; consequently they absorb far more 



