in the Air and in the Sea. 27 



On the other hand, Miihry accounts for the compensation- 

 current, flowing in the middle latitudes from west to east, by 

 the aspirating or attracting force of the equatorial current — that 

 is, by the tendency of water to find its level, which impels it to 

 fill up the void caused by the primary current. He therefore 

 makes the aspirating force operate in a vast arc across the 

 entire ocean, and chiefly between the 40th and 50th parallels of 

 latitude — a decided circulus vitiosus. 



The meridional currents* are mostly accounted for by the 

 constant difference of temperature of the equatorial and polar 

 regions ; and Miihry attributes to the cold and heavy polar flow 

 the primary, and to the warmer and lighter compensating anti- 

 polar flow the secondary action. At the same time, in conse- 

 quence of the velocity of the earth's rotation progressively dimi- 

 nishing in the direction of the poles, all the cold or polar streams 

 receive a deflection of their direction to the west, and the warm 

 antipolar currents a deflection eastward. Franklin and Rennell 

 explained also the meridional currents by the action of the 

 trade-winds ; for they believed that by the driving force of these 

 winds the waters are accumulated in the Gulf of Mexico and 

 are discharged in the Gulf-stream, — a view that probably now 

 possesses scarcely any adherents. 



Having briefly indicated the existing explanations of the 

 origin of the great ocean- currents and the trade- winds, we will 

 now endeavour to ascertain in what manner each of the three 

 forces before mentioned is capable of acting upon the currents, 

 how their influence must express itself, and, finally, how far the 

 explanations hitherto given correspond with the facts. 



A. Alteration or the Specific Gravity or the Water 

 and Air. 



a. Difference of Temperature. 

 Every material substance possesses the property of occupying 

 a greater space when its temperature is raised, while still re- 

 taining the given number of its molecules and its weight. From 

 this it follows that, after a rise of temperature of a body, fewer 

 of its particles can find room in a given space; so that the 

 specific gravity must diminish with rise of temperature. Sub- 

 stances of different kinds differ widely in their degrees of ex- 

 pansion, and hence also in the alteration of their specific gravity. 

 According to determinations by Ermanf, sea- water expands 

 0*00027 of its volume with every degree between 0° and 12° R. 

 On this ground it has been calculated J that the entire mass of 



* These, which flow in the direction of the meridian, Miihry calls lati- 

 tudinal, t Pogg. Ann. vol. xx. p. 114. 

 | Bischof, Lehrbuch der chem. u. phys. Geologie, p. /• 



