190 SPAEKS FROM A GEOLOGIST'S HAMMER. 



* 



The answer to the first question involves the determi- 

 nation of the physical causes of ocean currents. Since 

 the appearance of Maury's Physical Geography of the Sea 

 it has been generally conceded that the circulation of the 

 waters is simply an interchange between the arctic and 

 inter-tropical regions, resulting from a difference in densi- 

 ties. Mr. Croll, however, has pointedly demonstrated that 

 this hypothesis is untenable, and that Maury's own rea- 

 sonings result in mutual nullification. Increased density 

 (resulting from greater saltness) in intertropical regions, 

 caused by excess of evaporation, would equalize diminished 

 density (rarefaction) caused by excess of heat. Hence no 

 resultant diminished density in the intertropical regions 

 exists to initiate a flow of denser (colder) water from the 

 arctic regions ; and the circulation, which certainly is a 

 fact, cannot be explained by the theory of Lieutenant 

 Maury. 



Dr. W. B. Carpenter has more recently propounded, and 

 defended with characteristic positiveness and persistence, 

 a theory somewhat different from Maury's. He appeals 

 chiefly to the expansive effect of the excessive heat of the 

 intertropical regions. The expansion of the intertropical 

 waters creates, as he maintains, a sort of protuberance. 

 The- waters, seeking always a statical equilibrium, would 

 flow, superficially, down a gentle slope, from the equator 

 toward the poles; while this flow would be compensated 

 by an undercurrent setting from the polar to the inter- 

 tropical regions. This much vaunted theory seems to me 

 inadequate, untenable and contradictory. 1. It takes no 

 account of the influence of excessive evaporation in the 

 intertropical regions, which, in a general way, may be 

 assumed to reduce the volume of the water quite as much 



