C ROLL'S ''CLIMATE AND TIMEP 813 



cates of this theory are separated into two arbitrary classes : 1. Those 

 who consider the difference in density between equatorial and polar 

 waters to be due to difference in saltness ; and, 2, Those who attrib- 

 ute the difference in density to difference in temperature. Lieutenant 

 Maury's theory, stated in his " Physical Geography of the Sea," is 

 rejected because that eminent meteorologist recognized both of the 

 above-named causes of difference in density, while they are very 

 nearly equal and quite antagonistic, and because the actual differences 

 in specific gravity due to these causes are mathematically demonstra- 

 ble to be incompetent to produce so powerful currents ; being, in fact, 

 as Dr. Croll pointedly insists, only one seventh of that necessary to 

 produce the slightest motion. Dr. Carpenter's theory, based chiefly 

 on differences of density due to differences in temperature, is discussed 

 at still greater length than Lieutenant Maury's, and is rejected on the 

 grounds — 1. Of being counteracted by differences in saltness (in which 

 view Dr. Croll has the support of Sir Wyville Thomson) ; and, 2. Of 

 resting on assumed causes utterly inadequate either (a) to produce 

 existing currents, or {h) to convey northward so great an amount of 

 heat as that shown to be given out by the Gulf Stream ; the apparent- 

 ly paradoxical conclusion, that the Gulf Stream actually conveys an 

 absolutely greater quantity of heat to high latitudes than the whole 

 Atlantic could convey, being explained by the proposition that the 

 Gulf Stream obtains the greater portion of its heat in the southern 

 hemisphere, while according to Dr. Carpenter's hypothesis tlie circula- 

 tion should be independent in each hemisphere. 



In Chapter XIII. the "wind theory" of oceanic circulation is 

 enunciated and elaborated, and in the succeeding chapter its relations 

 to climatal variations are discussed. It is first pointed out that the 

 various ocean-currents are not due to the trade-winds alone, as was 

 until recently supposed by advocates of the wind theory, but to the 

 general impulse of the prevailing winds of the globe, viewed as a sin- 

 gle grand system, and acting, not upon several separate and indepen- 

 dent oceans, but upon a single grand oceanic system, the various parte 

 of which are most intimately related. The correspondence between 

 the supposed cause and the observed effect seems to be all that could 

 be desired. " All the principal currents of the globe are in fact mov- 

 ing in the exact direction in which they ought to move, assuming the 

 winds to be the sole impelling cause. In short, so perfect is the agree- 

 ment between the two systems, that, given the system of winds and 

 the conformation of sea and land, and . . . the system of oceanic 

 circulation might be determined a jyriori. Or, given the system of the 

 ocean-currents, together with the conformation of sea and land ; and 

 the direction of the prevailing winds could also be determined a 

 j)7'iori. Or, thirdly, given the system of winds and the system of 

 currents, and the conformation of sea and land might be roughly 

 determined." Return currents are formed through the tendency of 



