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 attribute 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 demonstrable 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 (b) to convey northward so great an amount of heat as that shown to be given out by the Gulf Stream; the apparently 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 the circulation 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 single grand system, and acting, not upon several separate and independent oceans, but upon a single grand oceanic system, the various parts 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 moving 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 agreement 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 priori. 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 priori. 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
