“It is not necessary,” he says, “to associate with oceanic currents the idea that they must of necessity, as on land, run from a higher to a lower level. So far from this being the case, some currents of the sea actually run up hill, while others run on a level. The Gulf-stream is of the first class” (§ 403). “The top of the Gulf-stream runs on a level with the ocean; therefore we know it is not a descending current” (§ 18). And in § 9 he says that between the Straits of Florida and Cape Hatteras the waters of the Gulf-stream “are actually forced up an inclined plane, whose submarine ascent is not less than 10 inches to the mile.” To the same effect see §§ 25, 59. It is perfectly true that “it is not necessary to associate with ocean-currents the idea that they must of necessity, as on land, run from a higher to a lower level.” But the reason of this is that ocean-currents do not, like the currents on land, owe their motion to the force of gravitation. If ocean-currents result from difference of specific gravity between the waters in tropical and polar regions, as Maury maintains, then it is necessary to assume that they are descending currents. Whatever be the cause which may give rise to a difference of specific gravity, the motion which results from this difference is due wholly to the force of gravity; but gravity can produce no motion unless the water _descend_. This fact must be particularly borne in mind while we are considering Maury’s theory that currents are the result of difference of specific gravity. Ocean-currents, then, according to that writer, owe their existence to the difference of specific gravity between the waters of inter-tropical and polar regions. This difference of specific gravity he attributes to two causes—(1) to difference as to _temperature_, (2) to difference as to saltness. There are one or two causes of a minor nature affecting the specific gravity of the sea, to which he alludes; but these two determine the general result. Let us begin with the consideration of the first of these two causes, viz.:—