current in their favour; most of them experience no current at all; but, now and then, some do find a current setting to the northward and westward, and operating against them at the rate of 20 and occasionally of 50 miles a day. The intertropical regions of the Atlantic, like those of the other oceans (§ 401), abound with conflicting currents, which no researches yet have enabled the mariner to unravel so that he may at all times know where they are and tell how they run, in order that he may be certain of their help when favourable, or sure of avoiding them if adverse.
409. The Greenland current.—There are other currents, such as the Greenland Current, the cold current from Davis' Strait, the ice-bearing current from the antarctic regions, all setting into the Atlantic and the Gulf Stream, one branch of which finds its way into the Arctic Sea; the other (§89) finds its way back to the south partly as Rennell's current, all of which have been fully treated of in Chap. II., or are delineated on Plates VI. and IX. Judging by these, there would seem to be a larger flow of polar waters into the Atlantic than of other waters from it; and I cannot account for the preservation of the equilibrium of this ocean by any other hypothesis than that which calls in the aid of under currents. They, I have no doubt, like the water-ways, the mineral veins, the passages in the bowels of the earth, bear in their secret ways, an important part in the grand system of the terrestrial economy.
CHAPTER IX.
§ 420-460.—THE SPECIFIC GRAVITY OF THE SEA, AND THE OPEN WATER IN THE ARCTIC OCEAN.
420. Interesting physical inquiries.—The crust of the planet upon which we live, with the forces that have been and are at work upon it, is the most interesting subject of physical inquiry and study that can claim the attention of diligent students. Precisely as the progress of man has been upward and onward, precisely has he looked more earnestly and with deeper longings towards the mysteries that encircle this crust. It is but a shell, and at most we can reach only a little way either above or below its very surface, and jet upon the tablets of this thin shell are
