a general oceanic circulation no higher a character than that of a good working _hypothesis_ consistent with our present knowledge of facts, and therefore entitled to be _provisionally_ adopted for the purpose of stimulating and directing further inquiry.” I am unable to agree with him, however, on this latter point. It seems to me that there is no necessity for adopting any hypothetical mode of circulation to account for the facts, as they can be quite well accounted for by means of that mode of circulation which does _actually exist_. It has been determined from direct observation that surface-currents flow from equatorial to polar regions, and their paths have been actually mapped out. But if it is established that currents flow from equatorial to polar regions, it is equally so that return currents flow from polar to equatorial regions; for if the one _actually_ exists, the other of necessity _must_ exist. We know also on physical grounds, to which I have already referred, and which fall to be considered more fully in a subsequent chapter, that a very large portion of the water flowing from polar to equatorial regions must be in the form of under currents. If there are cold under currents, therefore, flowing from polar to temperate and equatorial regions, this is all that we really require to account for the cold water which is found to occupy the bed of the ocean in those regions. It does not necessarily follow, because cold water may be found at the bottom of the ocean all along the equator, that there must be a direct flow from the polar regions to every point of the equator. Water brought constantly from the polar regions to various points along the equator by means of under currents will necessarily accumulate, and in course of time spread over the bottom of the inter-tropical seas. It must either do this, or the currents on reaching the equator must bend upwards and flow to the surface in an unbroken mass. Considerable portions of some of those currents may no doubt do so and join surface-currents; but probably the greater portion of the water coming from polar regions extends itself over the floor of the equatorial seas. In a letter in _Nature_, January 11, 1872, I endeavoured to show that the surface-currents of the ocean are not separate and independent of one another, but form one grand system of circulation, and that the impelling cause keeping up this system of circulation is not the _trade-winds_ alone, as is generally supposed, but the _prevailing winds of the entire globe considered also as one grand system_. The evidence for this opinion, however, will be considered more fully in the sequel.