Maury, a recognised authority, finds in the saline properties of the sea one of the principal forces from which the currents in the ocean proceed. “The brine of the ocean,” says he, “is the ley of the earth; from it the sea derives dynamical powers, and the currents their main strength.” Let us suppose a long tank or, say, swimming‐bath, divided in the middle by a water‐tight wall, on one side of which should be fresh and on the other salt water, at equal levels. It is obvious that were the division removed the waters would not stand side by side as before, for the denser water would have a tendency not merely to mingle with the lighter, but to form a current _under_ it. So salt waters of different densities. [Illustration: CHART OF THE ATLANTIC OCEAN.] “The ocean,” says Figuier, “is a scene of unceasing agitation; ‘its vast surface rises and falls,’ to use the image suggested by Schleiden, ‘as if it were gifted with a gentle power of respiration; its movements, gentle or powerful, slow or rapid, are all determined by differences of temperature.’” Heat increases its volume, and therefore lightens it; cold increases its density, and it will naturally descend. These are, then, among the obvious reasons of its currents. The duration and force of winds and the tides are both disturbing influences. Such an oceanic marvel as the great _Gulf Stream_ could only be explained after a careful study of all the operating causes of its existence. Dr. Maury has well described it. He says:—“There is a river in the bosom of the ocean: in the severest droughts it never fails, and in the mightiest floods it never overflows; its banks and its bottom are of cold water, while its current is of warm; it takes its rise in the Gulf of Mexico, and empties itself into the Arctic seas; this mighty river is the Gulf Stream. In no other part of the world is there such a majestic flow of water; its current is more rapid than the Amazon, more impetuous than the Mississippi, and its volume is more than a thousand times greater.” This great current of water particularly influences the climates of Northern Europe, and especially those of Britain and Ireland.