Page:Physical Geography of the Sea and its Meteorology.djvu/59

 Rh constant and uniform throughout. On such a globe, the equilibrium remaining undisturbed, there would be neither wind nor current. Let us now suppose that all the water within the tropics, to the depth of one hundred fathoms, suddenly becomes oil. The aqueous equilibrium of the planet would thereby be disturbed, and a general system of currents and counter-currents would be immediately commenced—the oil, in an unbroken sheet on the surface, running towards the poles, and the water, in an under-current, towards the equator. The oil is supposed, as it reaches the polar basin, to be reconverted into water, and the water to become oil as it crosses Cancer and Capricorn, rising to the surface in the inter-tropical regions, and retuning as before. Thus, without wind, we should have a perpetual and uniform system of tropical and polar currents; though without wind, Sir John Herschel maintains, we should have no "considerable currents" whatever in the sea. In consequence of the diurnal rotation of the planet on its axis, each particle of oil, were resistance small, would approach the poles on a spiral turning to the east with a relative velocity greater and greater, until, finally, it would reach the pole, and whirl about it at the rate of nearly a thousand miles the hour. Becoming water and losing its velocity, it would approach the tropics by a similar, but reversed spiral, turning towards the west. Owing to the principle here alluded to, all currents from the equator to the poles should have an eastward tendency, and all from the poles towards the equator a westward. Let us now suppose the solid nucleus of this hypothetical globe to assume the exact form and shape of the bottom of our seas, and in all respects, as to figure and size, to represent the shoals and islands of the sea, as well as the coast lines and continents of the earth. The uniform system of currents just described would now be interrupted by obstructions and local causes of various kinds, such as unequal depth of water, contour of shore lines, &c.; and we should have at certain places currents greater in volume and velocity than at others. But still there would be a system of currents and counter-currents to and from either pole and the equator. Now, do not the cold waters of the north, and the warm waters of the gulf, made specifically lighter by tropical heat, and which we