105. Current into the Caribbean Sea.—Now that we may form some idea as to the influence which the salts left by the vapour that the trade-winds, north-east and south-east, take up from sea-water, is calculated to exert in creating currents, let us make a partial calculation to show how much salt this vapour held in solution before it was taken up, and, of course, while it was yet in the state of sea-water. The north-east trade-wind regions of the Atlantic embrace an area of at least three million square miles, and the yearly evaporation from it is (§ 103), we will suppose, fifteen feet. The salt that is contained in a mass of sea-water covering to the depth of fifteen feet an area of three million square miles in superficial extent, would be sufficient to cover the British islands to the depth of fourteen feet. As this water supplies the trade-winds with vapour, it therefore becomes salter, and as it becomes salter, it becomes heavier; and therefore we may infer that the forces of aggregation among its particles are increased.
106. Amount of salt left by evaporation.—Whatever be the cause that enables these trade-wind waters to remain on the surface, whether it be from the fact just stated, and in consequence of which the waters of the Gulf Stream are held together in their channel; or whether it be from the fact that the expansion from the heat of the torrid zone is sufficient to compensate for this increased saltness; or whether it be from the low temperature and high saturation of the submarine waters of the intertropical ocean; or whether it be owing to all of these influences together that these waters are kept on the surface, suffice it to say, we do know that they go into the Caribbean Sea (§ 103) as a surface current. On their passage to and through it, they intermingle with the fresh waters that are emptied into the sea from the Amazon, the Orinoco, and the Mississippi, and from the clouds, and the rivers of the coasts round about. An immense volume of fresh water is supplied from these sources. It tends to make the sea-water, that the trade-winds have been playing upon and driving along, less briny, warmer, and lighter: for the waters of these large intertropical streams are warmer than sea-water. This admixture of fresh water still leaves the Gulf Stream a brine stronger than that of the extratropical sea generally, but not quite so strong (§ 102) as that of the trade-wind regions.
107. Currents created by storms.—The dynamics of the sea confess the power of the winds in those tremendous currents which
