Page:The ocean and its wonders.djvu/183

 There is an under-current setting from the Atlantic, northward through Davis' Straits, into the Arctic Ocean, and a surface-current setting out. The fact is proved beyond a doubt by the observations of arctic explorers, who have seen immense icebergs drifting rapidly northward against a strong current. This apparent anomaly could only be accounted for by the fact that a powerful under-current carried them northward; and as at least seven times more of these bergs must have been under than above water, we can easily understand how the under-current, acting on the larger mass of each berg, had power to carry it against the surface-current.

This under-current is warm, while the upper-current is cold. Now we know that, according to Nature's laws, heated water, like heated air, rises to the surface, and cold water sinks to the bottom. How, then, comes this warm current to be underneath the cold, as soundings have proved it to be? It is owing to the fact that the under-current is much salter, and therefore heavier (despite its warmth), than the surface-current; which latter, being mingled with the drainage and ice-masses of the arctic regions, is comparatively fresh, and therefore light as well as cold.

The hot and salt waters of the tropics are carried north by the Gulf Stream. There are here two counteracting agents at work. Heat inclines the Gulf Stream to rise; saltness inclines it to Rh