This oceanic circuit of warm water has a counterpart in the aërial circuit of heated air. The vapour which rose in the tropics overflows, and as it cools and gets beyond the region of the trade winds, descends mainly over the Northern Atlantic, carrying with it its greater velocity of rotation, and so causing westerly winds, which reach our shores after blowing over a wide expanse of ocean heated by the Gulf Stream, thus bringing us warmth and wet, while the corresponding counter-currents which blow over continental Europe and Asia from the north-east bring cold and drought. The extreme effects of this may be seen by comparing the Black Sea at Odessa, where ice often stops navigation, with the North Sea at the Lafoden islands, where the cod-fishing is carried on in open boats in the middle of winter. We in England are in the happy position where on the whole the mild and genial west winds prevail, though not exclusively, so as to give us the drenching rains of Western Ireland and Scotland, or to prevent spells of a continental climate which give us bracing frosts in winter, and alternations of cold and heat in summer. If we turn from these temperate regions to those in which exactly opposite conditions prevail, we find them still in the icy chains of a glacial period. Greenland, for instance, which is a typical case, shows us what happens when a continental mass of land stands at a high elevation in high latitudes with no Gulf Stream, but instead of it cold currents from a Polar ocean, and seas around it frozen or covered with icebergs for nine months out of the year. We have a dome of solid ice piled up to the height of 9000 feet or upwards, and sending millions upon millions of tons of glaciers down to the sea to be floated off as icebergs. The only trace we can see here of the old great glacial period is that these conditions were formerly more intense. Thus the glaciation of some of the mountain sides and islands off the coast of Greenland seem to show that the ice formerly stood 2000 or 3000 feet higher than at present, a result which would be attained if the whole continental mass, which is now slowly subsiding, had then been elevated to that extent. The southern hemisphere affords a still more striking example of this on a larger scale, for we have there, in all probability, higher land in higher latitudes, surrounded by frozen seas, and washed by cold currents. I pass from this however, as beyond these general facts the special conditions of the Antarctic Circle are not known to us like those of Greenland.