From inspection of the accompanying map the main point of Mr. Croll's reasoning may be understood. It will be seen that the direction of the currents in the central Atlantic is largely determined by the contour of the northeastern coast of South America. From some cause the southeast trade-winds are stronger than the northeast, and their force is felt in pushing the superficial currents of warm water farther north than Cape St. Roque, the eastern extremity of Brazil. As the direction of the South American coast trends rapidly westward from this point to the Isthmus of Panama, the resultant of the forces is a strong current northwestward into the _cul-de-sac_ of the Gulf of Mexico, from which there is only the one outlet between Cuba and the peninsula of Florida. Through this the warm water is forced into the region where westerly winds prevail, and spreads its genial influence far to the northward, modifying the climate of the British Isles, and even of far-off Norway. [Illustration: Fig. 100.--Map showing course of currents in the Atlantic Ocean: _b_ and _b'_ are currents set in motion by opposite trade-winds; meeting, they produce the equatorial current, which divides into _c_ and _c'_, continuing on as _a_ and _a'_ and _e_.] But why are the southeast trade-winds of the Atlantic stronger than the northeast? The ultimate reason, of course, is to be found in the fact that the northern hemisphere is warmer than the southern. The atmosphere over the northern-central portion of the Atlantic region is more thoroughly rarefied by the sun's heat than is that over the region south of the equator. The strong southeast trades are simply the rush of atmosphere from the South Atlantic to fill the vacuum caused by the heat of the sun north of the equator. But, again, why is this? Because, says Mr. Croll, we are now in that stage of astronomical development favourable to the increased warmth of the northern hemisphere. In the northern hemisphere the summers are longer than the winters. Perihelion occurs in winter and aphelion in summer. This is the reason why the North Atlantic is warmer than the South Atlantic, and why the trade-winds of the south are drawn to the north of the equator. Ten thousand five hundred years ago, however, the conditions were reversed, and the greater rarefaction of the atmosphere would have taken place south of the equator, thus drawing the trade-winds in that direction.