THE AFRICAN GRASSLANDS : SPHERES OF INFLUENCE IT is now evident how the general course of history in Europe and Asia has been controlled by the geo graphical conditions, and specially how three types of civilization have gradually been evolved on the margin of the great plain. In all the story so far, we have taken account in Africa only of a narrow strip along the Mediterranean ; of the rest it has not been necessary to say anything except to show that the discovery of the way to the Indies by the Cape was one of the out standing facts of history.
Here, then, is a set of apparently extraordinary facts. While history began in Egypt,' while many of the early scenes were enacted on the northern shores of Africa, yet the rest of the continent was unknown to the civilized world till within the last five centuries. Not only so, but notwithstanding the fact that the way to the Indies round the coasts of Africa had been discovered before Columbus sailed across the Atlantic, yet Africa remained the dark continent till the last half-century, while Spanish and Portuguese America was conquered and Northern America had become the seat of a great civilization ranking with that of Europe.
We have spoken, in Chapter II, of the absence of stimulus to native races to save energy in equatorial Africa; let us now consider the facts more in detail, to see whether they give any clue to this extraordinary history.
The relief map tells us that Africa as a whole stands high above sea-level, and that the margins descend steeply to great depths. If you consult maps which show variations in temperature through the year, you will see that no part of Africa at sea-level is cold; at any time of the year most of the land is warm, and considerable areas are hot. The area of greatest heat is, however, not constant, but there is a swing with the sun north and south of the equator, the heat from a given pencil of rays being spread over less area when it is received on a surface at right-angles to the axis of the pencil than in any other position ; or, in other words, more heat is received on a given area when the rays are vertical than when they are received at an angle. Partly owing to the less average height of the land in the north, partly owing to the greater amount of land not only in North Africa itself but also to north and north-east of the continent, the higher temperature is found in the north.
Connected in some way with this swing of the heat belt is the well-marked north and south swing of the rain zones seen in the accompanying maps. Rain is caused by the cooling of air, which is forced to rise to regions where it expands, and in so doing reduces its temperature. This rising may be caused by meeting with an obstruction such as a landslope up which it must go. It is for this reason that the western edge of the Dekkan is so wet in summer, for the south-west winds are compelled to ascend and cool themselves by expand ing. The westerly winds also blowing against the high lands of Britain cause the west to be wet, while areas equally high but farther to the east are comparatively dry, for there is no further ascent. It is not only land, however, which causes wind to rise; a current of heavier air pushing under another which is lighter, usually because it is warmer, causes the latter to rise, and again this must cool itself. This interference of currents can be the only cause of rain over the ocean, or over land so flat that air is not forced upwards merely by flowing over it, and it may of course be a contributory cause of rain on slopes also. The rainfall over the warmer part of Africa may be caused somewhat in the same way. The air in the equatorial belt is heated, and cooler winds from north and south may press in under it : these tend ing to keep moving in the same direction in space, are turned round so as to come in general from some easterly direction. But it is by no means certain that conditions are so simple as this ; there are, on the one hand, indica tions that the air in equatorial regions sinks and rises in comparatively thin streaks rather than in great masses, and, on the other, investigations of the upper air are showing that a good many beliefs as to the relation be tween pressure, winds and rainfall founded on informa tion obtained near ground-level must be revised in the light of fuller knowledge.