Page:Encyclopædia Britannica, Ninth Edition, v. 1.djvu/279

Rh FEATURES,] AFRICA 257 ing the powers of evaporation, are often fatal to the vege table and animal creation in the regions visited by them. ,ai ns In Africa the dependence of the winds and rains upon. ithin the the movement of the land beneath the sun is more clearly opics. marked than in any other intertropical region of the globe. The high temperature caused by the vertical heat of the sun over a particular area induces an indraught of air to that place, an ascending current is produced which carries up with it the warm and moist air; condensed in the higher regions of the atmosphere, the moisture falls as rain, and the condensation makes way for a further in draught. It is thus that in Africa the winds and rains follow as a rule the pendulating movement of the continent beneath the sun, and the rainy season of any space begins almost immediately after the sun has reached its zenith. Between the tropics and the equator the sun comes twice to the zenith of each belt during the year, at the tropical lines the sun is only once in the zenith; thus it follows that a double rainy season is observed in all places lying in the central belt of the tropics, and a single rainy season in those which are nearer the skirts of the zone. These wet and dry seasons correspond to the cooler and hotter periods of the year, and take the place of the summer and winter of the temperate regions. Various circumstances tend to interfere with and modify the working of this general rule of the rotation of seasons. In Southern Africa that rainy season which follows the apparent move ment of the sun northward, is greater than that which ensues after his passage south, since in the former case the winds are drawn inwards from the ocean and carry greater quantities of moisture, whereas in the latter the winds are drawn from the land north of the equator, and their mois ture is already in great part spent. In the northern and n the eastern regions of Africa the winds and rains are governed asts. as much by the heating and cooling of the Asiatic con tinent as by that of Africa itself, but in the central and western portions of the continent the rule is well exem plified. Thus in Damara Land, bordering on the southern tropic, there is one short rainy season from February till April, beginning only with the northing sun; at Loanda in Angola the greater rains last from February till May, the lesser rainy season, when the sun has passed this place going south, occurs in November only. At Annobon island, surrounded by wide sea, April and May are the rainy months of the northing sun, October and November of the southing. The Guinea coast, facing the sea to southward, has its greater rainy season from March to June, when the northing sun draws the ocean winds on to the coast; and its lesser rains occur in October and November, when the sun has passed southward from the land. Nearing the northern tropical line, the coast-land from Sierra Leone to the Senegal river has a simple wet and dry season during the year. On the eastern coast-land the rains are move dependant on the direction of the monsoon winds; about the mouths of the Zambeze and on the Mozambique coast the rains begin in November, after the north-east monsoon wind has set in over the northern part of the Indian Ocean, bringing with it the vapours drawn from the sea to condense on the coast slopes. The rains continue here till March, when the south-west monsoon begins to blow away from the land towards the then heated surface of Asia. At Zanzibar there is a double rainy season, a stronger in the months of March, April, and May, with the northing sun, beginning immediately after the south-west monsoon has set in, and a weaker in September and October with the southing sun. Under (lie equator on the east coast the rains begin in April with the south-west monsoon, continuing till June, and during this period the sky is obscured by heavy clouds. The second rainy season here is only marked by a few showers in September and October. &quot;Wliils the north-east mpn- In the soon is blowing the sky remains of a cloudless blue. In interior the interior of the continent, between these tropical coasts, the rainy seasons appear rather to precede than follow the advancing sun. In the region of the central Zambeze the greater rains last through February, March, and April, the lesser occurring in October and November. The worst droughts are experienced in December and January. Nearer the centre of the continent the two rainy seasons become so lengthened as almost to merge into one period of rains, extending over about eight months of the year. In the newly-explored country south- west of the Tanganyika, Dr Livingstone found that the rains began in October, and that the last showers fell in May; but there is probably a drier period between these limits. At the Tanganyika Lake the rainy season begins in September, lasting till May, and the same rainy reason has been observed in the interior. country of the west coast immediately north of the equator. Between these points, in Manuyema country, Dr Livingstone found that the rains continued till July, or almost through the year. Northward in the interior the rainy seasons are again clearly divided into a greater and lesser, and in the regions west of the Upper Nile between 5 and 10 N. lat, the stronger rains occur from August till October, the weaker come with the northing sun in April and May. The plateau of Abyssinia, rising high above the general level of the north of Africa, and inter cepting and condensing the moist winds, has also a double rainy season, a greater from June to September, when the sun is passing southward ; a lesser in February and April, with the northing sun. The rainy seasons in Central Africa are ushered in and accompanied by violent thunderstorms and by occasional falls of hail. The quantity of the rain fall, which is excessive in the regions near the equator, diminishes rapidly to north and south of this belt as tho dry regions on the borders of the tropics are approached. The Sahara, and also the Kalahari of Southern Africa, are almost rainless regions, but wherever a sufficient elevation occurs to intercept a cooler stratum of the atmosphere, rain is not wanting, even in the midst of the Great Desert. A striking instance of this is related by Mr Richardson. That traveller relates that when on the borders of the mountain knot of &quot;Air, in about latitude 19 N., on the 30th Sept. 1850, there was a cry in the encampment, The wady is coming. Going out to look, I saw a broad white sheet of foam advancing from the south between the trees of the valley. In ten minutes after a river of water came pouring along, and spread all around us, converting the place of our encampment into an isle of the valley. The current in its deepest part was very powerful, capable of carrying away sheep and cattle, and of uprooting trees. This is one of the most interesting phenomena I have witnessed during my present tour in Africa. The scene, indeed, was per fectly African. Rain had been observed falling in the south ; black clouds and darkness covered that zone of the heavens, and an hour afterwards came pouring down this river of water into the dry parched-up valley.&quot; The causes of want of rainfall in the vast region of the Dry re- Sahara appear to be mainly these that the winds advanc- gions. ing towards it come from a cooler and moister to a warmer and drier region, indeed to the hottest and driest of all, and so are constantly losing in moisture and gaining in temperature as they approach ; the high plateau of Abyssinia forms an effective screen from the winds of tho Indian Ocean, wringing out their moisture before the Sahara is reached, and on the Atlantic side the north-east trade wind constantly blows away from the land ; a barrier of mountains also deprives the Sahara of rain from the south-west. Another cause of dryness is the low level of great areas of the Sahara. &quot;Ye have seen that wherever I. 33.