Far be it from me to bear hard upon those attempts with which the ancients endeavoured to solve those phænomena, when, for want of a sufficient progress in experimental philosophy and observation, they were generally destitute of the proper means; but there is no excuse for a man’s either believing or writing, that earth, impregnated with so small a quantity of any mixture as not to be discernible to the eye, smell, or taste, could periodically swell the waters of a river, then almost dry, to such an immensity, as to cover the whole plains of Egypt, and discharge millions of tons every day into the sea, at the same time that it contributed to the health of the people and the fertility of the land. It puts me in mind of an assertion of M. de Maillet, almost as absurd as de la Chambre’s treatise, that the Nile, which in Egypt is the only fountain of pleasure, of health, and plenty, has a mixture of one tenth of mud during the time of the inundation: pleasant and wholesome stream, truly, to which Fleetditch would be Hippocrene.
But whatever were the conjectures of the dreamers of antiquity, modern travellers and philosophers, describing without system or prejudice what their eyes saw have found that the inundation of Egypt has been effected by natural means, perfectly consonant with the ordinary rules of Providence, and the laws given for the government of the rest of the universe. They have found that the plentiful fall of the tropical rains produced every year at the same time, by the action of a violent sun, has been uniformly, without miracle, the cause of Egypt being regularly overflowed.
The sun being nearly stationary for some days in the tropic of Capricorn, the air there becomes so much rarified, that the heavier winds, charged with watery particles, rush in upon it from the Atlantic on the west, and from the Indian Ocean on the east. The south wind, moreover, loaded with heavy vapour, condensed in that high ridge of mountains not far south of the Line, which forms a spine to the peninsula of Africa, and, running northward with the other two, furnish wherewithal to restore the equilibrium.
The sun, having thus gathered such a quantity of vapours as it were to a focus, now puts them in motion, and drawing them after it in its rapid progress northward, on the 7th of January, for two years together, seemed to have extended its power to the atmosphere of Gondar, when, for the first time, there appeared in the sky white, dappled, thin clouds, the sun being then distant 34° from the zenith, without any one cloudy or dark speck having been seen for several months before. Advancing to the Line with increased velocity, and describing larger spirals, the sun brings on a few drops of rain at Gondar the 1st of March, being then distant 5° from the zenith; these are greedily absorbed by the thirsty soil, and this seems to be the farthest extent of the sun’s influence, capable of causing rain, which then only falls in large drops, and lasts but a few minutes: the rainy season, however, begins most seriously upon its arrival at the zenith of every place, and these rains continue constant and increasing after he has passed it, in his progress northward. Before this, green boughs and leaves appear floating in the Bahar el Abiad, and shew that, in the latitude where it rises, the rains are already abundant. The Galla, who inhabit, or have passed that river, give account of its situation, which lies, as far as I could ever calculate, about 5° from the Line.
In April, all the rivers in Amhara, Begemder, and Lasta, first discoloured, and then beginning to swell, join the Nile in the several parts of its course nearest them; the river then, from the height of its angle of inclination, forces itself through the stagnant lake without mixing with it. In the beginning of May, hundreds of streams pour themselves from Gojam, Damot, Maitsha, and Dembea, into the lake Tzana, which had become low by intense evaporation, but now begins to fill insensibly, and contributes a large quantity of water to the Nile, before it falls down the cataract of Alata. In the beginning of June, the sun having now passed all Abyssinia, the rivers there are all full, and then is the time of the greatest rains in Abyssinia, while it is for some days, as it were, stationary in the tropic of Cancer.
These rains are collected by the four great rivers in Abyssinia; the Mareb, the Bowiha, Tacazzé, and the Nile. All these principal, and their tributary streams, would, however, be absorbed, nor be able to pass the burning deserts, or find their way into Egypt, were it not for the White River, which, rising in a country of almost perpetual rain, joins to it a never-failing stream, equal to the Nile itself.
In the first days of May, the sun, in his way to the northern tropic, is vertical over the small village of Gerri, the limit of the tropical rains. Not all the influence of the sun, which has already past its zenith, and for many days has been as it were stationary within a few degrees of it over Syene, in the tropic of Cancer, can bring them one inch farther to the northward, neither do any dews fall there as might be reasonably expected from the quantity of fresh and exhalable water that is then running in the Nile, though it passes close by that village, and after, through that wild and dreary desert. The fact is certain, and surely curious; the cause perhaps unknown, although it may be guessed at.
I conceive, that mountains are necessary to occasion either rain or dew, by arresting and stopping the great quantity of vapour which is here driven southward before the Etesian winds. Now, all that country between Gerri and Syene is flat and desert, so that this interruption is wanting; and it is owing to the same cause, that the bounds of the tropical rains do stop farther to the southward as you travel westward, and in place of lat. 16°, which is their limits at Gerri, they are confined within lat. 14° in that part of the kingdom of Sennaar which lies south and west of that capital, where all is free from mountains till you come to those of Kuara and Fazuclo.
Yet although the sun’s influence when at its greatest, is not strong enough to draw the boundaries of the summer’s rain farther north than Gerri, all the time that it is in the tropic of Cancer at its greatest distance, these rains are then at their heaviest throughout all Abyssinia; and Egypt, and all its labours, would soon be swept into the Mediterranean did not the sun now begin to change its sphere of action by hastening its progress southward.
From Syene the sun passes over the desert, and arrives at Gerri; here he reverses the effects his influence had when on his passage northward; for whereas, in his whole course of declination northward, from the Line to Gerri, he brought on the rains at every place where he became vertical, so now he cuts off those rains the instant he returns to the zenith of each of those places passing over Abyssinia in his journey southward, till arrived at the Line, in the autumnal equinox, his influence ceases on the side of Abyssinia, and goes to extend itself to the southern hemisphere. And so precisely is this stupendous operation calculated, that, on the 25th of September, only three days after the equinox, the Nile is generally found at Cairo to be at its highest, and begins to diminish every day after.