In his epic compendium A Shropshire Lad, A.E. Houseman laments the diaspora of English dead which have accumulated over centuries of British imperial endeavour:
It dawns in Asia, tombstones show
And Shropshire names are read;
And the Nile spills his overflow
Beside the Severn's dead.
This overflow of the Nile was until relatively recently the second great mystery of this massive waterway.
The first, of course, was its source; the river was so long that it was not until 150 years or so ago that Sir Samuel Baker discovered that the White Nile springs south of the equator from the highlands of Burundi, before flowing through a series of lakes and across a desert, to join the Blue Nile at Khartoum.
But it is what happens from Khartoum downriver that has been the Nile's enigma down the centuries. The level of the river rises and falls with rhythmic regularity throughout the year; a surge moves northwards to reach Aswan by the end of June, and arrive at Memphis and the delta region by September.
Then the waters gradually subside, until the river reaches its lowest level in the month of April. Over the centuries, these seasonal floods have been the lifeblood of Egyptian civilisation. They fed large areas of land with rich nutrients that made the valley fertile, and their failure could - and sometimes did - bring hardship, famine, pestilence and death. But their origin remained a mystery.
Herodotus of Halicarnassus confessed that he was stumped. "Concerning the nature of this river," he wrote, "I was not able to learn anything either from the priests or from anyone besides, though I questioned them very pressingly.
"For the Nile is flooded for a hundred days, beginning with the summer solstice and after this time it diminishes and is during the whole winter very small. And on this head I was unable to obtain anything satisfactory from any one of the Egyptians, when I asked what is the power by which the Nile is in its nature the reverse of other rivers."
The Egyptians had their own theories on the matter which they seem to have hidden from the curious foreigner: they attributed the rise and fall of the Nile to the Dog Star Sirius, which is prominent at the height of summer. Nowadays we have a much more plausible explanation.
We assign the seasonal flooding of the Nile to the tropical rains of the equatorial south where the White Nile begins its journey and, even more importantly, to the melting snows of the Abyssinian uplands where the Blue Nile starts.
