I hope my reader will not expect that I should enter into the discussion of the part St John is thought to have in this event, my business is only with natural causes.
Memphis and Alexandria, and all the ancient cities of Lower Egypt, stand upon cisterns, into which the Nile, upon its overflowing, was admitted, and there remained till it had deposited all its sediment, and became fit for drinking. These cisterns are now full of filth; though in disrepair, the water, when the Nile is high insinuates itself into them through the broken conduits.
In February and March the sun is on its approach to the zenith of one extremity of Egypt, and of course has a very considerable influence upon the other. The Nile being now fallen low, the water in the cisterns putrifies, and the river itself has lost all its volatile and finer parts by the continued action of a vertical sun; so that, instead of being subject to evaporation, it becomes daily more and more inclined to putrefaction. About St John’s day[178] it receives a plentiful mixture of the fresh and fallen rain from Ethiopia, which dilutes and refreshes the almost corrupted river, and the sun near at hand exerts its natural influence upon the water, which now is become light enough to be exhaled, though it has still with it a mixture of the corrupted fluid, so that it rises but a small height during the first few days of the inundation, then falls down and returns to the earth in plentiful and abundant dews; and that this is really so, I am persuaded from what I observed myself at Cairo.
My quadrant was placed on the flat roof, or terrass, of a gentleman’s house where I was taking observations; I had gone down to supper, and soon after returned, when I found the brass limb of the quadrant covered with small drops of dew, which were turned to a perfect green, or copperas colour; and this green had so corroded the brass in an hour’s time, that the marks remained on the limb of the quadrant for six months; and the cavities made by the corrosion were plainly discernible through a microscope.
It is in February, March, or April only, that the plague begins in Egypt. I do not believe it an endemial disease, I rather think it comes from Constantinople with merchandise, or passengers, and at this time of the year that the air having attained a degree of putridity proper to receive it by the long absence of dews, the infection is thereto joined, and continues to rage till the period I just spoke of, when it is suddenly stopped by the dews occasioned by a refreshing mixture of rain-water, which is poured out into the Nile at the beginning of the inundation.
The first and most remarkable sign of the change brought about in the air is the sudden stopping of the plague at Saint John’s day; every person, though shut up from society for months before, buys, sells, and communicates with his neighbour without any sort of apprehension; and it was never known, as far as I could learn upon fair inquiry, that one fell sick of the plague after this anniversary: it will be observed I don’t say died; there are, I know, examples of that, though I believe but few; the plague is not always a disease that suddenly terminates, it often takes a considerable time to come to a head, appearing only by symptoms; so that people taken ill, under the most putrid influence of the air, linger on, struggling with the disease which has already got such hold that they cannot recover; but what I say, and mean is, that no person is taken ill of the plague so as to die after the dew has fallen in June; and no symptoms of the plague are ever commonly seen in Egypt but in those spring months already mentioned, the greater part of which are totally destitute of moisture.
I think the instance I am going to give, which is universally known, and cannot be denied, brings this so home that no doubt can remain of the origin of this dew, and its powerful effects upon the plague.
The Turks and Moors are known to be predestinarians; they believe the hour of man’s death is so immutably fixed that nothing can either advance or defer it an instant. Secure in this principle, they expose in the market-place, immediately after Saint John’s day, the clothes of the many thousands that have died during the late continuance of the plague, all which imbibe the moist air of the evening and the morning, are handled, bought, put on, and worn without any apprehension of danger; and though these consist of furs, cotton, silk, and woollen cloths, which are stuffs the most retentive of the infection, no accident happens to those who wear them from this their happy confidence.
I shall here sum up all that I have to say relating to the river Nile, with a tradition handed down to us by Herodotus, the father of ancient history, upon which moderns less instructed have grafted a number of errors. Herodotus[179] says, that he was informed by the secretary of Minerva’s treasury, that one half of the water of the Nile flowed due north into Egypt, while the other half took an opposite course, and flowed directly south into Ethiopia.
The secretary was probably of that country himself, and seems by his observation to have known more of it than all the ancients together. In fact, we have seen that, between 13° and 14° N. latitude, the Nile, with all its tributary streams, which have their rise and course within the tropical rains, falls down into the flat country, (the kingdom of Sennaar), which is more than a mile lower than the high country in Abyssinia, and thence, with a little inclination, it runs into Egypt.