subject to many difficulties, nor is it easy to lay down rules which
might be applied to every particular case. Experience is here more
useful than speculation. We must not only know the general effects of
rivers, but we must also know in particular the river we have to do
with, if we would reason justly, make useful observations, and draw
stable conclusions. The remarks I have above given are mostly new; it is
to be wished that others may be collected, and then, possibly, in time,
we may obtain a sufficient knowledge of the subject to lay down certain
rules to confine and direct rivers, and prevent the ruin of bridges,
banks, and other damages which the violent impetuosity of the water
occasions.
The greatest rivers in Europe are the Wolga, which is about 650 leagues
in its course from Reschow to Astracan, on the Caspian Sea; the Danube,
whose course is about 450 leagues from the mountains of Switzerland to
the Black Sea; the Don, which is 400 leagues in its course from the
source of the Sosnia, which it receives, to its mouth in the Black Sea;
the Dnieper, whose course is about 350 leagues, and which also runs into
the Black Sea; the Duine is about 400 leagues in its course, and empties
itself into the White Sea, &c.
The greatest rivers in Asia are the Hoanho of China, whose course is 850
leagues, taking its source at Raja-Ribron, and falls into the sea of
China, in the middle of the gulph Changi: the Jenisca of Tartary, which
is about 800 leagues in extent, from the lake Seligna to the northern
sea of Tartary; the river Oby, which is about 600 leagues from Lake
Kila, to the Northern Sea, beyond the Strait of Waigats. The river
Amour, of eastern Tartary, which is about 575 leagues in its course,
reckoning it from the source of the river Kerlon, to the sea of
Kamschatka. The river Menan, whose mouth is at Poulo Condor, may be
measured from the surface of the Longmu which falls into it; the Kian,
whose course is about 550 leagues from the source of the river Kinxa,
which it receives, to its mouth in the China Sea; the Ganges is also
about 550 leagues, and the Euphrates 500, taking it from the source of
Irma, which it receives. The Indus about 400 leagues, and which falls
into the Arabian Sea, on the east of Guzarat. The Sirderious, which is
about 400 leagues long, and falls into Lake Aral.
The greatest rivers in Africa are Senegal, which is 1125 leagues long,
comprehending the Niger, which in fact is a continuation of it, and the
source of Gombarou, which falls into the Niger. The Nile 970 leagues
long, and which derives its source in Upper Ethiopia, where it makes
many windings. There are also the Zaira, the Coanza, and the Couma,
which are known as far as 400 leagues, but extend much farther; the
Quilmanci, whose course is 400 leagues, and which derives its source in
the kingdom of Gingiro.
The greatest rivers of America, and which are also the greatest in the
world, are the river Amazons, whose course is 1200 leagues, if we go up
as far as the Lake near Guanuco, 30 leagues from Lima, where the
Maragnon takes its source; and even reckoning from the source of the
river Napo, some distance from Quito, the course of the river Amazons is
more than a thousand leagues.
It might be said that the course of the river St. Lawrence, in Canada,
is more than 900 leagues from its mouth to the lake Ontaro, from thence
to lake Huron, afterwards to the lake Alemipigo, and to the lake
Assiniboils; the waters of these lakes falling one into another, and at
last into St. Lawrence.
The river Mississippi more than 700 leagues long from its mouth to any
of its sources, which are not remote from the lake of the Assiniboils.
The river de la Plata is more than 800 leagues long, from the source of
the river Parana, which it receives.
The river Oroonoko runs more than 575 leagues, reckoning from the source
of the river Caketa, near Pasto, part of which falls into the Oroonoko,
and part flows also towards the river Amazons.
The river Madera, which falls into the Amazons, is more than 660
leagues.
To know nearly the quantity of water the sea receives by all the rivers
which fall into it, let us suppose that one half of the globe is covered
by the sea, and that the other half is land, which is nearly the fact;
let us suppose also, that the mediate depth of the sea is 230 fathom.
The surface of all the earth being 170,981,012 square miles; and that of
the sea 85,490,506 square miles, which being multiplied by 1/4, the
depth of the sea gives 21,372,626, cubical miles for the quantity of
water contained in the ocean. Now, to calculate the quantity of water
which the ocean receives from the rivers, let us take some great river,
whose rapidity and quantity of waters are known; for example, the Po,
which runs through Lombardy, and waters a tract of land 380 miles long;
according to Riccioli, its breadth, before it divides into many
trenches, is 100 perches of Boulogne, or 1000 feet, its depth 10 feet,
and it runs four miles an hour; therefore the Po supplies the sea with
200,000 cubical perches of water in an hour, or 4 millions 800 thousand
in a day; but a cubical mile contains 125 millions cubical perches;
therefore 26 days is required to convey a cubical mile of water to the
sea: it remains therefore only to determine the proportion between the
river Po and all the rivers of the earth taken together, which is
impossible to do precisely. But to know it pretty exactly, let us
suppose that the quantity of water which the sea receives by the large
rivers in all countries is proportional to the extent and surface of
these countries, and that consequently the country watered by the Po,
and other rivers which fall therein, is in the same proportion on the
surface of the whole earth, as the Po is to all the rivers of the earth.
Now by the most correct charts, the Po, from its source to its mouth,
traverses a tract 380 miles long, and the rivers which fall therein, on
each side, proceed from the springs and rivers 60 miles distant from the
Po; therefore this great river, and the others it receives, waters a
tract 380 miles long, and 120 miles broad, which makes 450,600 square
miles, but the surface of all the dry land is 85,490,506 square miles;
consequently all the water which the rivers carry to the sea, will be
1974 times greater than the quantity which the Po furnishes; but as 26
rivers equal to the Po furnish a cubical mile of water to the sea in a
day, of course 1874 rivers like the Po would supply the sea with 26,308
cubical miles of water in a year, and that in the space of 812 years all
the rivers would supply the sea with 21,372,626 cubical miles of water;
that is to say, as much as there is in the ocean, and therefore 812
years is only required to fill it.[312:A]
The result of this calculation is, that the quantity of water evaporated
from the sea, and which the winds convey on the earth, is about 245
lines, or from 20 to 21 inches a year, or about two thirds of a line
each day: this is a very trifling evaporation even when trebled, in
order to estimate the water which refalls in the sea, and which is not
conveyed over the earth. Mr. Halley, in the Phil. Transactions, page
192, evidently shews, that the vapours which rise above the sea, and
which the winds convey over all the earth, are sufficient to supply all
the rivers in the world.
Next to the Nile the river Jordan is the most considerable in the
Levant, or even in Barbary; it supplies the Dead Sea with about six
million tons of water every day; all this water, and more, is raised by
evaporation; for, according to Halley's calculation of 6914 tons
evaporated from each mile, the Dead Sea, which is 72 miles in length by
18 broad, must every day lose near nine million tons of water, that is,
not only all the water it receives from the river Jordan, but also that
of the small rivers which come into it from the mountains of Moab and
elsewhere; consequently there is no necessity for its communicating with
any other sea by subterraneous canals.[313:A]
The most rapid rivers are the Tigris, the Indus, the Danube, the Yrtis,
in Siberia, the Malmistra, in Silesia, &c. but, as we have already
observed, the proportion of the rapidity of rivers depends upon the
declivity and upon the weight and quantity of water; by examining the
globe, we shall find that the Danube is much less inclined than the Po,
the Rhine, or the Rhone, for the Danube has a much longer course than
any of these other rivers, and falls into the Black Sea, which is higher
than the Mediterranean, and perhaps more so than the ocean.
All large rivers receive many others in the extent of their course; for
example, the Danube receives more than 200 rivulets and rivers; but by
reckoning only such as are considerable rivers, we shall find that the
Danube receives 31, the Wolga 32, the Don 5 or 6, the Nieper 19 or 20,
the Duine 11 or 12; so likewise in Asia the Hoanho receives 34 or 35,
the Jenisca 60, the Oby as many, the Amour about 40, the Kian, or river
Nankin about 30, the Ganges upwards of 20, the Euphrates 10 or 11, &c.
In Africa, the river Senegal receives upwards of 20 rivers: the Nile
does not receive any rivers for upwards of 500 miles from its mouth; the
last which falls therein is the Moraba, and from this place to its
source it receives about 12 or 13 rivers. In America, the river Amazons
receives more than 60, all of which are very considerable; the river St.
Lawrence about 40, by reckoning those which fall into the lakes; the
Mississippi more than 40, the Plata more than 50, &c.
There are high countries on the earth, which seem to be points of
division marked by nature for the distribution of the waters. In
Europe, the environs of Mount St. Goddard are one of these points;
another is situate between the provinces of Belozera and Wologda, in
Muscovy, from whence many rivers descend, some of which go to the White
Sea, others to the Black, and some to the Caspian. In Asia there are
several, in the country of Mogul Tartary, from whence rivers flow into
Nova Zembla, others to the Gulph Linchidolin, others to the sea of
Corea, others to that of China: and so likewise the Little Thibet, whose
waters flow towards the sea of China; the Gulph of Bengal, the Gulph of
Cambay, and the Lake Aral; in America, the province of Quito; whose
rivers run into the North and South Seas, and the Gulph of Mexico.
In the old continent there are about 430 rivers, which fall directly
into the ocean, or into the Mediterranean and Black Seas; but in the new
continent not more than 145 rivers are known, which fall directly into
the sea: in this number I have comprehended only the great rivers, like
the Somme in Picardy.
All these rivers carry to the sea a great quantity of mineral and saline
particles, which they have washed from the different soils through
which they have passed. The particles of salt, which are easily
dissolved, are conveyed to the sea by the water. Some philosophers, and
among the rest Halley, have pretended that the saltness of the sea
proceeded only from the salts of the earth, which the rivers transport
therein. Others assert, that the saltness of the sea is as ancient as
the sea itself, and that this salt was created that the waters might not
corrupt; but we may justly suppose that the sea is preserved from
corruption by the agitations produced by the winds and tides, as much as
by the salt it contains; for when put in a barrel it corrupts in a few
days; and Boyle relates, that a mariner, who was becalmed for 13 days,
found, at the end of that time, the water so infected, that if the calm
had not ceased, the greatest part of his people would have perished. The
water of the sea is also mixed with a bituminous oil, which gives it a
disagreeable taste, and renders it very unhealthful. The quantity of
salt contained in sea water is about a fortieth part, and is nearly
equally saline throughout, at top as well as bottom, under the line, and
at the Cape of Good Hope; although there are several places, as off the
Mosambique Coast, where it is salter than elsewhere.[317:A] It is also
asserted not to be so saline under the Arctic Circle, which may proceed
from the amazing quantities of snow, and the great rivers which fall
into those seas, and because the heat of the sun produces but little
evaporation in hot climates.
Be this as it may, I conceive that the saltness of the sea is not only
caused by the banks of salt at the bottom of the sea, and along the
coasts, but also by the salts of the earth, which the rivers continually
convey therein; and that Halley had some reason to presume that in the
beginning of the world the sea had but little or no saltness; that it is
become so by degrees, and in proportion as the rivers have brought salts
therein; that this saltness is every day increasing, and that
consequently, by computing the whole quantity of salt brought by all the
rivers, we might attain the knowledge of the age of the world by the
degrees of the saltness of the sea.
Divers and pearl fishers assert, according to Boyle, that the deeper
they descend into the sea, the colder is the water; and that the cold is
so intense at considerable depths, that they cannot remain there so long
under water, but are obliged to come up again much sooner than when
they descended to only a moderate one. It appeared to me that the weight
of the water might be as much the cause of compelling them to shorten
their usual time as the intenseness of the cold, when they descend to a
depth of 3 or 400 fathoms; but, in fact, divers scarcely ever descend
above an hundred feet. The same author relates, that in a voyage to the
East-Indies, beyond the line, at about 35 degrees south latitude, a
sounding lead of 30 or 35lb weight was sunk to the depth of 400 fathoms,
and that being pulled up again, it had become as cold as ice. It is also
a frequent practice with mariners to cool their wine at sea by sinking
their bottles to the depth of several fathoms, and they affirm the
deeper the bottles are sunk, the cooler is the wine.
These circumstances might induce us to presume that the sea is salter at
the bottom than at the surface; but we have testimonies which prove the
contrary, founded on experiments made to fill vessels with sea water,
which were not opened till they were sunk to a certain depth, and the
water was found to be no salter than at the surface. There are even some
places where the water at the surface is salt, and that at the bottom
fresh; and this must always be the case where there are springs at the
bottom of the sea, as near Goo, Ormus, and even in the sea of Naples,
where there are hot springs at the bottom.
There are other places where sulphurous springs and beds of bitumen have
been discovered at the bottom of the sea, and on land there are many of
these springs of bitumen which run into it.
At Barbadoes there is a pure bitumen spring, which flows from the rocks
into the sea: salt and bitumen, therefore, are predominant matters in
the sea water: but it is also mixed with many other matters; for the
taste of water is not the same in every part of the sea; besides, the
agitation and the heat of the sun alters the natural taste which the sea
should have; and the different colour of different seas, at different
times, prove that the waters of the sea contain several kinds of
matters, either which it loosens from its own bottom, or are brought
thither by rivers.
Almost all countries watered by great rivers are subject to periodical
inundations, those which are low, and derive their sources from a great
distance, overflow the most regularly. Every person almost has heard of
the inundations of the Nile, which preserves the sweetness and whiteness
of its waters, though extended over a vast tract of country, and into
the sea. Strabo and other ancient authors have written that it had seven
mouths, but there now remain only two which are navigable; there is a
third canal which descends to Alexandria, and fills the cisterns there,
and a fourth which is still smaller; but as they have for a long time
neglected to clean their canals, they are nearly choaked up. The
ancients employed a great number of workmen and soldiers, and every
year, after the inundation, they carried away the mud and sand which was
in these canals. The cause of the overflowing of the Nile proceeds from
the rains which fall in Ethiopia. They begin in April and do not cease
till September; during the first three months, the days are serene and
fair, but as soon as the sun goes down the rains begin, nor stop till it
rises again, and are generally accompanied with thunder and lightning.
The inundation begins in Egypt about the 17th of June; it generally
increases during 40 days, and diminishes in about the same time; all the
flat country of Egypt is overflowed; but this inundation is much less
now than it was formerly, for Herodotus tells us, that the Nile was 100
days in swelling, and as many in abating: if this is true, we can only
attribute the cause thereof to the elevation of the land, which the mud
of the waters has heightened by degrees, and to the diminution of the
mountains in Africa, from whence it derives its source. It is very
natural to believe that these mountains have diminished, because the
abundant rains which fall in these climates during half the year sweep
away great quantities of sand and earth from the mountains into the
valleys, from whence the torrents wash them into the Nile, which carries
great part into Egypt, where it deposits them in its overflowings.
The Nile is not the only river whose inundations are regular; the river
Pegu is called the _Indian Nile_, because it overflows regularly every
year; it inundates the country for more than 30 leagues from its banks;
and, like the Nile, leaves an abundance of mud, which so greatly
fertilizes the earth, that the pasturage is excellent for cattle, and
rice grows in such great abundance, that every year a number of vessels
are laden with it, without leaving a scarcity in the country.[321:A] The
Niger, or what amounts to the same, the upper part of the Senegal,
likewise overflows and covers all the flat country of Nigritia; it
begins nearly at the same time as the Nile, and increases also for 40
days: the river de la Plata, in Brasil, also overflows every year, and
at the same time as the Nile. The Ganges, the Indus, the Euphrates, and
some others, overflow annually; but all rivers have not periodical
overflowings, and when inundations happen it is the effect of many
causes, which combine to supply a greater quantity of water than common,
and, at the same time, to retard its velocity. We have before observed,
that in almost all rivers the inclination of their beds diminishes
towards their mouths in an almost insensible manner; but there are some
whose declivity is very sudden in some places, and forms what is termed
a _cataract_, which is nothing more than a fall of water, quicker than
the common current of the river. The Rhine, for example, has two
cataracts, the one at Bilefield, and the other near Schafhouse: the Nile
has many, and among the rest two which are very violent, and fall from a
great height between two mountains; the river Wologda, in Muscovy, has
also two near Ladoga; the Zaire, a river of Congo, begins by a very
large cataract, which falls from the top of a mountain; but the most
famous is that of Niagara, in Canada, that falls from a perpendicular
height of 156 feet, like a prodigious torrent, and is more than a
quarter of a mile broad: the fog, or mist, which the water makes in
falling, is perceived at five miles distance, and rises as high as the
clouds, forming a very beautiful rainbow when the sun shines thereon.
Below this cataract there are such terrible whirlpools, that nothing can
be navigated thereon for six miles distance, and above the cataract the
river is much narrower than it is in the upper lands[323:A]. The
description given of it by _Father Charlevoix_ is as follows:
"My first care, when I arrived, was to visit the most beautiful cascade
that is, perhaps, in nature; but I immediately discovered that Baron la
Hontain was deceived so greatly, both in its height and figure, that one
might reasonably imagine he had never seen it.
"It is true, that if we measure its height by the three mountains you
are obliged to ascend in going to it, there is not much abatement to be
made of the 600 feet, which the map of M. Delisse gives it, who
doubtless advanced this paradox only on the credit of the Baron la
Hontain, and Father Honnepin; but after I arrived at the top of the
third mountain, I observed that in the space of three leagues, which I
afterwards had to go to this fall of water, although you are forced
sometimes to ascend, you must nevertheless descend still more, and this
is what travellers do not appear to have paid proper attention to. As we
can only approach the cascade on one side, nor see it but in the
profile, it is not easy to measure its height by instruments:
experiments have been made to do it by a long cord, tied to a pole, and
after having often attempted this manner, it was found to be only 115 or
120 feet high; but it is impossible to ascertain whether the pole was
not stopped by some projection of the rock; for although when drawn up
again the end of the cord was always wet, yet that is no proof, since
the water which precipitates from the mountain, flies up again in foam
to a very great height: for my own part, after having considered it on
every side that I could examine it to advantage, I think that we cannot
allow it to be less than 140 or 150 feet.
"Its figure is that of a horse-shoe, and its circumference is about 400
paces; but exactly in its middle, it is divided by a very narrow
island, about half a quarter of a league long. It is true these two
parts join again; that which was on my side, and of which I could only
have a side view, has several projecting points, but that which I beheld
in front, appeared to be perfectly even." The Baron has also mentioned a
torrent, which, if not the offspring of his own invention, must fall
into some channel upon the melting of the snow.
There is another cataract three miles from Albany, in the province of
New-York, whose height is 50 feet perpendicular, and from which there
arises a mist that occasions a faint rainbow.[325:A]
In all countries where mankind are not sufficiently numerous to form
polished societies, the ground is more irregular, and the beds of rivers
more extended, less equal, and often abound with cataracts. Many ages
were required to render the Rhone and the Loire navigable. It is by
confining waters, by directing their course, and by cleansing the bottom
of rivers, that they obtain a fixed and regular course; in all countries
thinly inhabited Nature is rude, and often deformed.
There are rivers which lose themselves in the sands, and others which
seem to precipitate into the bowels of the earth: the Guadalquiver in
Spain, the river Gottenburg in Sweden, and the Rhine itself, lose
themselves in the earth. It is asserted, that in the west part of the
island of St. Domingo there is a mountain of a considerable height, at
the foot of which are many caverns, into which the rivers and rivulets
fall with so much noise, as to be heard at the distance of seven or
eight leagues.[326:A]
The number of rivers which lose themselves in the earth is very few, and
there is no appearance that they descend very low; it is more probable
that they lose themselves, like the Rhine, by dividing among the
quantity of sand; this is very common to small rivers that run through
dry and sandy soils, of which we have several examples in Africa,
Persia, Arabia, &c.