there, that the chain of the Alps was formerly covered by the sea, and
that above this chain there was a great thickness of vitrifiable sand,
which rendered the whole mountains a flat and level country. In this
depth of sand, there would necessarily be formed granite, free-stone,
flint, and all matters which take their origin and figure in sand,
nearly in a similar manner to that of the crystallisation of salts.
These blocks once formed would support their original positions, after
the rains and torrents had carried away the sand which surrounded them,
and being left bare formed all those peaks or pointed eminences we see
in so many places. This is also the origin of those high and detached
rocks found in China and other countries, as in Ireland, where they are
called the Devil's stones, and whose formation as well as that of the
peaks of mountains, had hitherto appeared so difficult to explain;
nevertheless the explanation which I have given is so natural, that it
directly presents itself to the mind of those who examine these objects,
and I must here quote what Father Tatre says, "From Yanchu-in-yen, we
came to Hoytcheou, and on the road met with something particular, rocks
of an extraordinary height, of the shape of a large square tower, and
situate in the midst of vast plains: I cannot account for it, unless by
supposing they were formerly mountains, from which the rain having
washed away the earth that surrounded them, thus left the rocks entirely
bare. What strengthens this conjecture is, that we saw some which,
towards the base, are still covered with earth to a considerable
height."
The summits of the highest mountains are composed of rocks, of granite,
free-stone, and other hard and vitrifiable matters, and this often as
deep as two or three hundred fathoms; below which we often meet with
quarries of marble, or hard stone, filled with fossil-shells, and whose
matter is calcinable; as may be remarked at Great Chartreuse, in
Dauphiny, and on Mount Cenis, where the stone and marble, which contains
shells, are some hundred fathoms below the summits, points and peaks of
high mountains; although these stones are more than a thousand fathom
above the level of the sea. Thus mountains, whereon we see points or
peaks, are generally vitrifiable rock, and those whose summits are flat,
mostly contain marble and hard stones filled with marine productions. It
is the same with respect to hills, for those containing granite, or
free-stone, are mostly intersected with points, eminences, cavities,
depths, and small intermediate valleys; on the contrary, those which
are composed of calcinable stone are nearly equal in height, and are
only interrupted by greater and more regular vallies, whose angles are
correspondent; and they are crowned with rocks whose position is regular
and level.
Whatever difference may appear at first between these two species of
mountains, their forms proceed from the same cause, as we have already
observed; only it may be remarked, that the calcinable stones have not
undergone any alteration nor change since the formation of the
horizontal strata; whereas those of vitrifiable sand have been changed
and interrupted by the posterior production of rocks and angular blocks
formed within this sand. These two kinds of mountains have cracks which
are almost always perpendicular in those of calcinable stones; but those
of granite and free-stone appear to be a little more irregular in their
direction. It is in these cracks metal, minerals, crystals, sulphurs,
and all matters of the second class are found, and it is below these
cracks that the water collects to penetrate the earth, and form those
veins of water which are every where found below the surface.
FOOTNOTES:
[279:A] See Phil. Trans. Abr. Vol. VI. part ii. p. 153.
ARTICLE X.
OF RIVERS.
We have before said that, generally speaking, the greatest mountains are
in islands and in the projections in the sea. That in the old continent
the greatest chains of mountains are directed from west to east, and
that those which incline towards the north or south are only branches of
these principal chains; we shall likewise find that the greatest rivers
are directed as the greatest mountains, and that there are but few which
follow the course of the branches of those mountains. To be assured of
this, we have only to look on a common globe, and trace the old
continent from Spain to China. We shall find, by beginning at Spain,
that the Vigo, Douro, Tagos, and Guadiana run from east to west, and the
Ebro from west to east, and that there is not one remarkable river whose
course is directed from south to north, or from north to south, although
Spain is entirely surrounded by the sea on the west side, and almost so
on the north. This observation on the directions of rivers in Spain not
only proves that the mountains in this country are directed from west to
east, but also that the southern lands, which border on the straits, are
higher than the coasts of Portugal; and on the northern coast, that the
mountains of Galicia, the Asturias, &c. are only a continuation of the
Pyrennees, and that it is this elevation of the country, as well north
as south, which does not permit the rivers to run into the sea that way.
It will also be seen, by looking on the map of France, that there is
only the Rhone which runs from north to south, and nearly half its
course, from the mountains to Lyons, is directed from the east towards
the west; but that on the contrary all the other great rivers, as the
Loir, the Charantee, the Garonne, and even the Seine, have a direction
from east to west.
It will be likewise perceived, that in Germany there is only the Rhine,
which like the Rhone shapes the greatest part of its course from north
to south, but that the others, as the Danube, the Drave, and all the
great rivers which fall into them, flow from the west to east into the
Black Sea.
It will be perceived that this Black Sea, which should rather be
considered as a great lake, has almost three times more extent from east
to west than from north to south, and consequently its direction is
similar to the rivers in general. It is the same with the Mediterranean,
whose length from east to west is about six times greater than from
north to south.
The Caspian Sea, according to the chart drawn by the order of Czar Peter
I. has more extent from the south to the north than from east to west;
whereas in the ancient charts it appears almost round, or rather more
broad from east to west than from south to north; but if we consider the
lake Aral as a part of the Caspian Sea, from which it is separated only
by plains of sand, we shall find the length is from the western coast of
the Caspian Sea as far as the greatest border of Lake Aral.
So likewise the Euphrates, the Persian gulph, and almost all the rivers
in China run from west to east; all the rivers in Africa beyond Barbary
flow from east to west, or from west to east, and there are only the
rivers of Barbary and the Nile which flow from south to north. There
are, in fact, great rivers in Asia which partly run from north to
south, as the Wolga, the Don, &c. but by taking the whole length of
their course, we find, that they only turn from the south to run into
the Black and Caspian seas, which are only inland lakes.
It may therefore in general be said, that in Europe, Asia, and Africa,
the rivers, and other mediterranean waters, extend more from east to
west than from north to south, which proceeds from the chains of
mountains being for the most part so directed, and that the whole
continent of Europe and Asia is broader in this direction than the
other; for there are two modes of considering the direction of
mountains. In a long and narrow continent like South America, in which
there is only one principal chain of mountains which stretches from
south to north, the river not being confined by any parallel range,
necessarily runs perpendicular to the course of the mountains, that is
from east to west, or from west to east; in fact, it is in this
direction all the rivers of America flow. In the old as well as the new
continent most of the waters have their greatest extent from west to
east, and most of the rivers flow in this direction; but yet this
similar direction is produced by different causes; for instance, those
in the old continent flow from east to west, because they are bounded
by mountains whose direction is from west to east; whereas those in
America preserve the same course from there being only one chain of
mountains that extends from north to south.
In general, rivers run through the centre of vallies, or rather the
lowest ground betwixt two opposite hills or mountains; if the two hills
have nearly an equal inclination, the river will be nearly in the middle
of the intermediate valley, let the valley be broad or narrow. On the
contrary, if one of the hills has a more steep inclination than the
other, the river will not be in the middle of the valley, but much
nearer the hill whose inclination is greatest, and that too in
proportion to the superiority of its declivity: in this case, the lowest
ground is not in the middle of the valley, but inclines towards the
highest hill, and which the river must necessarily occupy. In all places
where there is any considerable difference in the height of the
mountains, the rivers flow at the foot of the steepest hills, and follow
them throughout all their directions, never quitting their course while
they maintain the superiority of height. In the length of time, however,
the steepest hills are diminished by the rain acting upon them with a
greater degree of force, proportionate to their height, and consequently
carry away the sand and gravel in more considerable quantities, and with
greater violence; the river is then constrained to change its bed, and
seek the lowest part of the valley: to this may be added, that as all
rivers overflow at times, they transport and deposit mud and sand in
different places, and that sands often accumulate in their own beds, and
cause a swell of the water, which changes the direction of its course.
It is very common to meet in vallies with a great number of old channels
of the river, particularly if it is subject to frequent inundations, and
carries off much sand and mud.
In plains and large vallies, where there are great rivers, the beds are
generally the lowest part of the valley, but the surface of the water is
very often higher than the ground adjacent. For example, when a river
begins to overflow, the plain will presently be inundated to a
considerable breadth, and it will be observed that the borders of the
river will be covered the last; which proves that they are higher than
the rest of the ground, and that from the banks to a certain part of
the plain, there is an insensible inclination, so that the surface of
the water must be higher than the plain when the river is full. This
elevation on the banks of rivers proceeds from the deposit of the mud
and sand at the time of inundations. The water is commonly very muddy in
the great swellings of rivers; when it begins to overflow, it runs very
gently over the banks, and by depositing the mud and sand purifies
itself as it advances into the plain; so that all the soil which the
currents of the river does not carry along, is deposited on the banks,
which raises them by degrees above the rest of the plain.
Rivers are always broadest at their mouths; in proportion as we advance
in the country, and are more remote from the sea, their breadth
diminishes; but what is more remarkable, in the inland parts they flow
in a direct line, and in proportion as they approach their mouths the
windings of their course increase. I have been informed by M. Fabry, a
sensible traveller, who went several times by land into the western part
of North America, that travellers, and even the savages, are seldom
deceived in the distance they are from the sea if they follow the bank
of a large river; when the direction of the river is straight for 15 or
20 leagues, they know themselves to be a great distance from the coast;
but, on the contrary, if the river winds, and often changes its
direction, they are certain of not being far from the sea. M. Fabry
himself verified this remark in his travels over that unknown and almost
uninhabited country. In large rivers there is a considerable eddy along
the banks, which is so much the more considerable as the river is less
remote from the sea, which may also serve as a guide to judge whether we
are at a great or short distance from the mouth; and as the windings of
rivers increase in proportion as they approach the sea, it is not
surprising that some of them should give way to the water, and be one
reason why great rivers generally divide into many arms before they gain
the sea.
The motion of the waters in rivers is quite different from that supposed
by authors who attempt to give mathematical theories on this subject;
the surface of a river in motion is not level when taken from one bank
to the other, but according to circumstances the current in the middle
is considerably higher or lower than the water close to the banks; when
a river swells by a sudden melting of snow, or when by some other cause
its rapidity is augmented, if the direction of the river is straight,
the middle of the water where the current is rises, and the river forms
a convex curve, of a very sensible elevation. This elevation is
sometimes very considerable; M. Hupeau, an able engineer of bridges,
once measured the river Avieron, and found the middle was three feet
higher than near the bank. This, in fact, must happen every time the
water has a very great rapidity; the velocity with which it is carried,
diminishing the action of its weight in the middle of the current, so
that it has not time to sink to a level with that near shore, and
therefore remains higher. On the other hand, near the mouths, it often
happens that the water which is near the banks is higher than that of
the middle, although the current be ever so rapid. This happens wherever
the action of the tides is felt in a river, which in great ones often
sensibly extends as far as one or two hundred leagues from the sea; it
is also a well known fact that the current of a river preserves its
motion in the sea to a considerable distance; there is, in this case,
therefore, two contrary motions in a river; the middle, which forms the
current, precipitates itself towards the sea, and the action of the
tide forms a counter-current, which causes the water near the banks to
ascend, while that in the middle descends, and as then all the water
must be carried down by the current in the middle, that of the banks
continually descends thereto, and descends so much the more as it is
higher, and counteracted with more force by the tide.
There are two kinds of ebbings in rivers; the first above-mentioned is a
strong power occasioned by the tide, which not only opposes the natural
motion of the river, but even forces a contrary and opposite current.
The other arises from an inactive cause, such as a projection of land,
an island, &c. This does not commonly occasion a very sensible
counter-current, yet it is sufficient to impede the progress of boats
and craft, and necessarily produces what is called a dead water, which
does not flow like the rest of the river, but whirls about in such a
manner that when boats are drawn therein they require great strength to
get them out. These dead waters are very perceptible at the arches of
bridges in rapid rivers. The velocity of the water increases in
proportion as the diameter of the channel through which it passes
diminishes, the impelling force being the same; the velocity of a
river, therefore, increases at the passage of a bridge, in an inverse
proportion of the breadth of the arches to the whole breadth of the
river; the rapidity being very considerable in coming through the arch,
it forces the water against the banks, from whence it is reflected with
such violence as to form dangerous eddies and whirlpools. In going
through the bridge St. Esprit, the men are forced to be careful not to
lose the stream, even after they are past the bridge, for if they suffer
the boat to go either to the right or left, it might be driven against
the shore, or forced into the whirling waters, which would be attended
with great danger. When this eddy is very considerable, it forms a kind
of small gulph, the middle of which appears hollow and to form a kind of
cylindrical cavity, around which the water whirls with rapidity: this
appearance of a cylindrical cavity is produced by the centrifugal force,
which causes the water to endeavour to remove itself from the centre of
the whirlpool. When a great swell of water happens, the watermen know it
by a particular motion; they then say the water at the bottom flows
quicker than common: this augmentation of rapidity at the bottom,
according to them, always announces a sudden rise of the water. The
motion and weight of the upper water communicates this motion to them;
for in certain respects we must consider a river as a pillar of water
contained in a tube, and the whole channel as a very long canal where
every motion must be communicated from one end to the other. Now,
independent of the motion of the upper waters, their weight alone might
cause the rapidity of the river to increase, and perhaps move it at
bottom; for it is known, that by putting many boats at one time into the
water, at that instant we increase the rapidity of the under part of the
river, as well as retard that of the upper.
The rapidity of running waters does not exactly, nor even nearly, follow
the proportion of the declivity of their channels. One river whose
inclination is uniform and double that of another, ought, according to
appearance, to flow only as rapid again, but in fact it flows much
faster. Its rapidity, instead of being doubled, is sometimes triple,
quadruple, &c. This rapidity depends much more on the quantity of water
and the weight of the upper waters than on the declivity. When we are
desirous to hollow the bed of a river, we need not equally distribute
the inclination throughout its whole length, in order to give a greater
rapidity, as it is more easily effected by making the descent much
greater at the beginning, than at the mouth, where it may almost be
insensible, as we see it in natural rivers, and yet they preserve a
rapidity so much the greater as the river is fuller of water; in great
rivers, where the ground is level, the water does not cease flowing, and
even rapidly, not only with its original velocity, but also with the
addition of that which it has acquired by the action and weight of the
upper waters. To render this fact more conceivable, let us suppose the
Seine between the Pont-neuf and Pont-royal to be perfectly level, and
ten feet deep throughout: let us then suppose that the bed of the river
below Pont-royal and above Pont-neuf were left entirely dry, the water
would instantly run up and down the channel, and continue to do so until
it had recovered an equilibrium; for the weight of the water would keep
it in motion, nor would it cease flowing until its particles became
equally pressed and have sunk to a perfect level. The weight of water
therefore greatly contributes to its velocity, and this is the reason
that the greatest rapidity of the current is neither of the surface nor
at the bottom of the water, but nearly in the middle of its depth, being
pressed by the action of its weight at its surface, and by the re-action
from the bottom. Still more, if a river has acquired a great rapidity,
it will not only preserve it in passing a level country, but even
surmount an eminence without spreading much on either side, or at least
without causing any great inundation.
We might be inclined to think that bridges, locks, and other obstacles
raised on rivers, considerably diminishes the celerity of the water's
course; nevertheless that occasions but little difference. Water rises
on meeting with any obstacle, and having surmounted it, the elevation
causes it to act with more rapidity in its fall, so that in fact it
suffers little or no diminution in its celerity, by these seeming
retardments. Sinuosities, projections, and islands, also but very little
diminish the velocity of the course of rivers. A considerable diminution
is produced by the sinking of the water, and, on the contrary, its
augmentation increases its velocity; thus if a river is shallow the
stream passes slowly along, and if deep with a proportionate rapidity.
If rivers were always nearly of an equal fulness, the best means of
diminishing their rapidity, and confining them within their banks, would
be to enlarge their channel; but as almost all rivers are subject to
increase and diminish, to confine them we must retrench the channel,
because in shallow waters, if the channel is very broad, the water which
passes in the middle hollows a winding bed, and when it begins to swell
follows the direction it took in this particular bed, and striking
forcibly against the banks of the channel destroys them and does great
injuries. These effects of the water's fury might be prevented by
making, at particular distances, small gulphs in the earth; that is, by
cutting through one of these banks to a certain distance in the land. In
order that these gulphs might be advantageously placed, they should be
made in the obtuse angle of the river, for then the current of the water
in turning would run into them, and of course its velocity would be
diminished. This mode might be proper to prevent the fall of bridges in
places where it is not possible to make bars near the bridge which
sustain the action of the weight of the water.
The manner in which inundations are occasioned merits peculiar
attention. When a river swells, the rapidity of the water always
increases till it begins to overflow the banks; at that instant the
velocity diminishes, which causes inundations to continue for several
days; for when even a less quantity of water comes after the overflowing
than before, the inundation will still be made, because it depends much
more on the velocity of the water than on the quantity; if it was not so
rivers would overflow for an hour or two and then return to their beds,
which never occurs; the inundations always remaining for several days;
whether the rain ceases, or a less quantity of water is brought, because
the overflowing has diminished the velocity, and consequently, although
the like quantity of water is no longer carried in the same time as
before, yet the effects are the same as if the greater quantity had come
there. It might be remarked on the occasion of this diminution, that if
a constant wind blows against the current of the river, the inundation
will be much greater than it would have been without this accidental
cause, which diminishes the celerity of the water; on the contrary, if
the wind blows in the same directions with the current, the inundation
will be much less, and will more speedily decline.
"The swelling of the Nile, says M. Granger, and its inundations, has a
long time employed the learned; most of them have looked upon it as
marvellous, although nothing can be more natural, and is every day to be
seen in every country throughout the world. It is the rains which fall
in Abyssinia and Ethiopia which cause the swelling and inundation of
that river, though the north wind must be regarded as the principal
cause. 1. Because the north wind drives the clouds which contain this
rain into Abyssinia. 2. Because, blowing against the mouths of the Nile,
it causes the waters to return against the stream, and thus prevents
them from running out in any great quantity: this circumstance may be
every season observed, for when the wind, being at the north, suddenly
veers to the south, the Nile loses in one day more than it gathers in
four."
Inundations are generally greatest in the upper part of rivers, because
the velocity of a river continues always increasing until it arrives at
the sea, for the reasons we have related. Father Costelli, who has
written very sensibly on this subject, remarks, that the height of the
banks made to confine the Po from overflowing diminishes as they advance
towards the sea; so that at Ferrara, which is 50 or 60 miles from the
sea, they are near 20 feet high above the common surface of the Po, but
that at 10 or 12 miles from it they are not above 12 feet, although the
channel of the river is as narrow there as at Ferrara[306:A].