594 
considerable body' of water; in award, that 
it can contain vast caverns ; and yet every 
thing seems to indicate the contrary. A fact 
that is observed in the betoirs of the rivers 
concerning which we have spoken, and par- 
ticularly of the Rille, proves in some mea- 
sure that there are considerable lakes of wa- 
ters in the mountains which limit its course : 
this fact is, that in winter the greatest part of 
their betoirs become springs, which supply 
anew the river’s channel with as much water 
as they had absorbed from it during the sum- 
mer, ' Now from whence can that water 
come, unless from ihe reservoirs or lakes 
that are inclosed in the mountains, which 
being tower than tin- river in summer, absorb 
its water, and being higher in winter by the 
rain they receive, send it back again in 
their turn ? 
M. Guettard strengthens this conjecture 
by several instances that render it very pro- 
bable : he remarks at the same time, that 
this alternate effect of the betoirs swallowing 
up the water and restoring it again, causes 
p chaps an invincible obstacle to the re- 
straining of the water within the channel of 
the river. It has indeed been several times 
attempted to stop those cavities ; but the 
waier returns with such violence in winter, 
tint it generally carries away the materials 
with which they were stopped. 
The river of Sap- Andre is iost in part, as 
we have before said, in the same manner as 
the [then and the Rille ; but there is some- 
thing more remarkable in it than in those 
rivers ; to wit, that at the extremity of its 
course, where there is no perceptible cavity, 
it is ingulphed, but without any fall ; the 
water passes between the pebbles, and it is 
impossible to force a stick into that place any 
further than into the betoirs of which we have 
spoken. What makes this river take that 
subterraneous direction, is an impediment 
which its stream meets with in that place; 
it is there stopped by a rising ground six 
or seven feet high, whose bottom it has very- 
likely undermined, to gain a free passage, 
not having been able to make its way over 
it. At some distance it appears again ; but 
iii winter, as there is a greater quantity of 
water, it passes over that eminence, and 
keeps an uninterrupted course. 
Lastly, the Drome, after having lost some 
of its water in its course, vanishes entirely 
near the pit of Soiicv ; in that place it meets 
with a sort of subterraneous cavity near 25 
feet wide, and more than 15 deep, where the 
river is in a manner stopped, and into which 
it enters, though without any perceptible 
motion, and never appears again. 
M. Guettard finishes this memoir with 
some observations upon the Lerre. I his 
river is lost in the same manner as the Kille ; 
and though it is very near Paris, this singu- 
larity is unknown to almost every body ; 
was it not fur the account of M. l’Abbe le 
Boeuf, M. Guettard would have been also 
ignorant of it. And as he thinks the chief 
object of a naturalist’s observation ought to be 
the public good, he examines the means 
which might be employed to restrain the 
water of the lerre. The same object has 
made him add a description of the manner 
how the Rhone is lost, or rather how its 
course ^disturbed ; for it is now very certain 
that it does not lose itself, but that its channel 
RIVERS. 
is extremely confined, in the place where it 
was pretended that it lost itself, by two 
mountains, between whose feet it runs. M. 
Guettard makes it appear that it might not 
be impossible to widen that place, and give 
a sufficient channel to the river; which 
would render it navigable, and be of vast 
utility to all the country. 
The many advantages which accrue to a 
country from an abundance of rivers, especi- 
ally large navigable ones, are too obvious to 
require any particular detail ; but the disad- 
vantages and calamities occasioned by them 
are frequently no less obvious and fatal. 
Whole tracts of country are sometimes over- 
flowed on a sudden, and every thing swept 
.away at once ; or if the deluge proceeds not 
such a length, yet by the quantity of stag- 
nating water which is left, marshes are pro- 
duced, which bring on diseases in the neigh- 
bouring parts. It becomes therefore an ob- 
ject well worthy the public attention, how to 
secure the banks of rivers, or to form their 
channels in such a manner that Ihe super- 
fluous water may be carried off into the 
ocean without producing the mischievous 
effects abovementioned. In a treatise on 
rivers and canals published in the Phil. 
Trans, vol. 69. by Mr. Mann, he treats this 
subject at great length. Having laid down 
a number of theorems concerning the de- 
scent of the water in rivers, he points out a 
method of determining whether the motion 
of a river in any particular place is derived 
from the inclination of the bottom of its chan- 
nel, or merely from the pressure of the upper 
parts of the water upon the lower. “ For t his 
purpose,” says he, “ a pole must be thrust 
down to the ‘bottom, and held perpendicu- 
larly to the current of the water, with its 
upper end above the surface; if the water 
swells and rises immediately against the pole, 
it shows that its flowing is by virtue of a pre- 
ceding declivity ; if, on the contrary, the 
water stops for some moments before it 
begins to rise against the pole, it is a proof 
that it flows by means of the compression of 
the upper waters upon the lower.” 
The best and most simple method of mea- 
suring the velocity of the current of a river, 
according to our author, is as follows : 
“ Take a cylindrical piece of dry light wood, 
and of a length something less than the depth 
of the water in the river ; round one end of 
it let there be suspended as many small 
weights as may be necessary to keep up the 
cylinder in a perpendicular situation in the 
water, and in such a manner that the other 
end of it may just appear above the surface 
of the water. Fix to the centre of that end 
which appears above water a small and 
straight rod precisely in the direction of the 
cylinder’s axis; to the end that, when the 
instrument is suspended in the water, the 
deviations of the rod from a perpendicularity 
to the surface of it may indicate which end of 
the cylinder advances the fastest, whereby 
may be discovered the different velocities oT 
the water at different depths ; for if the rod 
inclines forwards according to the direction 
of the current, it is a proof that the surface 
of the water has the greatest velocity ; but if 
it inclines back, it shows that the swiftest 
current is at the bottom ; if it remains per- 
pendicular, it is a sign that the velocities at 
the surface and bottom are equal. 
“ This instrument being placed in the 
current of a river or cam) receives all the 
percussions of the water throughout the whole 
depth, and will have an equal velocity with 
that of the whole current ironi the surface to 
the bottom at the place where it is put in ; 
and by that means may be found, both with- 
ease and exactness, the mean velocity of that 
part of the river for any determinate distance 
and time. 
“ Rut to obtain the mean velocity of the 
whole section of the river, the instnimi j-.s 
must be put successively both in the middle 
and towards the sides, because the velocities 
at those places are often veil different iron*' 
each other. Having by this means found 
the difference of time required for the cur- 
rents to run over an equal space, or the diffe- 
rent distances run over in equal times ; the - ] 
mean proportional of .d ! these trials, which is ] 
found by dividing the common sum of them* ! 
all by the number of trials, will be the mean 
velocity of the river or canal. 
“ If it is required to find the velocitv of the 
current only at the surface, or at the ‘middle, 
or at the bottom, a sphere of wood, of such a ■ 
weight as will remain suspended in equF > 
librium with the water at the surface or. 1 
depth which we want to measure, will he j 
better for the purpose than a c\ tinder, be- 1 
cause it is only affected by the water 'of that ; 
part of the current where it remains suspend- 
ed. 
It is a very easy guide both to the cylinder 
and the globe in that part which we want to? J 
measure, by means of two threads, or small 
cords, which two persons must hold and dr- 1 
rect, one on each side of the river; taking, 
care at the same time neither to retard nor 
accelerate the. motion of the instrument.” 
Our author next proceeds to deduce from 
his theory the best methods of removing the 
defects and inconveniencies which must ne- j 
cessarily happen to rivers and canals in a 
series of years.. From his theory he draws- ■ 
the following conclusion : that the deeper 
the waters are in their bed in proportion to 
its breadth, the more their motion is accele- 
rated ; so that their velocity increases in an. 
inverse ratio of the breadth of the bed, and 
also of the greatness of the section ; whence- 
are deduced the two following universal prac- 
tical rules: 1st. To augment the velocity of 
water in a river or canal, without augment- 
ing the declivity of the bed, we must increase 
the depth and diminish the breadth of its bed- j 
2dly. But to diminish the velocity of water in 
a river or canal, we must, on the contrary, 
increase the breadth and diminish the depth 
of its bed. 
The above proposition is perfectly con- 
formable to observation and experience : for 
it is constantly seen, that the current is the 
swiftest where the waters are deepest and the 
breadth of the bed the least, and that they 
flow' slowest where their depth is the least 
and the breadth of the bed the greatest. 
“ The velocitv of the waters ” says M, de 
Button, “ augments in the same proportion 
as the section of the channel through which 
they pass diminishes, the force of impulsion 
from the back waters being supposed always 
the same. Nothing,” continues he, “ pro- 
duces so great a diminution in the swiftness 
of a current as its growing shallow ; and on ' 
the contrary, the increase of the volume of 
water augments its velocity more than any 
