RIV 
Velocities at the top and bottom are eqiial. 
This instrument, being placed in the cur- 
rent of a river or canal, receives all the per- 
cussions of the water throughout the whole 
depth, and will have an equal velocity with 
that of the whole current from the surface 
to the bottom at the place where it is put 
in, and by that means may be found, both 
with exactness and ease, the mean velocity 
cf that part of the river for any determinate 
distance and time. But to obtain the mean 
velocity of tlie whole section of the river, 
the instrument must be put successively 
both in the middle and towards the sides, 
because the velocities at those places are 
often very different from each other. Hav- 
ing by this means found the several veloci- 
ties, from the spaces run over in certain 
times, the arithmetical mean proportional 
of all these trials, which is found by divid- 
ing the common sum of them, all by the 
number of the trials, will be the mean velo- 
city of the river or canal. And if this me- 
dium velocity be multiplied by the area of 
the transverse section of the waters at any 
place, the product will be the quantity 
running through tliat place in a second of 
time. 
If it be required to find the velocity of 
the current only at the surface, orat the mid- 
dle, or at the, bottom, a sphere of wood 
loaded, or a common bottle corked with a 
little water in it, of such a weight as will 
remain suspended in equilibrium with the 
water at the surface or depth which we 
want to measure, wilt be better for the 
purpose than the cylinder, because it is 
only affected by the water of that sole part 
of the current where it remains suspended. 
It follows from what has been said in the 
former part of this article, that the deeper 
the waters are in their bed in proportion 
to its breadth, the more their motion is ac- 
celerated; so that their velocity increases 
in the inverse ratio of the breadth of the 
bed, and also of the magnitude of the sec- 
tion ; whence, in order to augment the 
velocity of water in a river or canal, with- 
out augmenting the declivity of the bed, we 
must increase the depth of the channel, and 
diminish its breadth. And these principles 
are agreeable to observation ; as it is well 
known, that the velocity of flowing waters 
depends much more on the quantity and 
depth of tlie water, and on the compression 
of the upper parts on the lower, than on 
the declivity of the bed ; and therefore tlie 
declivity of a river must be made much 
greater in the beginning than toward the 
VOL. V. 
ROA 
end of its course; where it should be almost 
insensible. 
RIVINA, in botany, a genus of the 
Tetrandria Monogynia class and order. 
Natural order of Holoracese. Atriplices, 
Jussieu. Essential character : calyx four- 
leaved, permanent; berry containing one 
lens-shaped seed. There are four species. 
road, an open way, or public passage, 
forming a communication between one 
place and another. The Romans took the 
most pains in forming roads, and the labour 
and expenses they were at in rendering 
them spacious, firm, straight, and smooth, 
is incredible. They usually strengthened 
the ground by ramming it, laying it with 
flints, pebbles, or sand, and sometimes with 
a lining of masonry, rubbish, bricks, &c. 
bound together with mortar. In some 
places in the Lionois, F. Menestrier ob- 
serves that he has found huge clusters of 
flints cemented with lime, reaching tea or 
twelve feet deep, and making a mass as 
hard and compact as marble, and which, 
after resisting the injuries of time for 1600 
years, is still scarce penetrable by all the 
force of hammers, mattocks, &c. and yet 
the flints it> consists of are not bigger than 
eggs. The most noble of the Roman roads 
was the Via Appia, which was carried to 
such a vast length, that Procopius reckons 
it five days journey to the end of it, and 
Leipsins computes it at 3S0 miles : it is 12 
feet broad, and made of square free-stone, 
generally a foot and a half on each side ; 
and though this has lasted for above 1800 
years, yet in many places it is for several 
miles together as intire as when it was first 
made. 
The ancient roads are distinguished into 
military roads, double roads, subterraneous 
roads, &c. the military roads were grand 
roads, formed by the Romans for marching 
their armies into the provinces of the em- ' 
pire ; the principal of these Roman roads 
in England, are Watling-street, Ikenild- 
street, Foss-way, and Erminage-street. — 
Double roads, among the Romans, were 
roads for carriages, with two pavements, 
the one for those going one way, and the 
other for those returning the other ; these 
were separated from each other by a cause- 
W'ay raised in the middle, paved with bricks 
for the conveniency of foot passengers ; 
with borders and mounting stones from 
space to space, and military columns to mark 
the distance. Subterraneous roads are those 
dug through a rock, and left vaulted ; as 
that of Puzeoli near Naples, whicli is nearly 
O 1 
