<M6 H Y D v 
its deciduous offspring, or eggs, from the sides. 
The hydra viridis inhabits the stagnant waters 
and slowly running streams of Europe, gene- 
rally on the surface of plants, and appears 
like a little transparent green jelly, when con- 
tracted and transparent: when expanded it 
is a linear body, fixed at one end, and sur- 
rounded at the other by tentacula, or arms 
placed in a circle round the mouth, and gra- 
dually producing its young from the sides, 
which at first seem small papillae, increasing 
in length, till they assume the form of the 
parent, and then dropping off. Like all its 
tribe, it has the power of reproducing parts 
which have been destroyed, and if cut or di- 
vided in any direction, each separate part 
becomes a perfect polype. See Adamson 
the Microscope. There are live species. 
H\ DRACHNA, a genus of insects of 
the order aptera : the generic character is, 
head, thorax, and abdomen united or 'con- 
nate; feelers two, jointed; eyes two, four, or 
six ; legs eight, formed for swimming. The 
genus hydrachna, allied in the closest man- 
ner to that of acarus, under which the only 
species known to Linnaeus were arranged, 
was lirst instituted by the ingenious Muller, 
by whose industrious researches many new 
and curious kinds have been discovered. 
Among the larger insec'ts of this genus is 
the hydrachna flaccida, so called, in order 
the more clearly to distinguish it from one or 
two others with which it may sometimes be 
confounded. Its size is that of a small pea, 
and its colour a very bright red: its shape is 
nearly globular, but the skin is of such a na- 
ture as to yield to every inclination of the 
body, so that the whole, when taken out of 
the water, has an irregularly flaccid appear- 
ance. This is most remarkable in the full- 
grown animal, which is also of a much more 
torpid nature than the rest of the genus, 
which are animals of great celerity of motion ; 
■and indeed the young or unadvanced indivi- 
duals of the present species are of a more 
compact appearance, and swim with a greater 
degree of swiftness than the larger ones. The 
hydrachna flaccida is not very uncommon in 
stagnant waters. 
Of all the hydrachna: yet discovered by far 
the most elegant is the hydrachna geogra- 
phic, so named from the fancied map-like 
distribution of its variegations. It is one of the 
largest of the genus, equalling the size of the 
former: it is occasionally seen in clear ponds 
and other stagnant waters, but is one of the 
rarer kinds: its shape is globular, and its co- 
lour a polished black, decorated with car- 
mine-coloured spots and patches, which, in a 
certain light, are accompanied by a kind of 
gilded lustre. 
Hydrachna roeseliana is named after Roe- 
sel, by whom it is figured in his well-known 
work on insects. It is of equal, or even supe- 
rior size to that immediately preceding, which 
in many points it seems greatly to resemble; 
the ground-colour however in this is red, wfitli 
black variegations, disposed in a different 
manner from those of the former. It is found, 
though not very frequently, in stagnant wa- 
ters 
Among the smaller, or middle-sized hy- 
drachna, one ol the most common is the liy- 
drachnae extendens, which is of the size of a 
very small hemp-seed, and of a bright red 
colour, without any variegations; it is ex- 
HYD 
tremely nimble in its motions, and always 
carries the hinder pair of legs, which exceed 
the rest in length, in an extended posture. 
Hydrachnjt araneoides is a small species, 
of a brown colour, clouded with fed, and 
marked on the back by a very large oval 
patch of the same colour. It is found, like 
the rest, in stagnant waters, and has the habits 
of a young spider. 
Some ot the genus are distinguished by a 
kind of cylindric process at the end of the 
abdomen; of this kind is the hydrachna buc- 
cinator. It is a very small species, of a dark- 
brown colour, with a large rufous patch at 
the upper part of the body, the cylindric pro- 
cess being of a dull yellow. It is a native of 
stagnant waters. 
The eggs of the hydrachnse, which are 
small and round, are deposited in flat clus- 
ters, sometimes on the bodies of the naps, 
and other water insects. r Ihe young, when 
first excluded, are furnished with six legs 
only, but after the first and second change of 
their skins, become eight legged insects. 
HYDRARGYRUM, name given in the 
pharmacopeia to mercury or quicksilver. 
HYDRANGEA, a genus of the digunia 
order, in the decandria class of plants, and in 
the natural method ranking under the 13th 
order, succulents. The capsule is bilocular, 
birostrated, and cut round, or parting hori- 
zontally. There are three species, of which 
the hortensis is a very handsome and now a 
popular greenhouse plant. 
HYDRASTIS, a genus of the polygamia 
order, in the polyandria class of plants, and 
in the natural method ranking with those of 
which the order is doubtful. There is neither 
calyx nor nectarium ; there are three petals, 
and the berry is composed of monospermous 
acini. There is one species an herb of Ca- 
nada. 
HYDRAULICS. The science which has 
for its object the motion of fluids is called 
hydraulics; and its immediate application is 
to furnish us with tire means of conducting 
water from one situation to another, by ca- 
nals of aqueducts, and to elevate it by 
pumps, jets-d’eaux, and other hydraulic en- 
gines, either for the purposes of ornament 
or use. 
In treating of this subject we shall com- 
mence with the simplest principles, and shall 
first speak of the discharge of fluids through 
small apertures. 
When water flow's from a vessel which lias 
a hole or aperture in the bottom, small in 
comparison to the width of the vessel, the 
water descends vertically, and the surface 
appears smooth, but at three or four inches 
from the bottom the particles turn from this 
direction, and proceed on all sides with a 
motion more or less oblique towards the 
aperture. The same effect takes place when 
water flows through an aperture laterally. 
The tendency of the particles towards the 
aperture is a necessary consequence of their 
perfect mobility; for they will certainly be 
directed towards the point where there is the 
least resistance, and that point is the aperture. 
It is also to be observed, that in this case, 
at a small distance from the bottom, a kind 
of funnel is formed in the water, the point of 
which corresponds to the center of the aper- 
ture ; when, however, the water flows through 
| a lateral orifice or aperture, there is formed 
II Y D 
only a kind of half funnel, which does not ap« 
pear to commence till the. surface is near 
touching the upper side of the hole. It is 
probable that the funnel begins to, form itself 
from the first moment of the flow ; but it does 
not become perceptible till the surface is 
only at a small distance from the bottom, 
It appears also, that the funnel commences 
higher or lower, according to the width of 
the bottom ; and that the formation of it is 
less prompt or less perceptible, according to 
the proportion of the aperture to the extent 
ol tire bottom. The funnel is also augmented 
by any roughness which may exn>t at the 
sides or bottom of the vessel. 
Water flows out of a small hole in the bot- 
tom of a vessel with a velocity equal to that 
which a ponderous body acquires in falling 
from a height equal to the vertical height of 
the sin face of the- fluid above the aperture. 
The same law takes place in a lateral ori- 
fice; lor the pressure oi the fluid is equal (at 
the same depth) in all directions, and conse- 
quently produces the same degree of velo- 
city. bee Hydrostatics. 
A fluid, in running out of an aperture, ac- 
quires a velocity sufficient to make it re- 
mount to a vertical height equal to that of 
the fluid above the aperture, in the same 
manner as a falling body acquires a velocity 
capable of making it ascend to the height 
from which it descended. 
It is evident, from the theory of falling bo- 
dies, that it the velocity of the fluid in running 
through the aperture was uniformly continu- 
ed, the fluid would move through a space 
double the height of the fluid above the 
aperture, in the same time that a falling body 
would employ in descending from that height. 
1 he height being the same, the velocity of 
the fluid in running out of the orifice will al- 
ways be the same, whatever the species of the 
fluid may be, and whatever' its density. It 
is true, that when the fluid has more density 
it presses more forcibly, but then the mass is 
more considerable; and it is evident, that 
when the moving powers are proportioned to 
the masses which they put in motion, the ve- 
locities are equal. 
The quantities of a fluid discharged in the 
same space of time through different orifices 
supposing the vessels equally full during the 
whole ot the experiment, are to each other as 
the products of the areas of the apertures by 
the square roots of the heights. For instance 
it has been proved by experiment, that a cir- 
cular orifice of an inch diameter, made in a 
thin vessel or partition, and under a surface 
of fluid four feet in height, will furnish iu 
one minute of time, 5436 cubic inches French. 
If, therefore, it was an object to ascertain 
how much a circular orifice of two inches 
diameter, under nine feet of height from the 
surface of the water, frould furnish in the 
same, the following proportion must be em- 
ployed (it must be observed, that the orifice 
ot two inches is four times as great as an ori- 
fice of one inch, because the areas of circles 
aie as the squares of their diameters): 
lX\/ 4*4 Xy/ 9** 5436 " x 
Dr at length 
s : 12 : : 5436 : 3261a 
12 
2)653232 
