HYD 
portions are first joined together by a 
slender neck, which gradually fills up and 
disappears, the food passing from one part 
into the other; and thus we may form 
polypes, not only from different portions 
of the same animal, but from those of dif- 
ferent animals. We may fix the head of 
one to the body of another, and the com- 
pound animal will grow, eat and multiply, 
as if it had never been divided. By push- 
ing the body of one into the mouth of ano- 
ther, so far that their heads may be brought 
into contact, and kept in that situation tor 
some time, they will at last unite into one 
animal, only having double the usual num- 
ber of arms. The hydra tusca may be 
turned inside out like a glove, at the same 
time that it continues to eat and live as 
before. The lining of the stomach now 
forms the outer skin, and the former epi- 
dermis constitutes the lining of the sto- 
mach. See Adams on the Microscope. 
HYDRACHNA, a genus of insects of 
the order Coleoptera. Head, thorax, and 
abdomen united ; two feelers, jointed ; 
from two to six eyes; eight legs, ciliateand 
formed for swimming. The insects of this 
genus are inhabitants of the water, and 
swim with considerable swiftness: they prey 
on the larva of Tipulae, and Monoculi: 
the eggs are red and at first spherical, but 
afterwards become semi-lunar ; larva six- 
footed and furnished with a singular pro- 
boscis. There are about fifty species. H. 
geographica, so called from the fancied 
map-like distribution of its variegations. It 
is one of the largest of the genus, and is 
occasionally seen in the clear ponds, and 
other stagnant waters. This is reckoned 
one of the most beautiful of the British 
insects. 
HYDRANGEA, in botany, a genus of 
the Decandria Digynia class and order. 
Natural order of Succulent®. Saxifragse, 
Jussieu. Essential character : capsule two- 
celled, two beaked, containing many seeds; 
corolla five petalled; calyx, five-cleft, su- 
perior. There are three species. 
HYDRARGYRUM, an old name given 
to mercury. 
HYDRASTIS, in botany, a genus of the 
Polyandria Polygynia class and order. Na- 
tural order of Ranunculaceae, Jussieu. Es- 
sential character : calyx none; petals three ; 
nectary none; berry composed of one seed- 
ed acini, or granulations. There is but 
one species, viz. H. canadensis, Canadian 
yellow rost. 
HYDRATE, in chemistry, lately intro- 
HYD 
duced by Proust to express the chemical 
union of water with any substance, and es- 
pecially with certain metallic oxides. The 
hydrate of copper is a blue-green oxide of 
this metal, which differs from the brown 
oxide, only in containing a large quantity 
of water, which a low red heat will expel. 
HYDRAULICS teach us to ascertain the 
velocity and impetus of fluids when in mo- 
tion, and serve as the basis for computing 
the powers of various machinery acted up- 
on by running water. 
The first principle we shall inculcate in 
this service is, that water being an inelastic 
fluid, (though many have thrown away much 
time in the attempt to prove the contrary,) 
can only be set in motion by two causes: 
viz. the increased pressure of the air, as in 
the air-vessels of fire-engines, and by gra- 
vitation ; that is, where it is liberated from 
confinement, and allowed to descend to an 
inferior level. In the former case, water 
may be made to rise by machinery suited 
to the purpose ; in the latter, it will inviola- 
bly seek a lower situation. 
The velocity of water, proceeding through 
a hole in the side of a vessel, is ever propor- 
tioned to the distance of the aperture from 
the level of the fluid, the square root of the 
intermediate space being the guide. It must, 
however, be recollected, that in consequence 
of the decrease of that space, as the water 
is let out, the pressure becomes gradually 
less ; therefore the medium, or mean dis- 
tance, between the surface and the vent 
whence the water issues, will be found, in 
general, a correct standard. Hence we see, 
that, in order to force double the quantity 
of water through the lowest of two aper- 
tures, the distance must be quadrupled. 
For if a hole made at C in the pipe A B, 
'fig. t, will supply one gallon of water in a 
minute ; to draw double that quantity in the 
same time, the lower hole, D, must measure 
from the surface, B, four times as much as 
from C to the surface. 
This establishes the above position, and 
proves besides, that the force, is equal to the 
velocity, as indeed we know to result in 
every branch of mechanism. To shew this, 
let the pipe, A B, be perforated in several 
parts, as at CDE; the first, i. e. C, be- 
ing one foot ; that at D being four feet ; and 
that at E being seven feet below the sur- 
face, B ; between E and A we will suppose 
only one foot interval, so that D may be in 
the centre of the height, A B. Draw the 
horizontal line, A F, and from D describe 
the semi-circle, B G A, having D G equal 
