Encyclopaedia Britannica; or, A dictionary of arts and sciences, compiled upon a new plan … > Volume 2, C-L

H Y D ( 804 ) H Y D
Imperfeiflly, as in the chalcedony * and giving a gene¬
ral cloudinefs or miltinefs to the Hone, but of fo im
perfedt and irregular an admixture, as not to be capa¬
ble of fo goodapolilh as the chalcedony ; and appear-
1 ing of a dulky and foul furface, till thrown into water,
in which they become lucid, and in fome degree tranf-
parent, either in part or totally ; alfa changing their
colour, -which returns to them on being taken out of
the water.
To this genus belong the oculus belt of authors, or
whitilh-grey hydrophanes, variegated with yellow, and
with a black central nucleus ; and the oculus mundi,
or lapis mutabilis, which is hkewife a whitilh-grey kind
without veins.
HYDROPHOBIA, an averfion or dread of water; a
HYDROS
THEfcienc# of Hydrostatics treats of the nature
gravity, preffure, and motion of fluids in general;
and of weighing folids in them.
A fluid is a body that yields to the leafl: preflure or
difference of preffures. Its particles are fo exceedingly
fmall, that they cannot be difcerned by the bell of micto-
fcopes ; they are hard, fince no fluid, except air or fleam,
can be preffed into a lefs fpace than it naturally pofleffes;
and they are round and fmooth, fince they are lb eafily
moved among one another.
All bodies, both fluid and folid, prefs downwards by
the force of gravity: but fluids have this wonderful pro¬
perty. that their preffure upwards and fidewife is equal
to their preffure downwards ; and this is always in pro¬
portion to their perpendicular height, without any re¬
gard to their quantity : for, as each particle is quite free
to move, it will move towards that part or fide on
which the preffure is leaft. And hence, no particle or
quantity of a fluid ,can be at reft, till it is every way e-
qually preffed.
(Plate XCIX. fig. 2.) To Ihew by experiment that
fluids prefs upward as well as downward, let AB be a
long upright tube filled with water near to its top ; and
CD a fmall tube open at both ends, and immerfed into
the water in the large ofte : if the immerfion be quick,
you will fee the water rife in the fmall tube to the fame
height that it ftands in the great one, or until the fur
faces of the water in both,are on the fame level: which
{hews that the water is preffed upward into the fmali tube
by the weight of what is in the great one; otherwife it
could never rife therein contrary to its natural gravity ;
unlefs the diameter of the bore were fo fmall, that the
attraAion of the tube would raife the water ; which will
never.happen, if the tube be as wide as that in a common
barometer. And as the water rifes no higher in the
fmall Kibe than till its furface be-on a level • ith the fur
face of the water in the great one, this (hews that the
preffure is not in proportion to the quantity of water in
the great tube, but in proportion to its perpendicular
terrible fymptom of the rabies canina. See Medicine.
HYDROPHYJLLUM, in botany, a genus of the peman-
dria monogynia clafs. The corolla is bell-fliaped ; the
ftigma is bifid ; and the capfule is roandilh with two
valves. There are two fpecies, none of them natives
of Britain.
HYDROPS, in medicine. SccMedicine.
HYDROSCOPE, an inftrument anciently ufed for the
meafuring of time.
The hydrofcope was a kind of water-clock, confid¬
ing of a cylindrical tub, conical at bottom : the cylin¬
der was graduated, or marked out with divifions, to
which the top of the water becc.ming fucceflively con¬
tiguous, as it trickled out at.the vertex of the cone,
pointed out the hour.
T A T I C S.
height therein: for there is much more water in the
great tube all, around the Imall one, than what is rai-
fed to the fame height in the fmall one, as it ftands in the
great.
Take out the fmall tube, and let the water run out of
it; then it will be filled with air. Stop its upper end
with the cork C, and it will be full of air all below the
cork : this done, plunge it again to the bottom of the
water in the great tube, and you wdl fee the water rife
up in it to the height E; which {hews that the air is a
body, otherwife it could not hinder the water from rifing
up to the fam$ height as it did before, namely, to A ;
and in fo doing it drove the air out at the top ; but now
the air is confined by the cork C : and it alfo ftiews
that the air is a compreflible body; for if it were not
fo, a drop of water could not enter into the tube.
The preffure of fluids being equal in all directions, it
follows, that the fides of a veffel are as much preffed by a
fluid in it, all around in any given ring of points, as the
fluid below that ring is preffed by the weight of all that
Rands above it. Hence the preffure upon every point in
the fides, immediately above the bottom, is equal to the
preffure upon every point of the bottom. To fliew this
by experiment, let a hole be made at E (fig. ;.) in the
fide of the tube AB clofe by the bottom; and another
hole of the fame fize in the bottom, at C; then pour
water into the tube, keeping it full as long as you chufd
the holes fhould run, and have two two hafons ready to
receive the water that runs through the two holes, until
you think there is enough in each bafon ; and you will
find, by meafuring the quaintities. that they are equal;
which {hews that the water run with equal fpeed through
both holes; which it could not have done, if it had not
been equally preffed through them both: for if a hole
of the fame fize be made in the fide of the tube, as about
f, and if all three are permitted to run together, you will
find that the quantity run through the hole at/is much
iefs than what has run in the fame time through either of
the holes C or e.
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