II Y D 
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fs composed of many strong fleshy fibres, from 
which arise many leaves with footstalks hve 
or six inches long, jagged into three, five, or 
seven lobes, almost to the mid-rib, indented 
on their edges. The flowers are produced in 
loose clusters hanging downward, are bell- 
shaped, and of a dirty white colour. 
HYDROSCOPE, an instrument antiently 
used for the measuring of time. The hydro- 
scope was a kind of water-clock, consisting 
of a cylindrical tube, conical at bottom: the 
cylinder was graduated, or marked out with 
divisions, to which the top of the water be- 
coming successively contiguous, as it tiickled 
out, the vertex of the cone pointed out the 
hour. 
H Y I>RO STATIC AL balance, a kind 
of balance, contrived for the easy and exact 
finding the specific gravities of bodies, both 
liquid and solid. See Hydrostatics. . 
HYDROSTATICS have for their object 
the weight and pressure of fluids; and m this 
branch of science the art of detei mining the 
specific gravities ot bodies is usually included, 
but this we have already been under a neces- 
sity of anticipating. Archimedes, among 
the antients, accomplished the most remark- 
able discoveries in this science. He is ho- 
noured even at this day, as the inventor of 
the ingenious hydrostatic process, by wmch 
the purity or baseness of a crown of gold was 
ascertained. Among the moderns we are in- 
debted to Gailileo, Torricelli, Descartes, 
Pascal, G uglielminf, and Mariotte, for the j 
best information on this subject; and by their | 
experiments (which are as curious as they ; 
are decisive) we are instructed in what y e j 
may expect or fear from the power ot fluids 
violently acted upon by the principle of gra- 
vity, and in what manner and upon what 
principles we may employ, for the use of 
man, the hydraulic machines. 
It has been observed in another place (bee 
Gravitation), that the propensity which 
bodies have of approaching towards the earth, 
or perhaps towards its centre, is the only 
cause of what we term weight or gravity, and 
that it is by the continual efforts which they 
make to obey that law, that they press upon 
every obstacle which impedes their progress. 
As fluids, like solid bodies, are impelled by 
their gravity, so in this case they press upon 
every object which opposes their fall; but 
from their nature they press in a different 
manner from solid bodies; hence arise the 
peculiar phenomena concerning which we are 
now to inquire. 
Fluids are substances, the component parts 
of which are moveable among themselves, 
having scarcely any cohesion one with ano- 
ther, and moving independently at each other. 
Some philosophers have included in this de- 
finition what they term the grosser fluids, as, 
for example, a heap of corn, a heap ot shot, 
of sand, &c. as well as the rarer and more 
elastic fluids, as common air, and all otner 
aeriform substances. The proper objects, how- 
ever of the hydrostatic science, are those fluids 
which, in common language, are termed li- 
quids, or those which always present to us a 
plane surface, level or parallel to the honzon. 
All liquid substances- are not equally so; 
hence it follows, that the laws of hydrostatics 
apply with less exactness in proportion as 
those substances depart from perfect fluidity. 
Water and oil both flow when the vessels, 
which contain them, are either overturned Or 
broken ; but the effusion of oil is slower than 
that of water, because the particies of oil have 
more cohesion among themselves. T he most 
singular effects in hydrostatics principally de- 
pend, perhaps, upon the extreme minuteness 
of the particles of fluids, but at least upon 
their great mobility. 
To preserve a lucid order in the considera- 
tion of this subject, it will be necessary to di- 
vide the objects of our inquiry into three 
branches. In the first place, therefore, we 
shall consider in what manner the principle 
of gravity acts on the particles of fluids, and 
the phenomena which it produces in the fluids 
themselves; as well as their action against 
the sides, the bottoms, and tops of the ves- 
sels in which they are contained- Secondly, 
in what manner fluids of different densities 
act upon each other; and thirdly, the action 
of fluids on bodies immersed in them. 
I. In pursuing the first object of this in- 
quiry, it may lie established as an axiom : 
1 .' That the parts of the same fluid act 
with respect to their weight or pressure, in- 
dependently o. each other. 
This property arises from their having 
scarcely any cohesion among themselves. It 
is otherwise with solid bodies ; their several 
parts adhering together, they press in one 
common mass ; hence the falling of solid bo- 
dies is productive of a diderent effect from 
that of liquids. We dread the falling of a 
pound of ice upon our heads, while we aie 
much more indifferent concerning that ot a 
pound of water. The latter, in its descent, 
is divided by the resistance of the ah, by 
which some of its parts are retaided moii 
than others ; and the swiftness of the whole 
mass is still more retarded by this division 
than it otherwise would be ; for by being thus 
divided it requires a larger surface, which 
abates its effect. On the. contrary, a solid 
body falls upon a small space, which receives 
its whole force. Hence it follows, that angu- 
lar bodies falling upon any part ot the human 
frame are more dangerous than flat 01 plane 
ones of the same weight, and descending 
from the same height _ 
It follows from this principle, that it an 
aperture is made at the bottom of a vessel 
full of any fluid, in order to prevent the 
flowing out of the liquor, it is only necessary 
to counteract the weight of that comma of 
fluid which has the aperture for its base, and 
that to counteract that weight it is the same 
whether the vessel is full oi liquor, or whether 
it contains only a column, the base of which 
shall be equal 'to the aperture at the bottom. 
Let the cylindrical vessel of glass A B 
(Plate Hydrostatics, fig. 9.) have a hole m 
the bottom at C, furnished with a cylindrical 
ferule of copper of an inch diameter D, 
which is to be stopped with a piston G, or 
the sucker of a pump well fitted to the fe- 
rule, and oiled, that it may yield to a mode- 
rate pressure. Let the piston be supported 
by a small rod G H, fastened at H to the 
silk which unites with the portion of the pul- 
ley M, with which the extremity of the lever 
M N is furnished, and which has for its cen- 
ter of motion the point L. I he othei por- 
tion of the pulley N, which terminates the 
other extremity of the lever, is also furnished 
with lines of silk, which supp- rt the small ba- 
son or scale I. Upon the copper ferule D 
then fit a cylindrical tube of glass I- E, the in- 
terior diameter of which is equal to that o 
the ferule, and its height equal to that of the 
vessel A B. When the apparatus 1S . 
in this manner, fill the tube E F wflh ^atuv 
and continue to put small weights udo the 
scale I, until the piston begins to me. A 
wards take away the glass tube E t, and place 
the piston G in the copper ferule D, and 
pour water into the large vessel A B, , and it 
will appear that the same weights as before m 
the bason I, will raise up the piston th 
larger vessel A B is entirely lull. Hence it 
follows that there is the same power to be 
counteracted, whether there rests upon > 
piston only a column of water of its own size, - 
or whether the vessel A B is entirely full. 
Such a column, therefore, presses upon its 
base independently ol the vest of tne wa 
contained in the vessel. . 
To account for this, let us suppose all the 
water in a vessel to be divided into se\ eral 
columns, 1, 2, 3, 4, 5, (fig- JO.) each com- 
posed of an equal number oi parts, h be 
bottom of the vessel, which selves for the 
base and support of all the columns, is open- 
ed in a, the column 3, being no longer sup- 
ported, will descend through the apertme, 
sliding between the two columns 2 and 4, 
which are supported by the parts ot the bot- 
tom of the vessel b and c, all the moveable 
parts of which become (it we may use the ex- 
pression) small rollers, which retard the ; fal 
only in a very slight degree. 1 his effect is 
the result of the small degree of cohesion be- 
tween the parts of the fluid. If the columns 
1 and 2 on the one part, and 4 and o on the 
other, were composed of parts adhering toge- 
ther, they would retard each other in their 
descent during their whole length, in the same 
manner as a wax candle would do; and L>) 
the fall of the column 3, a void vvould be 
made between them. But as all the particles 
are extremely minute, moving easily upon 
each other, they descend when the summit of 
the column 3 begins to descend, having no 
longer any support from that side; and the 
superficies of the whole mass descends m the 
same manner, though only one ot the co- 
lumns caused the flow from its a . tri 
the parts have a degree of viscosity, as those 
of oily fluids, or when the mass of the flow- 
ing liquor lias much more of breadth than o*. 
height, the void which the descending co- 
lumn leaves above it is easily perceived, for 
then the surface, instead of being plane and 
even, is hollow in the middle, and assumes a 
funnel-like form, because the adjacent parts 
do not arrive with sufficient swdtness to re- 
place those which descend through the aper- 
ture; besides the pressure ot the air above* 
the aperture is stronger than its resistance be- 
From what has been now stated, it is easy 
to perceive how fluids differ from solids, in the 
phenomena of gravitation. R the vessel A B 
(fD 9.) being full of water, and the tube El 
herns removed* it was required to raise up 
the piston G ; all *hat is necessary m this 
case is, to support the weight ot the column 
of water directly above the piston, because 
this column can move independently o. the 
remainder; but if the whole mass of water 
was converted into ice, then the mass ceasing 
to be a liquid, and all its parts adhering toge- 
tber, to raise up the piston ,t wou Id . be neees- 
Tiier, ivj Acviac uw i'— , . , . 
sarv to support the weight of the whole mass. 
2. Fluids press equally in all directions. 
In other words, they not only pres* from 
