HYDROSTATICS. 
-tube, 'which would evidently be an enormous 
weight. . . , . J* 
11. The effects of gravity on fluids ot dflte- 
V’.-nt densities will, from what lias piecedeo, 
uot be very diflicult to comprehend. 
It has been observed, that fluids are masses 
of small bodies moveable with great facility 
amoim themselves independently ot each 
otheri pressing separately, and in proportion 
to their masses. . ,,, 
It is moved also by chemical analysis, that 
even these minute particles are composed of 
particles still smaller. Now whether it results 
from the interposition of caloric in greater or 
less quantities, which we know is the cause of 
all fluidity, and also of the difference that ex- 
ists between the incompressible and elastic 
fluids; or whether it may depend upon the 
shape or size of the particles, which, as in so - 
lid bodies, may increase or diminish the po- 
rosity, it is certain, that there is a consider- 
able difference with respect to density m de- 
ferent fluids. . . , . . •< 
From this difference in point of densitj , a 
separation may be observed generally to take 
place, soon after mixing two heterogeneous 
fluids together, unless this effect is counter- 
acted bv some more powerful cause. It has 
been observed, that the particles, according 
to their weight, press independently of each 
other. Those therefore which have flic most 
density, having more power to gain posses- 
sion of the lower part ot the vessel which con- 
tains them, oblige the others to yield and le- 
eign their situation; and lienee a separation 
is effected. When oil and water, tor in- 
stance, have been well shaken together, and 
afterwards the whole is left in a state ot rest, 
the water, having more density than the oil, 
takes the lower position, and the oil rises to 
the surface. If this effect does not take place, 
it is owing to the intervention ot one of the 
following causes: First, a kind of elective at- 
traction, which may exist between the par- 
ticles of different fluids, as when water and 
wine are mixed together, the water, though 
heavier than the wine, does not separate it- 
self. Secondly, the viscosity ot one ot the 
substances, as when the whites ot eggs are 
beaten together, and by that means a con- 
siderable quantity of air mixes with them; 
the air, though much lighter, has not power 
to disengage itself from j lie matter in which it 
is enveloped, in order to effect its escape. 
If two fluids of different densities are placed 
in a state of equipoise with each other, and 
have the same base, their perpendicular 
heights above the horizon will be in a recipro- 
cal ratio to their densities or specific gra- 
vities. . . . 
If, for example, mercury is put into an in- 
verted' siphon, and water is poured into one 
of the branches, in. order to elevate the mer- 
cury in the other branch one inch above its 
level, it is necessary that the water should be 
about thirteen inches and an halt high. _ 1 he 
height of the wafer then will be thirteen 
times and a half that of the mercury; because 
the specific gravity of mercury is about thir- 
teen tunes and a half as great as that of 
water. . . , ,, 
This observation will also apply to the re- 
ciprocal action of air and water, or air and 
mercury upon each otfier. Many ot the phe- 
nomena of hydrostatics and hydraulics aie to 
be referred to the pressure of the atmosphere, 
for which we must refer to Pneumatics. 
It is, however, proper on the present occa- 
sion, to call the reader’s attention to some of 
the properties ot this fluid, and lie will easily 
remember, that as a fluid, air is possessed ot 
gravity, and consequently presses upon all 
bodies which oppose it; and it is necessary to 
add, that like water, it presses in all direc- 
tions. If, therefore, a small hole is made 
with a giinblet, cither in the side or bottom of 
a cask or vessel which is quite lull ot liquor, 
it will not run out, because the external air 
which presses against the hole, sustains the 
liquor, which has not a sufficient heignt to 
overcome its pressure. Hence the necessity 
of a vent peg, to enable liquor to be drawn 
out of a full cask. I he elasticity of the small 
quantity of air which is introduced at the 
vent presses the fluid, and overcomes the 
pressure of the air at the cock. 1 here is an 
instrument in common use, called a Valencia, 
for extracting small quantities of liquor out 
of the bung-holes of casks. It is a tube with 
a small aperture at the bottom and the top. 
When full, if the hole at the top is stopped 
‘ with the thumb or finger, so as to prevent 
the pressure ot the air at the top, the liquor 
will not run out of the hole at the bottom, 
being kept in by the force of the external 
It is proper to observe, that all the effects 
which depend upon the pressure of air, take 
place in a room where the column of air is 
terminated by the ceiling, as well as without 
doors where the column of air has the whole 
height of the atmosphere ; and the reason 
is, because the air in the room iias a commu- 
nication with that on the outside, supposing it 
to be only by means of the key-hole. Thus a 
barometer placed in a hall, will have its mei- 
cury as high as if it was placed iii an open 
field. 
The curious effects produced by siphons, 
all depend upon the pressure of the air. 
A siphon is a bent tube, A B C, (fig- 3) 
made of glass, metal, &c. One branch ot 
which A B, is shorter than the other b b . In 
order to make use of this instrument, place the 
extremity of the short branch in the vessel 
A, which may be supposed to contain any 
fluid matter, as water for instance. If the 
air then is drawn out of the siphon (fig. 4.), 
by means of the long branch x, the liquoi 
will begin to flow, and will not cease while 
the short branch A B remains immersed in 
the fluid. It is easy to see that the pressure 
of the air upon the surface of the fluid in the 
vessel, is the cause of its discharge through 
the siphon. For suppose G l the confines of 
the atmosphere, all the points of the surface 
A of the liquor will be equally pressed by the 
column' of air AF; if, therefore, at some 
point of this surface, the pressure is suspend- 
ed, the liquor must flow at that point, be- 
cause it finds less resistance there than in any 
other part ; this is therefore the obvious rea- 
son why the siphon becomes full immediately 
after the air is drawn out at the extremity C. 
If the two branches of the siphon were of 
equal lengths, as B A, B O, the flow tnrough 
the bent tube would not take place; because 
the column of air D G which would resist in 
D, being of an equal height with that which 
presses at A, would also be in equilibrium 
with it, in the same manner as the two co- 
lumns of the fluid B A, B D. But since B C, 
one of the legs, is longer than the other, 
though the column of the air G-C, which an- 
° <* E s 
953 
swers to it, is realty longer lliaii that which 
presses in A; yet it is not callable of pre- 
venting the passage of the fluid. lo under- 
stand this .more perfectly, let us consider the 
column of air GC to be divided into two 
parts, one of which G D, would form an equi- 
poise with the column of air F A, and would 
be capable of stopping the flow from the tube 
if the branch BC ended in Lb i be portion 
of flu id which fills the part I) C of the siphon, 
will find no other resistance in C than one co- 
lumn of air D C of the same length with it, 
which is evidently very inferior to it m 
weight. This portion of fluid then flows out, 
because it greatly exceeds in weight the co- 
lumn of air which is opposed to it. But while 
it continues to flow, nothing sustains that 
which is above it, which flows necessarily, 
while the pressure of the air at A furnishes a 
new supply of fluid to replace that which runs 
out. It is by these means, that the water in 
the siphon continues to flow without inter- 
mission; because the resistance of the air in 
C is as much exceeded, as the length of the 
branch B C of the siphon exceeds that ot the 
branch A B. In order to prove this, suppose 
there is added at C a tube to lengthen that 
branch, then it will plainly appear that in a 
given time more water will flow than would 
have been discharged without that augmenta- 
tion to the branch B C. - 
Since it is the pressure of the air which, 
elevates the fluid in the short branch BA, it 
follows, that the height of this branch is li- 
mited to thirty-two feet when the fluid is wa- 
ter, because the pressure of the atmosphere 
cannot elevate water higher; but when the 
liquor is mercury, the height of the short 
branch should not exceed thirty inches, be- 
cause the atmosphere cannot sustain meicury 
at a greater height. 
A siphon may be disguised in a cup, fig. 5, 
from which no liquor will flow, until it be 
raised above the bend of the siphon ; but 
when the efflux once begins, it will continue 
to flow till the vessel be emptied. This has 
been called Tantalus’s cup, because it is usual 
to place a hollow figure over the inner tube of 
such a length, that when the fluid has got 
nearly up to the lips of the figure, the siphon 
may begin to act, and empty the cup. 
Intermitting springs, which puzzled philo- 
sophers formerly, are found to be natural si- 
phons, which mav be thus explained: Let A 
11 <t. 6, be part of a hill, within which there is a 
cavity B B, anti from this cavity a vein, or 
channel running in the direction B CD. 1 he 
rain that falls upon the side of the hill will 
sink and strain through the small pores and 
crevices in the hill, and fill the cavity B B 
with water. W h e n the water rises to the 
level of C, the vein BC X) will oe full, and 
the water will run through it as a siphon, and 
will empty the cavity B B. It must then 
stop, and „\v hen the cavity is again filled, it 
will begin to run again. 
HI. The action of fluids on solid bodies 
immersed in them, has been tieated ot in 
Specific Gravity, which see. 
To finish the subject of hydrostatics, how- 
ever, we may add that it is evident that when 
a solid body 'is plunged into a fluid, it occupies 
a space in that fluid exactly equal to its owu 
magnitude. The quantity of fluid then so 
displaced, either equals in density, and conse- 
quently in weight, the solid which displaced 
k; or, on the contrary, one of the two must 
