FLU 
ancient naturalists, who were ignorant of 
the equal and general pressure of all fluids, 
that the component parts, or the particles 
of the same element, did not gravitate or 
rest on each other ; so that the weight of a 
vessel of water, balanced in air, would be 
entirely lost when the fluid was weighed in 
its own element. The following experi- 
ment seems to leave this question perfectly 
decided : take a common bottle* corked 
close, with some shot in the inside to make 
it sink, and fasten it to the end of a scale 
beam ; then immerse the bottle in water, 
and balance the weight in the opposite 
scale; afterwards open the neck of the 
bottle and let it fill with water, which will 
cause it to sink ; then weigh the bottle 
again. Now it will be found that the weight 
of the water which is contained in the bot- 
tle is equal to the difference of the weights 
in the scale, when it is balanced in air ; 
which sufficiently shews that the weight of 
the water is the same in both situations. 
As the particles of fluids possess weight as 
a common property of bodies, it seems rea- 
sonable that they should possess the conse- 
quent power of gravitation which belongs 
to bodies in general. Therefore, supposing 
that the particles which compose fluids be 
equal, their gravitation must likewise be 
equal; so that, in the descent of fluids, 
when the particles are stopped and support- 
ed, the gravitation being equal, one particle 
will not have more propensity than another 
to change its situation, and after the im- 
pelling force has subsided the particles will 
remain at absolute rest. 
From the gravity of fluids arises their 
pressure, which is always proportioned to 
the gravity. For if the particles of fluids 
have equal magnitude and weight, the gra- 
vity or pressure must be proportional to the 
depth, and equal in every horizontal line of 
fluid; consequently, the pressure on the 
bottom of vessels is equal in every part. 
The pressure of fluids upwards is equal to 
the pressure downwards at any given depth. 
For, suppose a column of water to consist 
of any given number of particles acting 
upon each other in a perpendicular direc- 
tion, the first particle acts upon the second 
with its own weight only ; and, as the se- 
cond is stationary, or fixed by the surround- 
ing particle, according to the third law of 
motion, that action and reaction are equal ; 
it is evident that the action or gravity in 
the first is repelled in an equal degree by 
the reaction of the second; and in like 
manner, the second acts on the third, with 
FLU 
its own gravity added to that of the first, 
but still the reaction increases in an equi- 
valent degree, and so on throughout the 
whole depth of the fluid. 
The particles of a fluid, at the same 
depth, press each other equally in all direc- 
tions. This appears to rise out of the very 
nature of fluids, for as the particles give 
way to every impressive force, if the pres- 
sure amongst themselves should be unequal 
the fluid could never be at rest, which is 
contrary to experience ; therefore, we con- 
clude, that the particles press each other 
equally, which keeps them in their own 
places. This principle applies to the whole 
of a fluid as well as a part. ' For if four or 
five glass tubes, of different forms, be im- 
mersed in water, when the corks in the 
ends are taken out, the water will flow 
through the various windings of the different 
tubes, and rise in all of them to the same 
height as it stands in the straight tube: 
therefore the drops of fluids must be equally 
pressed in all directions during their ascent 
through the various angles of the tube, 
otherwise the fluid could not rise to the 
same height in them all. 
From the mutual pressure and equal ac- 
tion of the particles of fluids, the surface 
will be perfectly smooth and parallel to the 
horizon. If from any exterior cause the 
surface of water has some parts higher than 
the rest, these will sink down by the natu- 
ral force of their own gravitation, and dif- 
fuse themselves into an even surface. See 
Hydrostatics. 
Fluids, motion of. The motion of fluids, 
viz. their descent or rise below or above 
the common surface or level of the source 
or fountain, is caused either, 1. By the 
natural gravity or pressure of the fluid con- 
tained in the reservoir, or fountain ; or, g. 
By the pressure or weight of the air on the 
surface of the fluid in (he reservoir, when it 
is at the same time either taken off or dimi- 
nished on some part in aqueducts, or pipes 
of conduit. 3. By the spring or elastic 
power of compressed or condensed air, as 
in the common water engine. 4. By the 
force of pistons, as in all kinds of forcing 
pumps, &c. 5. By the power of attrac- 
tion, as in the case of tides, &c. 
FLUIDITY. The state of bodies when 
their parts are very readily moveable in all 
directions with respect to each other. Many 
useful and curious properties arise out of 
this modification of matter, which form the 
basis of the mechanical science called hy- 
drostatics, and are of considerable impor- 
