952 
the top to the bottom like other bodies, bu t 
they also press, according to their weight, 
upon all bodies that oppose them in a lateral 
direction, and even from the bottom to the 
top. Hence, it a cask is tilled with liquid 
oil, the oil will run out it an aperture is made 
in the side, but when it is congealed it will 
not • un out on account of its having become 
i solid body, for solid bodies press only from 
tl:eir vertex to their base, and not laterally. 
To understand properly this lateral pres* 
sme ot fluids, and also that which they exert 
from their base towards their vertex, it is ne- 
cessary to consider them as a mass of small 
globules deposited in a vessel; and to re- 
member that these minute globules are not 
arranged regularly as upon a cord, but that 
Very frequently one column exercises its pres- 
sure between two others, and has a propensity 
to displace them, as may be seen in tig. 11. 
where the perpendicular pressure which is 
made opposite to the point </, is directed by 
the lateral columns towards the sides, e f, of 
the vessel, in such a manner, that if the ves- 
sel was open in those places the liquid would 
Tow out, on account of the great mobility of 
'its parts. It is by the same mode of reason- 
ing, that the pressure of fluids, from their 
base towards their vertex, is accounted for. 
ft is upon this principle that the water, ele- 
vated by the New River water-works,, after 
having descended from a bason in a vertical 
pipe,, and then after having flowed horizon- 
tally in a succession of pipes under the pave- 
; ment, is raised up again, through another 
pipe, as high as the fountain at the Temple 
Garden, ft is also upon this principle that a 
•vessel may be filled either at the mouth or at 
the bottom indifferently, provided that it is 
done through a pipe, the top of which is as 
high as the top of the vessel to be filled. 
Jlence it follows, that when piers, aqueducts, 
'reservoirs, or other hydraulic works for the 
retention of water are ‘to be constructed, it 
becomes necessary to proportion theirstrength 
•to the lateral pressure which they are likely 
to sustain, which becomes greater as the 
height of the water is more considerable. 
Nearly the same precautions are necessary to 
•be taken with respect to what some philoso- 
phers erfil the grosser fluids, which also have 
a propensity to expand, as well on account of 
the smallness of their parts as from the small 
degree ot cohesion which exists between 
them. Walls designed to support terraces 
ought to be sufficiently strong to resist the 
lateral pressure of the earth and rubbish 
which they are to sustain, since this pressure 
will he greater as the particles of earth, and 
of the other materials of which the terraces 
are composed, are less bound together, and 
in proportion as the terraces are more ele- 
vated. 
3. All the parts of the same fluid are in 
equilibrium with each other, whether thev are 
contained in one vessel or many, provided 
'they communicate with each other ; awl their 
surfaces also are always in a plane parallel to 
the horizon. 
This is a consequence of the principle* 
Vdiich has been before established: for, since 
the particle k (fig. 11.) would be raised from 
f hen as e towards the top, unless a column 
equal to the column i k, pressed upon it to 
retain it in its place; it follows that to be in 
equilibrium, the upper extremities of the two 
columns should be in the same horizontal 
HYDROSTATICS. 
plane, or in points equally distant from the 
centre of the earth; which points, however, 
cannot be found by a right line; for in the 
distance of a thousand fathoms there is about 
one foot difference in the perpendicular 
height. From tins property of fluids it fol- 
lows, that water conducted bv pipes placed in 
the earth, will remount as high as the place 
whence it flowed, whatever lise depth under 
ground through which it may have been con- 
ducted by the pipes. It is "customary to al- 
low half an inch of inclination in the length of 
six feet, to counteract the resistance produced 
by friction; but it is clear from what has 
been said, that this is not absolutely neces- 
sary, for however long the passage might he 
-the water would still ascend as high" as the 
place whence it came, but it would require a 
little longer time to accomplish the ascent. 
We are enabled, upon this principle, to ac- 
count for the springs which are sometimes 
found on the tops of mountains. Such waters 
flow from mountains still more elevated 
(whether they are far or near), by subterra- 
neous canals. It follows from this principle, 
that if there are many- reservoirs which com- 
municate together, it is necessary only to 
sec one of them to know the height ot the 
water in the others; tor it must necessarily be 
of the same height there as in all the rest." 
I' torn what has been observed, viz. that 
when all the parts of the same tluid are in 
equilibrium, their surfaces will also he in a 
plane parallel to the horizon, or, in other 
words, every part of the surface at an equal 
distance from the centre of the earth, it fol- 
low’s, that when the surface of water is very 
large, it becomes necessarily and sensibly 
convex. 1 his is easily perceived at sea, 
where the masts of ships are observed at a 
distance before any other part of the ship 
can be distinguished, 
It follow s from the equal pressure of fluids 
in all directions, that the horizontal bottom 
of a vessel sustains just the pressure of a co- 
lumn of the fluid, whose base is the area of 
the bottom ot the vessel, and whose perpen- 
dicular height is equal to the depth of the 
fluid. Thus in the vessel A B C tig. 12. 
the bottom B C does not sustain a pressure 
equal to the whole quantity of fluid contained 
in the vessel, but only of a column whose 
base is CB, and height C E. Also in the 
vessel F G II, the bottom G H, tig. 13, sus- 
tains a pressure equal to what it would be if 
the vessel were as w ide at the top as bottom. 
i his leads us to notice what is called the 
hydrostatical paradox, which is thus ex- 
pressed, “ that a quantity of fluid, however 
small, may be made to counterpoise a quan- 
tity however large.” Thus if to the w ide 
vessel A B, tig. 14, the tube C D is attached, 
communicating with A B, and then w ater be 
poured into either of them, it will stand at the 
same height in both, consequently there is an 
equilibrium between them. 
It may be thus illustrated : Let A B D G, 
tig. to, represent any cylindrical vessel, to the 
•inside of which is fitted a cover C, w hich will 
slide up and down without suffering any water 
to pass between the edges. In the cover is 
inserted a small tube, C F, which is open at 
top, and communicates with the inside of the 
cylinder beneath the cover at C. The cy- 
linder is filled with water, and the cover put 
on. Then if the cover is loaded w ith a weight, 
as a pound, it will be depressed, and the water 
rise in the lube to D, and the weight will be 
sustained. If another weight be added, the 
water will rise to F, and the weight sustained, 
and so on, according to the weight added, and 
the length of the tube. Now. the weight of the- 
water in the tube is but a few grains, yet it* 
lateral pressure se rves to sustain as much as 
the weight of a column of water whose base 
is equal to that in die tube. r l hus the co- 
lumn E d produces a pressure in the water 
contained in the cylinder, equal to what 
would have been produced by the column 
A ad D; and as this pressure is exerted 
equally every way, the cover will be pressed 
upwards with a force equal to the weight of 
A a d D ; consequently if A a d D weigh a 
pound, EC will sustain a pound: and 0 the 
like of any other heights and weights. 
One of the most usetul machines to shew 
that a small quantity of water is capable of 
great pressure, is the hydrostatic bellows. 
1 his machine (tig. 16.) consists of two thick 
ciiculai or o\ al boards, united to each other 
by leather, like a pair of common bellows, or 
a bai ber s pull. Into the lower board a pipe 
B, several feet high is fixed. Now, in shew- 
ing experiments with this simple machine, 
whirl! the reader might easily make, let wa- 
ter be poured into the pipe at its top, which will 
run into the bellows, and separate the board* 
a little: then, to shew how much a small 
quantity ot w ater will be able to ellect by pres- 
sure, let weights belaid upon the upper board. 
It we pour more water into the pipe, it will 
as before run into the bellows, and raise up 
the board with all the weights upon it. And 
though the water in the tube should weigh in 
all but a single pound, yet the pressure of 
tlsis small force upon the water below in the 
bellows, will support the weights, which are 
perhaps a hundred pounds f nor will they 
have weight enough to make them descend, 
and conquer the weight of the w ater, bv forc- 
ing it out of the mouth of the pipe. 
I pon the principle ot the upward pressure 
of fluids, a piece of lead may he made to 
swam in water, by immersing *it to a proper 
depth, and keeping the water from getting 
above it. Let C D, fig. 17, be a glass tube 
open throughout, and G a flat piece of lead 
half an inch thick, lilted exactly to the lower 
end of the tube, but not to go within it. By 
means of the packthread L, the lead is held 
dose to the bottom of the tube, and in this 
situation it is immersed in the water of the 
vessel K to somewhat more than eleven times 
its own thickness, because lead is more than 
eleven times heavier than water ; then tin? 
thread I. may be let go, but the lead will not 
tall, but be sustained by the upward pressure 
ot the water below it. If some water he 
poured upon the lead, or if the tube he raised 
a little, the lead will tall by its ow n weight, 
w hich will t hen be too heavy for the pressure 
of the water round the tube, upon the column 
of w ater below it. 
It is clear, from the foregoing principles, 
that a tun tilled with water, may be bury bv 
pressing it with some pounds additional 
weight of the fluid, through a tube, which 
may be supposed to be from twenty-five to 
thirty feet in height; for from what has been 
■said, it necessarily follows, that the small 
quantity of water which tiie tube contains, 
presses upon the bottom of the tun as much 
as it a column of water had been added a * 
wide .the tun itself, and as long as tl® 
