HYDROSTATICS. 
so is the gravity of water to the gravity of 
the body. 3. “ When a fluid 1 is to be weigh- 
ed.” Weigh the fluid in a cup, which is to 
be deemed an appendage, and treated ac- 
cording to the foregoing rule, observing, 
that as the whole weight is to the loss of 
weight; so is the gravity of the solid to 
the gravity of the fluid. 
We may ascertain the respective weights 
of two known ingredients in a given com- 
pound, thus: take the differences of every 
pair of the three specific gravities ; (viz. 
the specific gravities of the compound, and 
of each ingredient) : multiply each quantity 
by the difference of the other two ; then, 
as flie greatest product is to the whole 
weight of the compound ; so is each of 
the other two products to each respec- 
tive weight of the two ingredients. 
If a piece of glass, or of metal, be im- 
mersed by suspension in different fluids, 
it will lose in weight; that is, it will re- 
quire an equipoise, according to the weight 
of the fluids respectively : observing, that 
in the lightest fluid, say alcohol, it will lose, 
least weight. This is the principle on 
which the hydrometer acts, as will lie sub- 
sequently shown. 
Vessels Ailed with water weigh more 
than when empty : to prove this, let a 
bottle be loaded so as to sink in a pail 
of water deep enough for the water to 
cover its mouth; which should be pre- 
viously closed by a plug, in such manner 
as might be easily pushed in ; append the 
bottle, in equilibrio, to the hydrostatic 
balance, and drive in the plug: the water 
will follow and destroy the equilibrium. 
Fluids press every way alike, though their 
general tendency is to gravition. Tlius if a 
vessel be made" weaker in the side than 
at the bottom, and be so laden or op- 
pressed, by the weight of water, as to burst 
the vessel, the weakest part, wherever 
situated, will become the outlet ; but, so 
soon as liberated, the fluid will invariably 
descend ; unless acted upon by a syphon, 
as shown in treating of hydraulics. Tiie 
pressure upwards is, however, merely in 
conformity with circumstances attendant 
upon general pressure, and proves the 
tendency of fluids to find their own level. 
Thus if you take a glass tube of moderate 
diameter, open at both ends, and stop one 
closely with your finger ; when you im- 
merse the other end in any fluid, it will 
enter but little within the vacancy : be- 
cause the columns of air within the tube 
represses if. But when the finger is with- 
drawn, the water will ascend within the 
tube, to tiie level of the body in which it 
is immersed. 
As fluids press in all directions, it is evi- 
dent their whole weight cannot be applied 
against one part or side ; while on the other 
hand it is equally true, that, in some in- 
stances, the bottoms of vessels receive 3 
pressure which does not appear to be their 
due. Thus, in a pan whose base is narrower 
than its brim, the bottom sustains only the 
weight of a column equal to its are?, multi- 
plied by its height ; yet if the pan be of a 
bell-shape, having its base broader than its 
brim, the bottom will sustain n weight equal 
to its area also multiplied by its height. 
Consequently in a vessel of a conical form 
the base would be oppressed as much as if 
the sides were cylindrical. This is called 
the hydrostatic paradox ; but wail be easily 
reconciled by the consideration, that if a 
tube of glass be made with a curved bot- 
tom, so as to turn up in the form of the let- 
ter U, but with one leg or part much wider 
than the other, the water will rise equally in 
both. If to each a piston be fitted, tlieir 
weights being equal, and that one piston be 
first put into the wider leg of the tube, it 
will cause the fluid to rise in the other in 
proportion to its weight ; but on applying 
tiie lesser piston to the corresponding smal- 
ler tube, the two will be held in equilibrio. 
We have indeed further proof of the pres- 
sure of water upwards, by means of two 
boards, whose sides are joined by leather, 
as in a pair of bellows : these may be of any 
form or of any size. At the top ot one of 
the boards cut a hole, and insert a tube of 
about four or five feet in lengtii, so as to be 
perfectly tight : place on ihe board several 
weights, according to the size of the ma- 
chine, and pour water into the tube. The 
upper board will bear up against the weights, 
provided tiiey be not disproportionately 
heavy, and will admit the water between 
the top and bottom to the extent admitted 
by the pliable side's. Some water ought to 
be poured in before the weights are set on. 
A circle of about twenty inches in diameter 
will thus lift and support three weights, of 
100 lb. each. Where either dir or any other 
fluid is debarred from access between two 
planks annexed in the water, the lower one 
being kept to the bottom forcibly, they will 
not separate, unless a force equal to the 
weight of the superincumbent fluid be ap- 
plied ; because the lateral and superior parts 
of the fluid arc prevented from exerting their 
pressure, except in that direction which keeps 
