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SCIENCE which treats of the weight, motion, And as Auids have vacuities, or are not perfectly
and equilibria of liquid bodies. Under this head, dense ; it is also probable, that they are compounded not only accounts of the nature and properties of fluids of small spheres of different diameters, whose interin general are introduced, and the laws by which they ftices may be successively filled with apt materials for act; but also the art of weighing solid bodies in fluids, that purpose : and the smaller these interstices are, the in order to discover their specific gravities.
greater will the gravity of the fluid always be.
For instance, fuppose a barrel be filled with bullets
Secr. I. Of Fluids in general.
in the most compact manner, a great many small-shot
may afterwards be placed in the interstices of those Fuid de.
Sir Isaac Newton's definition of a fluid is, That it is balls, the vacuities of the shot may then be replenished find, &c. a body yielding to any force impreffed, and which with a certain quantity of sea-fand; the interftices of
hath its parts very easily moved one among another. the grains of the sand may again be filled with water;
and thus may the weight of the barrel be greatly
This definition supposes the motion spoken of pro- augmented, without increasing the general bulk.-
Its nature duced by a partial preffure; for in the case of an in- Now this being true with regard to solids, is appli
pro. compressible fluid, it is demonstrated by Dr Keil, that cable also to fluids. For instance, river water will perties. under a total or an equal pressure, it would be impof- dissolve a certain quantity of salt; after which it will fible that the yielding body should move.
receive a certain quantity of sugar ; and after that, a
The original and constituent parts of fryds are by certain quantity of alun, and perhaps other diffoluble
the moderns conceived to be particles small
, smooth, bodies, and not increase its firit dimenfions.
hard, and spherical: according to which opinion, The more perfect a fluid is, the more easily will it
every particle is of itself a solid or a fixed body; and, yield to all impressions, and the more easily will the
when considered fingly, is no fluid, but becomes fo parts unite and coalesce when separated. A perfect
only by being joined with other particles of the same fluid is that whose parts are put into motion by the
kind. 'From this definition, it hath been concluded leafl force imaginable : an imperfect one is that whose
by some philosophers, that some substances, such as parts yield to a small force, not the leaf. It is pro-
mercury, are essentially fiuid, on account of the par- bable, that in nature there is no perfect Auid, the ele-
ticular configuration of their particles ; but later dif ment of fire perhaps excepred; tince we see that the
coveries have evinced the fallacy of this opinion, and mutual attraction of the parts of all the fluids, subject
that fluidity is truly to be reckoned an effect of heat. to our experiments, renders them cohesive in some de-
gree; and the more they cling together, the less per-
That fluids have vacuities, will appear upon mixing feet their fluidity is. If, for instance, a glass be filled
salt with water, a certain quantity whereof will be with water above the brim, it will visibly rise to a
dissolved, and thereby imbibed, without enlarging the convex surface, which, was it a perfect fluid, free from
dimenfions. A Auid's becoming more buoyant, is a either tenacity or cohesion, would be impossible.
certain proof that its speciớc gravity is increased, and Mercury, the most perfect fluid we know, is not
of consequence that many of its vacuities are thereby exempt from this attraction ; for should the bottom
filled: after which it may still receive a certain quan- of a flat glass, having a gentle rising toward the mid-
tity of other dissoluble bodies, the particles whereof dle, be covered thin with quicksilver, a little motion of
are adapted to the vacancies remaining, without adding the machine will cause the fluid soon to separate from
any thing to its bulk, though the absolute weight of the middle, and lie round it like a ring, having edges
the whole fluid be thereby increased.
of a considerable thickness.
This might be demenitrated, by weighing a phial But if a like quantity thereof be poured into a gol-
of rain-water critically, with a nice balance : pour den cup, it will, on the contrary, appear higher con-
this water into a cup, and add salt to it; refund of liderably on the fides than in the middle. Which may
the clear liquor what will again fill the phial; an in- proceed in part, perhaps, from the gold's being of
crease of weight will be found under the same dimen. great denlity, and therefore capable of exerting there-
fions, from a repletion, as has been said, of the vacuities on a greater degree of attraction than other metals.
of the fresh water with saline particles.
Probably too it may happen from its having pores of
VOL. IX. Part I.
Preffure of an apter disposition and magnitude to receive the mi- part or side in which the pressure is leaft. And hence, Pressure of
nute mercurial particles, than those of iron and some no particle or quantity of a fluid can be at rest till it is
other metals; and therefore the attraction of cohesion every way equally pressed.
in this experiment may obtain alfo : and every one To fhow by experiment that Auids press upward as
knows how easily these two bodies incorporate, and well as downward, let A B be a long upright tube CCXXXIX.
make a perfe&t amalgama. But the reason commonly filled with water near to its top; and CD a linall tube fig. 2.
given for the two phenomena is, that merenry, in the open at both ends, and immersed into the water in the
first case, attracts itself more than it does glass; and, large one: if the immerlion he quick, you will fee
in the last case, mercury attracts gold more than it the water rise in the finall cube to the same height
that it stands in the great one, or until the surfaces
Sir Isaac Newton held all matter to be originally of the water in both are on the same level : which
homogeneous; and that from the different modifica- shows that the water is pressed upward into the small
tions and texture of it alone, all bodies receive their tube by the weight of what is in the great one; other-
various structure, composition, and form. In his defi- wise it could never rise therein, contrary to its natural
nition of a fluid, he seems to imply, that he thought gravity, unless the diameter of the bore were so
fluids to be composed of primary solids ; and, in the small, that the attraction of the tube would raise the
beginning of his Principia, he speaks of sand and pow. water; which will never happen, if the tube be as
ders as of imperfect fluids.
wide as that in a common barometer. And, as the Borelli has demonstrated, that the constituent parts water rises no higher in the small tube than till its of fluids are not Auid, but confittent bodies; and that surface be on a level with the surface of the water in
the elements of all bodies are perfectly firm and hard. the great one, this shows that the pressure is not in Florentine. The incompressibility of water, proved by the Flo- proportion to the quantity of water in the great tube, experiment.rentine experiment, is a fufficient evidence also, that but in proportion to its perpendicular height therein :
each primary, particle or fplerule thereof is a perfect for there is much more water in the great tube all
and impenetrable solid. Mr Locke too, in his Ejay around the small one, than what is raised to the same
on Human Underflanding, admits this to be fo.
height in the small one as it Itands in the great.
This famous experiment was first attempted by the 'Take out the small tube, and let the water run out
great lord Verulam, who inclosed a quantity of water of it; then it will be filled with air. Stop its upper
in lead, and found that it inclined rather to make its end with the cork C, and it will be full of air all be.
way through the pores of the metal, rhan be reduced low the cork: this done, plunge it again to the but.
into less compass by any force that could be applied. tom of the water in the great tube, and you will see
The academics of Florence made this experiment af. the water rise up in it to the height E. Which shows
terwards more accurately with a globe of silver, as that the air is a body, otherwise it could not hinder
being a metal less yielding and ductile than gold. the water from rising up to the fame height as it
This being filled with water, and well closed, they did before, namely, to A ; and in so doing, it drove
found, by hammering gently thereon, that the spheri- the air out at the top; but now the air is contined
city of the globe was altered to a less capacious fi- by the cork C: And it allo shows that the air is a
gure (as might geometrically be proved); but a part compressible body; for if it were not so, a drop of
of the water always like dew came through its lides water could not enter into the tube.
before this could be obtained. This has been attempted The preffure of fluids being equal in all directions,
by Sir Isaac Newton, and so many competent judges, it follows, that the sides of a vessel are as much pressed
on gold and several other metals since, with equal suc. by a fluid in it, all around in any given ring of points,
cess, that we do not hold any fuid in its natural state, as the fluid below that ring is preffet by the weight
except the air, to be either compressible or elastic.- of all that stands above it. Hence the pressure upon
In some experiments by Mr Canton, it hath been ob- every point in the sides, immediately above the bottom,
served, that water is more or less compressed accord. is equal to the pressure upon every poini of the bottom.
ing to the different constitution of the atmosphere; -To show this by experiment, let a hole be made at e Fig. 3.
whence it hath been concluded that the Florentine in the wide of the tube A B close by the bottom, and
experiment was erroneous: but it will not follow, that another hole of the same fize in the bottom at C;
water can be compressed by any artificial force, be. then pour your water into the tube, keeping it full as
cause nature hath a method of compreffing it; any long as you choose the holes should run, and have two
more than that folid metal can be compressed artiti. basons ready to receive the water that runs through
cially, though we know that very flight degrees of the two holes, until you think there is enough in
heat and cold will expand or contract its dimensions. cach bason ; and you will find by measuring the quan-
tities, that they are equal. Which shows that the wa
ter run with equal speed through both holes; which Sect. II. Of the Gravity and Pressure of Fluids. it could not have done, if it had not been equally
preffed through them both. For, if a hole of the Fluids prere
All bodies, both Auid and folid, press downwards same fize be made in the side of the tube, as about fx. as much by the force of gravity : but fluids have this wonder- and if all three are permitted to run together, you wyward as ful property, that their pressure upwards and fidewife will find that the quantity run through the hole at f downward. is equal to their preffure downwards; and this is al. is much less than what has run in the same time
ways in proportion to their perpendicular height, with through either of the holes C or e.
out any regard to their quantity: for, as tach par In the same figure, let the tube be re curved from
ticle is quite free to move, it will move towards that the bottom at C into the shape DE, and the hole at
Préffare of C be stopt with a cork. Then pour water into the Join each bottom to its vessel by a hinge D, so that Pressure of Fluids. tube to any height, as Ag, and it wil spout up in a it
like the lid of a box; and let each bor. Fluids.
jet EFG, nearly as high as it is kept in the tube AB, tom be kept up to its vessel by equal weights E and E
by continuing to pour in as much there as runs through hung to lines which go over the pulleys F and F
the hole E ; which will be the case whilst the surface (whose blocks are fixed to the sides of the vessels at f),
A g keeps at the same height. And if a little ball of and the lines tied to hooks at d and d, fixed in brass
cork G be laid upon the top of the jet, it will be sup- bottoms opposite to the hinges D and D. Things
ported thereby, and dance upon it. The reason why being thus prepared and fitted, hold the vessel AB
the jet rises not quite so high as the surface of the (fig. 6.) upright in your hands over a bafon on
water Ag, is owing to the resistance it meets with in table, and cause water to be poured into the vessel
the open air : for if a tube, either great or small, was slowly, till the pressure of the water bears down its
screwed upon the pipe at E, the water would rise in bottom at the fide d, and raises the weight E ; and
it until the surfaces of the water in both tubes were then part of the water will run out at d. Mark the
on the same level; as will be shown by the next expe- height at which the surface H of the water stood in
the vessel, when the bottom began to give way at d; The hydro
Any quantity of a Auid, how small foever, may be and then, holding up the other vessel AB (fig. 5.) in fiatic para. made to balance and support any quantity, how great the same manner, cause water to be poured into it at
soever. This is deservedly termed the hydrostatical H: and you will see, that when the water rises to A
paradox ; which we shall first show by an experiment, in this vefsel, juit as high as it did in the former, its
and then account for it upon the principle above men bottom will also give way at d, and it will lose part of
tioned, namely, that the presure of Auids is direally as the water.
their perpendicular height, without any regard to their The natural reason of this surprising phenomenon
is, that since all parts of a fuid at equal depths be-
Let a small glass tube DCG, open at both ends, low the surface are equally pressed in all manner of
CCXXXIX. and bended at B, be joined to the end of a great one directions, the water immediately below the fixed
fig. 4. AI at cd, where the great one is also open; so that part Bf (fig. 5.) will be pressed as much upward a.
these tubes in their openings may freely communicate gainit its lower surface within the vessel, by the action
with each other. Then pour water through a small of the column Ag, as it would be by a column of
necked funnel into the small tube at H; this water the same height, and of any diameter whatever ; (as
will run through the joining of the tubes at cd, and was evident by the experiment with the tube, fig. 4.)
rise up into the great tube; and if you continue pour. and therefore, fince action and reaction are equal and
ing until the surface of the water comes to any part, as contrary to each other, the water immediately below
A, in the great tube, and then leave off, you will see the surface Bf will be pressed as much downward by
that the furface of the water in the small tube will be it, as if it was immediately touched and pressed by a
just as high at D; so that the perpendicular altitude column of the height g A, and of the diameter Bf:
of the water will be the same in both tubes, however and therefore the water in the cavity BD df will be
small the one be in proportion to the other. This pressed as much downward upon its bottom CC, as
shows, that the small column DCG balances and sup- the bottom of the other vessel (fig. 6.) is preffed by
ports the great column Acd; which it could not do all the water above it.
if their pressures were not equal against one another in To illustrate this a little farther, let a hole be made
the recurved bottom at B.-If the small tube be at f in the fixed top Bf, and let a tube G be put into
made longer, and inclined in the fituation GEF, the it, then, if water be poured into the tube A, it will
surface of the water in it will stand at F, on the fame (after filling the cavity B d) rise up into the tube G,
level with the surface A in the great tube : that is, until it comes to a level with that in the tube A;
the water will have the same perpendicular height in which is manifestly owing to the pressure of the water
both tubes, although the column in the small tube is in the tube A, upon that in the cavity of the vessel
longer than that in the great one; the former being below it. Consequently, that part of the top Bf, in
oblique, and the latter perpendicular.
which the hole is now made, would, if corked up, be
Since then the pressure of Auids is dire&tly as their pressed upward with a force equal to the whole weight of
perpendicular heights, without any regard to their all the water which is supported in the tube G: and the
quantities, it appears,
that whatever the figure or fize same thing would hold at g, if a hole were made there. of vessels be, if they are of equal heights, and if the And so, if the whole cover or top Bf were full of holes, areas of their bottoms are equal, the pressures of equal and had tubes as high as the middle one A: put into heights of water are equal upon the bottoms of these them, the water in each tube would rise to the same vessels ; even though the one should hold a thousand height as it is kept in the tube A, by pouring more
or ten thousand times as much water as would fill the into it, to make up the deficiency that it sustains by Fig. 5,6. other. To confirm this part of the hydrostatical pa- supplying the others, until they are all full; and then
radox by an experiment, let two vessels be prepared the water in the tube A would support equal heights
of equal heights, but very unequal contents, such as of water in all the rest of the tubes. Or, if all the
. AB fig. 5. and A B in fig. 6. Let each vessel be open tubes except A, or any other one, were taken away,
at both ends, and their bottoms D d, D d be of equal and a large tube equal in diameter to the whole top
widths. Let a brass bottom CC be exactly fitted to Bf were placed upon it and cemented to it, and
each vessel, not to go into it, but for it to stand upon; then if water were poured into the tube that was left
and let a piece of wet leather be put between each in either of the holes, it would afcend through all the
vessel and its brass bottom, for the fake of closeness. reft of the holes, until itfilled the large tube to the