ABOUT WATER—HYDROSTATICS AND HYDRAULICS—LAW OF ARCHIMEDES—THE BRAMAH
PRESS—THE SYPHON.
At present we will pass from Air to Water, from Pneumatics to
Hydrostatics and Hydraulics. We must remember that Hydrostatics and
Hydraulics are very different. The former treats of the weight and
pressure of liquids when they are at rest, the latter treats of them in
motion. We will now speak of the properties of Liquids, of which Water
may be taken as the most familiar example.
We have already seen that Matter exists in the form of Solids, Liquids,
and Gases, and of course Water is one form of Matter. It occupies a
certain space, is slightly compressible; it possesses weight, and
exercises force when in motion. It is a fluid, but also a liquid. There
are fluids not liquid, such as air or steam, to take equally familiar
examples. These are elastic fluids and compressible, while water is
inelastic, and termed incompressible.
The chemical composition of water will be considered hereafter, but at
present we may state that water is composed of oxygen and hydrogen, and
proportions of eight of the former to one of the latter by weight; in
volume the hydrogen is as two to one.
From these facts, as regards water, we learn that volume and weight
are very different things,—that equal volumes of various things may
have different weights, and that volume (or bulk) by no means indicates
weight Equal volumes of feathers and sand will weigh very differently.
[The old “catch” question of the “difference in weight between a pound
of lead and a pound of feathers” here comes to the mind. The answer
generally given is that “feathers make the heavier ‘pound’ because
they are weighed by avoirdupois, and lead by troy weight.” This is an
error. They are both weighed in the same way, and pound for pound are
the same _weight_, though different in _volume_.]
Fluids in equilibrium have all their particles at the same distance
from the centre of the earth, and although within small distances
liquids appear perfectly level (in a direct line), they must, as the
sea does, conform to the shape of the earth, though in small levels the
space is too limited to admit of any deviation from the plane at right
angle to the direction of gravity.
Liquids always fall to a perfectly level surface, and water will seek
to find its original level, whether it be in one side of a bent tube,
in a watering pot and its spout, or as a fountain. The surface of the
water will be on the same level in the arms of a bent tube, and the
fountain will rise to a height corresponding with the elevation of the
parent spring whence it issues. The waterworks companies first pump
the water to a high reservoir, and then it rises equally high in our
high-level cisterns.
As an example of the force of water, a pretty little experiment may be
easily tried, and, as many of our readers have seen in a shop in the
Strand in London, it always is attractive. A good-sized glass shade
should be procured and placed over a water tap and basin, as per the
illustration herewith. Within the glass put a number of balls of cork
or other light material. Let a stop-cock, with a small aperture, be
fixed upon the tube leading into the glass. Another tube to carry away
the water should, of course, be provided, but it may be used over
again. When the tap is properly fixed, if the pressure of the water be
sufficient, it will rush out with some force, and catching the balls as
they fall to the bottom of the glass shade bear them up as a juggler
would throw oranges from hand to hand. If coloured balls be used the
effect may be enhanced, and much variety imparted to the experiment,
which is very easy to make.
[Illustration: Fig. 57.—Water jet and balls.]
Water exercises an enormous pressure, but the pressure does not depend
upon the amount of water in the vessel. It depends upon the vessel’s
height, and the dimensions of the base. This has been proved by filling
vessels whose bases and heights are equal, but whose shapes are
different, each holding a different quantity of water. The pressure at
the bottom of each vessel is the same, and depends upon the depth of
the water. If we subject a portion of the liquid surface to certain
force, this pressure will be dispersed equally in all directions, and
from an acquaintance with this fact the Hydraulic Press was brought
into notice. If a vessel with a horizontal bottom be filled with water
to a depth of one foot, every square foot will sustain a pressure of
62·37 lbs., and each square inch of 0·433 lbs.
[Illustration: Figs. 58, 59, 60, 61.—Pressure of Water.]
We will now explain the principle of this WATER PRESS. In the small
diagram, the letters A B represent the bottom of a cylinder which has a
piston fitted in it (P). Into the opposite side a pipe is let in, which
leads from a force-pump D, which is fitted with a valve E, opening
upwards. When the piston in D is pulled up water enters through the
valve; when the piston is forced down the valve shuts, and the water
rushes into the chamber A B. The pressure pushes up the large piston
with a force multiplied as many times as the area of the small piston
is contained in the large one. So if the large one be ten times as
great as the small one, and the latter be forced down with a 10 lb.
pressure, the pressure on the large one will be 100 lbs., and so on.
[Illustration: Fig. 62.—Water Press.]
The accompanying illustration shows the form of the Hydraulic or Bramah
Press. A B C D is a strong frame, F the force-pump worked by means of
a lever fixed at G, and H is the counterprise. E is the stop-cock to
admit the water (fig. 63).
[Illustration: Fig. 63.—Bramah Press.]
The principles of hydrostatics will be easily explained. The Lectures
of M. Aimé Schuster, Professor and Librarian at Metz, have taught us in
a very simple manner the principle of Archimedes, in which it is laid
down that “a body immersed in a liquid loses a portion of its weight
equal to the weight of the liquid displaced by it.” We take a body
of as irregular form as we please; a stone, for example. A thread is
attached to the stone, and it is then placed in a glass of water full
up to the brim. The water overflows; a volume of the liquid equal to
that of the stone runs over. The glass thus partially emptied is then
dried, and placed on the scale of a balance, beneath which we suspend
the stone; equilibrium is established by placing some pieces of lead
in the other scale. We then take a vase full of water, into which we
plunge the stone suspended from the scale, supporting the vase by means
of bricks. The equilibrium is now broken; to re-establish it, it is
necessary to fill up with water the glass placed on the scale; that
is to say, we put back in the glass the weight of a volume of water
precisely equal to that of the stone.
[Illustration: Fig. 64.—Demonstration of the upward pressure of
liquids.]
If it is desired to investigate the principles relating to connected
vessels, springs of water, artesian wells, etc., two funnels, connected
by means of an india-rubber tube of certain length, will serve for the
demonstration; and by placing the first funnel at a higher level, and
pouring in water abundantly, we shall see that it overflows from the
second.
A disc of cardboard and a lamp-glass will be all that is required to
show the upward pressure of liquids. I apply to the opening of the
lamp-glass a round piece of cardboard, which I hold in place by means
of a string; the tube thus closed I plunge into a vessel filled with
water. The piece of cardboard is held by the pressure of the water
upwards. To separate it from the opening it suffices to pour some
water into the tube up to the level of the water outside (fig. 64). The
outer pressure exercised on the disc, as well as the pressure beneath,
is now equal to the weight of a body of water having for its base the
surface of the opening of the tube, its depth being the distance from
the cardboard to the level of the water.
Syringes, pumps, etc., are the effects of atmospheric pressure.
Balloons rise in the air by means of the pressure of gas; a balloon
being a body plunged in gas, is consequently submitted to the same laws
as a body plunged in water.
Boats float because of the pressure of liquid, and water spurts from
a fountain for the same reason. I recollect having read a very useful
application of the principles of fluid pressure.
[Illustration: Fig 65.—Experiment on the convexity of a meniscus.]
A horse was laden with two tubs for carrying a supply of water, and
in the bottom of the tubs a valve was fixed. When the horse entered
the stream the tubs were partly immersed; the water then exercised its
upward pressure, the valve opened, and the tubs slowly filled. When
they were nearly full the horse turned round and came out of the water;
the pressure had ceased.
Thus the action of the water first opened the valve, and then closed it.
The particular phenomena observable in the water level in narrow
spaces, as of a fine glass tube, or the level of two adjoining waves,
capillary phenomena, etc., do not need any special appliance for
demonstration, and it is the same with the convexity or concavity of
meniscuses.
Fig. 65 represents a pretty experiment in connection with these
phenomena. I take a glass, which I fill up to the brim, taking care
that the meniscus be concave, and near it I place a pile of pennies.
I then ask my young friends how many pennies can be thrown into the
glass without the water overflowing. Everyone who is not familiar with
the experiment will answer that it will only be possible to put in one
or two, whereas it is possible to put in a considerable number, even
ten or twelve. As the pennies are carefully and slowly dropped in, the
surface of the liquid will be seen to become more and more convex, and
one is surprised to what an extent this convexity increases before the
water overflows.
The common _syphon_ may be mentioned here. It consists of a bent tube
with limbs of unequal length. We give an illustration of the syphon
(fig. 66). The shorter leg being put into the mixture, the air is
exhausted from the tube at _o_, the aperture at _g_ being closed with
the finger. When the finger is removed the liquid will run out. If the
water were equally high in both legs the pressure of the atmosphere
would hold the fluid in equilibrium, but one leg being longer, the
column of water in it preponderates, and as it falls, the pressure on
the water in the vessel keeps up the supply.
[Illustration: Fig. 66.—The Syphon.]
Apropos of the syphon, we may mention a very simple application of the
principle. Cut off a strip of cloth, and arrange it so that one end
shall remain in a glass of water while the other hangs down, as in the
illustration. In a short time the water from the upper glass will have
passed through the cloth-fibres to the lower one (fig. 67).
This attribute of porous substances is called _capillarity_, and shows
itself by _capillary attraction_ in very fine pores or tubes. The same
phenomenon is exhibited in blotting paper, sugar, wood, sand, and
lamp-wicks, all of which give familiar instances of capillarity. The
cook makes use of this property by using thin paper to absorb grease
from the surface of soups.
Capillarity (referred to on page 25) is the term used to define
capillary force, and is derived from the word _capillus_, a hair; and
so very small bore tubes are called capillary tubes. We know that when
we plunge a glass tube into water the liquid will rise up in it, and
the narrower the tube the higher the water will go; moreover, the water
inside will be higher than at the outside. This is in accordance with a
well-known law of adhesion, which induces concave or convex surfaces[9]
in the liquids in the tubes, according as the tube is wetted with the
liquid or not. For instance, water, as we have said, will be higher in
the tube, and concave in form; but mercury will be depressed below the
outside level, and convex, because mercury will not adhere to glass.
When the force of cohesion to the sides of the tube is more than twice
as great as the adhesion of the particles of the liquid, it will rise
up the sides, and if the forces be reversed, the rounded appearance
will follow. This accounts for the convex appearance, or “meniscus,” in
the column of mercury in a barometer.
Amongst the complicated experiments to demonstrate molecular
attraction, the following is very simple and very pretty:—Take two
small balls of cork, and having placed them in a basin half-filled with
water, let them come close to each other. When they have approached
within a certain distance they will rush together. If you fix one of
them on the blade of your penknife, it will attract the other as a
magnet, so that you can lead it round the basin (fig. 68). But if the
balls of cork are covered with grease they will _repel_ each other,
which fact is accounted for by the form of the _menisques_, which are
convex or concave, according as they are moistened, or preserved from
action of the water by the grease.
[Illustration: Fig. 67.—An improvised syphon.]
This attribute is of great use in the animal and vegetable kingdoms.
The rising of the sap is one instance of the latter.
Experience in hydrostatics can be easily applied to amusing little
experiments. For instance, as regards the syphon, we may make an image
of _Tantalus_ as per illustration (fig. 69). A wooden figure may be cut
in a stooping posture, and placed in the centre of a wide vase, as if
about to drink. If water be poured slowly into the vase it will never
rise to the mouth of the figure, and the unhappy _Tantalus_ will remain
in expectancy. This result is obtained by the aid of a syphon hidden in
the figure, the shorter limb of which is in the chest. The longer limb
descends through a hole in the table, and carries off the water. These
vases are called _vases of Tantalus_.
The principle of the syphon may also be adapted to our domestic
filters. Charcoal, as we know, makes an excellent filter, and if we
have a block of charcoal in one of those filters,—now so common,—we can
fix a tube into it, and clear any water we may require. It sometimes
(in the country) happens that drinking-water may become turgid, and in
such a case the syphon filter will be found useful.
[Illustration: Fig. 68.—Molecular attraction.]
The old “deception” jugs have often puzzled people. We give an
illustration of one, and also a sketch of the “deceptive” portion
(figs. 70 and 71). This deception is very well managed, and will create
much amusement if a jug can be procured; they were fashionable in the
eighteenth century, and previously. A cursory inspection of these
curious utensils will lead one to vote them utterly useless. They are,
however, very quaint, and if not exactly useful are ornamental. They
are so constructed, that if an inexperienced person wish to pour out
the wine or water contained in them, the liquid will run out through
the holes cut in the jug.
To use them with safety it is necessary to put the spout A in
one’s mouth, and close the opening B with the finger, and then
by drawing in the breath, cause the water to mount to the lips by the
tube which runs around the jug. The specimens herein delineated have
been copied from some now existent in the museum of the Sèvres china
manufactory.
The _Buoyancy of Water_ is a very interesting subject, and a great
deal may be written respecting it. The swimmer will tell us that it is
easier to float in salt water than in fresh. He knows by experience
_how difficult it is to sink_ in the sea; and yet hundreds of people
are drowned in the water, which, if they permitted it to exercise its
power of buoyancy, would help to save life.
[Illustration: Fig. 69.—Vase of Tantalus.]
The sea-water holds a considerable quantity of salt in solution, and
this adds to its resistance, or floating power. It is heavier than
fresh water, and the Dead Sea is so salt that a man cannot possibly
sink in it. This means that the man’s body, bulk for bulk, is much
lighter than the water of the Dead Sea. A man will sink in fresh, or
ordinary salt water if the air in his lungs be exhausted, because
without the air he is much heavier than water, bulk for bulk. So if
anything is weighed in water, it apparently loses in weight exactly
equal to its own bulk of water.
Water is the means by which the _Specific Gravity_ of liquids or solids
is found, and by it we can determine the relative densities of matter
in proportion. Air is the standard for gases and vapours. Let us
examine this, and see what is meant by SPECIFIC GRAVITY.
We have already mentioned the difference existing between two equal
volumes of different substances, and their weight, which proves that
they may contain a different number of atoms in the same space. We also
know, from the principle of Archimedes, that _if a body be immersed in
a fluid, a portion of its weight will be sustained by the fluid equal
to the weight of the fluid displaced_.
[Illustration: Fig. 70.—Deception jugs of old pattern.]
[This theorem is easily proved by filling a bucket with water, and
moving it about in water, when it will be easy to lift; and likewise
the human body may be easily sustained in water by a finger under the
chin.]
The manner in which Archimedes discovered the displacement of liquids
is well known, but is always interesting. King Hiero, of Syracuse,
ordered a crown of gold to be made, and when it had been completed
and delivered to His Majesty, he had his doubts about the honesty of
the goldsmith, and called to Archimedes to tell him whether or not
the crown was of gold, pure and simple. Archimedes was puzzled, and
went home deep in thought. Still considering the problem he went to
the bath, and in his abstraction filled it to the brim. Stepping in he
spilt a considerable quantity of water, and at once the idea struck
him that any body put into water would displace its own weight of the
liquid. He did not wait to dress, but ran half-naked to the palace,
crying out, “Eureka, Eureka! I have found it, I have found it! “What
had he found?—He had solved the problem.
[Illustration: Fig. 71.—Section of jug.]
He got a lump of gold the same weight as the crown, and immersed it
in water. He found it weighed nineteen times as much as its own bulk
of water. But when he weighed the kings crown he found it displaced
more water than the pure gold had done, and consequently it had been
adulterated by a lighter metal. He assumed that the alloy was silver,
and by immersing lumps of silver and gold of equal weight with the
crown, and weighing the water that overflowed from each dip, he was
able to tell the king how far he had been cheated by the goldsmith.
It is by this method now that we can ascertain the specific gravity of
bodies. One cubic inch of water weighs about half an ounce (or to be
exact, 252½ grains). Take a piece of lead and weigh it in air; it
weighs, say, eleven ounces. Then weigh it in a vase of water, and it
will be only ten ounces in weight. So lead is eleven times heavier than
water, or eleven ounces of lead occupy the same space as one ounce of
water.
[Illustration: Fig. 72.—Weighing metal in water.]
[The heavier a fluid is, or the greater its density, the greater will
be the weight it will support. Therefore we can ascertain the purity
or otherwise of certain liquids by using hydrometers, etc., which will
float higher or lower in different liquids, and being gauged at the
standard of purity, we can ascertain (for instance) how much water is
in the milk when supplied from the dairy.]
[Illustration: Fig. 73.—Hydrometer.]
But to return to SPECIFIC GRAVITY, which means the
“Comparative density of any substance relatively to water,” or as
Professor Huxley says, “The weight of a volume of any liquid or solid
in proportion to the weight of the same volume of water, at a known
temperature and pressure.”
Water, therefore, is taken as the unit; so anything whose equal
volume under the same circumstances is twice as heavy as the water,
is declared to have its specific gravity 2; if three-and-a-half times
it is 3·5, and so on. We append a few examples; so we see that things
which possess a higher specific gravity than water sink, which comes to
the same thing as saying they are heavier than water, and _vice versâ_.
To find the specific gravity of any solid body proceed as above, in the
experiment of the lead. By weighing the substance in and out of water
we find the weight of the water displaced; that is, the first weight
less the second. Divide the weight in air by the remainder, and we
shall find the specific gravity of the substance.
[Illustration: Fig. 74.—Over-shot wheel of mill.]
The following is a table of specific gravities of some very different
substances, taking water as the unit.
+----------+----------++----------+----------++------------+----------+
|Substance.| Specific ||Substance.| Specific ||Substance. | Specific |
| | Gravity. || | Gravity. || | Gravity. |
+----------+----------++----------+----------++------------+----------+
| Platinum | 21·5 || Iron | 7·79 || Water | 1·000 |
| Gold | 19·5 || Tin | 7·29 || Sea Water | 1·026 |
| Mercury | 13·59 || Granite | 2·62 || Rain Water | 1·001 |
| Lead | 11·45 || Oak Wood | 0·77 || Ice | ·916 |
| Silver | 10·50 || Cork | 0·24 || Ether | 0·723 |
| Copper | 8·96 || Milk | 1·032 || Alcohol | 0·793 |
+----------+----------++----------+----------++------------+----------+
But we have by no means exhausted the uses of water. Hydrodynamics,
which is the alternative term for hydraulics, includes the
consideration of many forms of water-wheels, most of which, as
mill-wheels, are under-shot, or over-shot accordingly as the water
passes horizontally over the floats, or acts beneath them. These wheels
are used in relation to the fall of water. If there is plenty of water
and a slight fall, the under-shot wheel is used. If there is a good
fall less water will suffice, as the weight and momentum of the falling
liquid upon the paddles will turn the wheel. Here is the Persian
water-wheel, used for irrigation (fig. 75). The Archimedian Screw,
called after its inventor, was one of the earliest modes of raising
water. It consists of a cylinder somewhat inclined, and a tube bent
like a screw within it. By turning the handle of the screw the water is
drawn up and flows out from the top.
[Illustration: Fig. 75.—Irrigation wheel in Egypt.]
The Water Ram is a machine used for raising water to a great height by
means of the momentum of falling water.
The Hydraulic Lift is familiar to us all, as it acts in our hotels, and
we need only mention these appliances here; full descriptions will be
found in Cyclopædias.
We have by no means exhausted the subject of Water in this chapter. Far
from it. But when we come to Chemistry and Physical Geography we shall
have more to tell, and our remarks as to the application of science to
Domestic Economy, in accordance with our plan, will also lead us up to
some of the uses of water. So for the present we will take our leave
of water in a liquid form, and meet it again under the application of
Heat, which subject will take us to Ice and Steam,—two very different
conditions of water.
FOOTNOTES:
[9] The curved surface of a column of liquid is termed a “meniscus,”
from the Greek word _meniskos_, meaning “a little lens.”