PHYSICS—THE MEANING OF PHYSICS—FORCES OF
NATURE—GRAVITY—COHESION—CHEMICAL ATTRACTION—CENTRE OF
GRAVITY—EXPERIMENTS—AUTOMATON TUMBLERS.
Having now introduced our readers to Science which they can find for
themselves in the open air, and the pursuit of which will both instruct
and amuse, we will proceed to investigate the Branch of Science called
PHYSICS.
PHYSICS may be briefly described as the Branch of Natural
Science which treats of such phenomena as are unaccompanied by any
important changes in the objects wherein such phenomena are observed.
For instance, the sounding of a bell or the falling of a stone are
physical phenomena, for the objects which cause the sound, or the fall,
undergo no change. Heat is set free when coal burns. This disengagement
of heat is a physical phenomenon; but the change during combustion
which coal undergoes is a _chemical_ phenomenon. So the objects may
be the same, but the circumstances in which they are placed, and the
forces which act upon them, may change their appearance or position.
This brings us at once to the _Forces of Nature_, which are three
in number; viz., Gravity, Cohesion, and Affinity, or Chemical
Attraction. The phenomena connected with the last-named forms the
Science of _Chemistry_. We give these three Forces these names. But
first we must see what is Force, for this is very important. Force
is a CAUSE—the cause of Motion or of Rest. This may appear
paradoxical, but a little reflection will prove it. It requires force
to set any object in motion, and this body would never stop unless some
other force or forces prevented its movement beyond a certain point.
Force is therefore the cause of a change of “state” in matter.
We have said there are three forces in nature. The first is Gravity,
or the attraction of particles at a distance from each other. We may
say that Gravity, or Gravitation, is the mutual attraction between
different portions of matter acting at all distances,—the force of
attraction being, of course, in proportion to the mass of the bodies
respectively. The greatest body is the Earth, so far as our purposes
are concerned. So the attraction of the Earth is _Gravity_, or what we
call _Weight_.
We can easily prove this. We know if we jump from a chair we shall
come to the floor; and if there were nothing between us and the actual
ground sufficient to sustain the force of the attracting power of the
earth, we should fall to the earth’s surface. In a teacup the spoon
will attract air bubbles, and large air bubbles will attract small
ones, till we find a small mass of bubbles formed in the centre of the
cup of tea. Divide this bubble, and the component parts will rush to
the sides of the cup. This form of attraction is illustrated by the
accompanying diagrams.
[Illustration: Fig. 14.]
Suppose two balls of equal magnitude, A and B (fig.
14). These being of equal magnitude, attract each other with equal
force, and will meet, if not opposed, at a point (M) half-way
between the two. But they do not meet, because the attraction of the
earth is greater than the attraction they relatively and collectively
exercise towards each other. But if the size of the balls be different,
the attraction of the greater will be more evident, as shown below,
where the points of meeting are indicated respectively (figs. 15 and
16). These experiments will illustrate the phenomena of _falling
bodies_. Gravity is the cause of this, because every object on the
surface of the earth is very much smaller than the earth itself, and
therefore all bodies _fall_ towards the centre of the earth. A certain
time is thus occupied, and we can find the _velocity_ or rapidity of a
falling body very easily. On the earth a body, if let fall, will pass
through a space sixteen feet in the first second; and as the attraction
of the earth still continues and is exercised upon a body already in
rapid motion, this rate of progress must be proportionately increased.
Just as when steam is kept up in an engine running down hill, the
velocity of the train will rapidly increase as it descends the gradient.
[Illustration: Figs. 15 and 16.]
A body falling, then, descends sixteen feet in the first second, and
for every succeeding second it assumes a greater velocity. The distance
the body travels has been calculated, and the space it passes through
has been found to _increase in proportion to the square of the time it
takes to fall_. For instance, suppose you drop a stone from the top of
a cliff to the beach, and it occupies two seconds in falling, if you
multiply 2 × 2, and the result by sixteen, you will find how high the
cliff is: in this (supposed) case it is (omitting decimals) sixty-four
feet high. The depth of a well can also be ascertained in the same way,
leaving out the effect of air resistance.
But if we go up into the air, the force of gravity will be diminished.
The attraction will be less, because we are more distant from the
centre of the earth. This decrease is scarcely, if at all, perceptible,
even on very high mountains, because their size is not great in
comparison with the mass of the earth’s surface. The rule for this is
_that gravity decreases in proportion to the square of the distance_.
So that if at a certain distance from the earth’s surface the force of
attraction be 1, if the distance be _doubled_ the attraction will be
only _one quarter_ as much as before—not one-half.
Gravity has exactly the same influence upon _all_ bodies, and the force
of the attraction is in proportion to the mass. All bodies of equal
mass will fall in the same time in a given distance. Two coins (or a
coin and a feather _in vacuo_) will fall together. But in the air the
feather will remain far behind the coin, because nearly all the atoms
of the former are resisted by the air, while in the coin only some
particles are exposed to the resistance, the _density_ of the latter
preventing the air from reaching more than a few atoms, comparatively
speaking. The theory of weight and gravitation, and experiments
relating to the falling of bodies, may be easily demonstrated with
ordinary objects that we have at hand. I take a halfpenny and a piece
of paper, which I cut in the shape of the coin, and holding them side
by side, I drop them simultaneously; the halfpenny reaches the ground
some time before the paper, a result quite in accordance with the laws
of gravitation, as one must bear in mind the presence of air, and the
different resistance it offers to two bodies differing in density. I
next place the paper disc on the upper surface of the piece of money,
and then drop them simultaneously. The two objects now reach the ground
at the same time, the paper, in contact with the halfpenny, being
preserved from the action of the air. This experiment is so well known
that we need not further discuss it; but it must be plainly evident
that it is capable of development in experiments on phenomena relating
to falling bodies.[7] When a body influenced by the action of a force
acts, in its turn, upon another, the latter reacts in an opposite
manner upon the first, and with the same intensity.
_The Attraction of Cohesion_ is the attraction of particles of bodies
to each other at very small distances apart. Cohesion has received
various names in order to express its various degrees. For instance,
we say a body is tough or brittle, or soft or hard, according to the
degrees of cohesion the particles exercise. We know if we break a glass
we destroy the cohesion; the particles cannot be reunited. Most Liquid
particles can be united, but not all. Oil will not mix with water.
The force of cohesion depends upon heat. Heat expands everything, and
the cohesion diminishes as temperature increases.
There are some objects or substances upon the earth the particles of
which adhere much more closely than others, and can only, with very
great difficulty, be separated. These are termed _Solids_. There are
other substances whose particles can easily be divided, or their
position altered. These are called _Fluids_. A third class seem to have
little or no cohesion at all. These are termed _Gases_.
Adhesion is also a form of attraction, and is cohesion existing on the
surfaces of two bodies. When a fluid adheres to a solid we say the
solid _is wet_. We turn this natural adhesion to our own purposes in
many ways,—we whitewash our walls, and paint our houses; we paste our
papers together, etc.
On the other hand, many fluids will hot adhere. Oil and water have
already been instanced. Mercury will not stick to a glass tube, nor
will the oiled glass tube retain any water. We can show the attraction
and repulsion in the following manner:—Let one glass tube be dipped
into water and another into mercury, you will see that the water will
ascend slightly at the side, owing to the attraction of the glass,
while the mercury will be higher in the centre, for it possesses no
attraction for the glass (fig. 17). If small, or what are termed
capillary (or hair) tubes, be used (fig. 18), the water will rise up in
the one tube, while in the other the mercury will remain lower than the
mercury outside the tube. (See _Capillarity_.)
[Illustration: Fig. 17.]
[Illustration: Fig. 18.]
_Chemical Attraction_ is the force by which two different bodies unite
to form a new and different body from either. This force will be fully
considered in CHEMISTRY, in a future part.
It is needless for us to dwell upon the uses of these Forces of Nature.
Gravity and Cohesion being left out of our world, we can imagine the
result. The earth and sun and planets would wander aimlessly about;
we should float away into space, and everything would fall to pieces,
while our bodies would dissolve into their component parts.
_The Balance and Centre of Gravity._—We have spoken at some length
about Gravity, and now we must say something respecting that point
called the _Centre of Gravity_, and the _Balance_, and upon the latter
we have a few remarks to make first, for a well-adjusted balance is
a most useful thing, and we will show you how to make one, and then
proceed to our illustrations of the Centre of Gravity, and explain it.
[Illustration: Fig. 19.—Torsion balance, which can easily be
constructed, capable of weighing a milligram one-tenth of full size.]
All those who cultivate experimental science are aware that it is
useful to unite with theoretical ideas that manual dexterity which is
acquired by the student accustoming himself to practical operations.
One cannot too strongly urge both chemists and physicists to exercise
themselves in the construction of the appliances they require, and
also to modify those already existing, which may be adapted to their
wants. In a large number of cases it is possible to manufacture, at
small expense, delicate instruments, capable of rendering the same
service as the most elaborate apparatus. Important scientific labours
have often been undertaken by men whose laboratories were most simple,
who, by means of skill and perseverance, knew how to do great things
with small resources. A delicate balance, for instance, indispensable
alike to chemist and physicist, can be manufactured at little cost in
different ways. A thin platinum wire and a piece of wood is all that
is needed to make a balance capable of weighing a milligram; and to
make a very sensitive hydrostatic balance, little is required besides
a glass balloon. Fig. 19 represents a small torsion balance of extreme
simplicity. A thin platinum wire is stretched horizontally through two
staples, from the wooden supports, A B, which are fixed in a
deal board. A very thin, delicate lever, C D, cut in wood,
or made with a wisp of straw, is fixed in the centre of the platinum
wire by means of a small clip, which secures it firmly. This lever
is placed in such a manner that it is raised perceptibly out of the
horizontal line. At D is fixed a paper scale, on which is put
the weight of a centigram. The lever is lowered to a certain point,
slightly twisting the platinum wire. Near the end of the lever a piece
of wood, F, is fixed, on which is marked the extreme point
of its movements. Ten equi-distant divisions are marked between these
two points, which represent the distance traversed by the lever under
the weight of the milligram. If a smaller weight than a centigram is
placed on the paper scale the lever falls, and balances itself after
a few oscillations. If it falls four divisions, it is evident that the
substance weighs four milligrams. Taking a rather thicker platinum
wire, to which a shorter lever must be adapted, one can weigh the
decigram, and so on. It would be an easy matter, also, to make, on the
same model, balances for weighing considerable weights. The platinum
wire should be replaced by iron wires of larger diameter, firmly
stretched, and the lever should be made of a piece of very resisting
wood. One can also, by adaptation, find the exact value of the most
trifling weights. By lengthening a very fine platinum wire several
yards, and adapting a long, slender lever, it will not be impossible to
ascertain the tenth of a milligram. In this latter case the balance can
be set when it is wanted.
Fig. 20 represents Nicholson’s Areometer, which any one may construct
for himself, and which, as it is here represented, constitutes another
kind of balance. A glass balloon, filled with air, is hermetically
closed with a cork, through which is passed a cylinder of wood,
surmounted by a wooden disc, D. The apparatus is terminated at
its lower end by a small tray, C, on which one can put pieces
of lead in variable quantities. It is then plunged into a glass filled
with water. The pieces of lead on the tray, C, are added by
degrees, until the stem of the areometer rises almost entirely above
the level of the water; it is next passed through a ring, which keeps
it in position, and which is fastened to the upper part of the glass by
means of four iron wires in the shape of a cross. The stem is divided
in such a way that the space comprised in each division represents the
volume of a cubic centimetre. Thus arranged, the apparatus constitutes
a balance. The object to be weighed is placed on the disc, D,
and the areometer sinks in the water, oscillates, and then remains in
equilibrium. If the stem sinks five divisions, it is evident that the
weight of the object corresponds to that of five cubic centimetres of
displaced water, or five grams.
[Illustration: Fig. 20.—Nicholson’s Areometer, contrived to serve as a
balance.]
It is obvious, therefore, from the preceding examples, that it is
not impossible to construct a weighing apparatus with ordinary and
very inexpensive objects. We can, in the same way, show that it is
possible to perform instructive experiments with no appliances at all,
or, at any rate, with common things, such as everyone has at hand.
The lamented Balard, whose loss science has had recently to deplore,
excelled in chemical experiments without a laboratory; fragments of
broken glass or earthenware were used by him for improvising retorts,
bottles and vases for forming precipitates, and carrying on many
important operations.
Scheele also operated in like manner; he knew how to make great
discoveries with the humblest appliances and most slender resources.
One cannot too earnestly endeavour to imitate such leaders, both in
teaching others and instructing oneself.
The laws relating to the weight of bodies, the centre of gravity, and
stable or unstable equilibrium, may be easily taught and demonstrated
by means of a number of very familiar objects. By putting into the
hands of a child a box of soldiers cut in elder-wood, the end of
each fixed into half a bullet, we provide him with the means of
making some easy experiments on the centre of gravity. According to
some authorities on equilibrium, it is not impossible, with a little
patience and delicacy of manipulation, to keep an egg balanced on
one of its ends. This experiment should be performed on a perfectly
horizontal surface, a marble chimney-piece, for example. If one can
succeed in keeping the egg up, it is, according to the most elementary
principles of physics, because the vertical line of the centre of
gravity passes through the point of contact between the end of the egg
and the surface on which it rests.
[Illustration: Fig. 21.—Experiment on “centre of gravity.”]
Fig. 21 reproduces a curious experiment in equilibrium, which is
performed with great facility. Two forks are stuck into a cork, and the
cork is placed on the brim of the neck of a bottle. The forks and the
cork form a whole, of which the centre of gravity is fixed over the
point of support. We can bend the bottle, empty it even, if it contains
fluid, without the little construction over its mouth being in the
least disturbed from its balance. The vertical line of the centre of
gravity passes through the point of support, and the forks oscillate
with the cork, which serves as their support, thus forming a movable
structure, but much more stable than one is inclined to suppose.
This curious experiment is often performed by conjurors, who inform
their audience, that they will undertake to empty the bottle without
disturbing the cork. If a woodcock has been served for dinner, or any
other bird with a long beak, take off the head at the extreme end of
the neck; then split a cork so that you can insert into it the neck of
the bird, which must be tightly clipped to keep it in place; two forks
are then fixed into the cork, exactly as in the preceding example, and
into the bottom of the cork a pin is inserted. This little contrivance
is next placed on a piece of money, which has been put on the opening
of the neck of the bottle, and when it is fairly balanced, we give
it a rotatory movement, by pushing one of the forks as rapidly as we
please, but as much as possible without any jerk (fig. 22). We then see
the two forks, and the cork surmounted by the woodcock’s head, turning
on the slender pivot of a pin. Nothing can be more comical than to
witness the long beak of the bird turning round and round, successively
facing all the company assembled round the table, sometimes with a
little oscillation, which gives it an almost lifelike appearance.
This rotatory movement will last some time, and wagers are often laid
as to which of the company the beak will point at when it stops. In
laboratories, wooden cylinders are often to be seen which will ascend
an inclined plane without any impulsion. This appears very surprising
at first, but astonishment ceases when we perceive that the centre of
gravity is close to the end of the cylinder, because of a piece of
lead, which has been fixed in it.
[Illustration: Fig. 22.—Another experiment on the same subject.]
[Illustration: Fig. 23.—Automatic puppets.]
Fig. 23 gives a very exact representation of a plaything which was
sold extensively on the Boulevards at Paris at the beginning of the
New Year. This little contrivance, which has been known for some time,
is one of the most charming applications of the principles relating
to the centre of gravity. With a little skill, any one may construct
it for himself. It consists of two little puppets, which turn round
axles adapted to two parallel tubes containing mercury. When we place
the little contrivance in the position of fig. 24, the mercury being
at _a_, the two dolls remain motionless, but if we lower the doll
S, so that it stands on the second step (No. 2) of the flight,
as indicated in fig. 25, the mercury descends to _b_ at the other end
of the tube; the centre of gravity is suddenly displaced; the doll
R then accomplishes a rotatory movement, as shown by the arrow
in fig. 25, and finally alights on step No. 3. The same movement is
also effected by the doll S, and so on, as many times as there
are steps. The dolls may be replaced by a hollow cylinder of cartridge
paper closed at both ends, and containing a marble; the cylinder,
when placed vertically on an inclined plane, descends in the same
way as the puppets. The laws of equilibrium and displacement of the
centre of gravity, are rigorously observed by jugglers, who achieve
many wonderful feats, generally facilitated by the rotatory motion
given to the bodies on which they operate, which brings into play the
centrifugal force. The juggler who balances on his forehead a slender
rod, on the end of which a plate turns round, would never succeed
in the experiment if the plate did not turn on its axis with great
rapidity. But by quick rotation the centre of gravity is kept near the
point of support. We need hardly remark, too, that it is the motion of
a top that tends to keep it in a vertical position.
[Illustration: Fig. 24.—First position of the puppets.]
[Illustration: Fig. 25.—Second position of the puppets.]
Many experiments in mechanical physics may occur to one’s mind. To
conclude the enumeration of those we have collected on the subject,
I will describe the method of lifting a glass bottle full of water
by means of a simple wisp of straw. The straw is bent before being
passed into the bottle of water, so that, when it is lifted, the centre
of gravity is displaced, and brought directly under the point of
suspension. Fig. 26 shows the method of operation very plainly. It is
well to have at hand several pieces of straw perfectly intact, and free
from cracks, in case the experiment does not succeed with the first
attempt.
Having now seen how this point we call the centre of gravity acts, we
may briefly explain it.
[Illustration: Fig. 26.—Lifting a bottle with a single straw.]
The centre of gravity of a body is that point in which the sum of the
forces of gravity, acting upon all the particles, may be said to be
united. We know the attraction of the earth causes bodies to have a
property we call _Weight_. This property of weight presses upon every
particle of the body, and acts upon them as parallel forces. For if a
stone be broken all the portions will equal the weight of the stone;
and if some of them be suspended, it will be seen that they hang
parallel to each other, so we may call these weights parallel forces
united in the whole stone, and equal to a single resultant. Now, to
find the centre of gravity, we must suspend the body, and it will hang
in a certain direction. Draw a line from the point of suspension, and
suspend the body again: a line drawn from that point of suspension will
pass through the same place as the former line did, and so on. That
point is the centre of gravity of that suspended body. If the form of
it be regular, like a ball or cylinder, the centre of gravity is the
same as the mathematically central point. In such forms as pyramids
it will be found near the largest mass; viz., at the bases, about
one-fourth of the distance between the apex and the centre of gravity
of the base.
When the centre of gravity of any body is supported, that body cannot
fall. So the well-known leaning towers are perfectly safe, because
their lines of direction fall within the bases. The centre of gravity
is in the centre of the leaning figure. The line of direction drawn
vertically from that point falls within the base; but if the tower were
built up higher, so that the centre of gravity were higher, then the
structure would fall, because the line of direction would fall without
the base.
We see that animals (and men) are continually altering the position of
the centre of gravity; for if a man bears a load he will lean forward,
and if he takes up a can of water in one hand he will extend the other
to preserve his balance or equilibrium.
[Illustration: Fig. 27.—Balancing a weight on a nail and key.]
The experiment shown in the accompanying illustration is apparently
very difficult, but it will be found easy enough in practice if
the hand be steady. Take a key, and by means of a crooked nail, or
“holdfast,” attach it to a bar of wood by a string tied tightly round
the bar, as in the picture. To the other extremity of the bar attach
a weight, and then drive a large-headed nail into the table. It will
be found that the key will balance, and even move upon the head of the
nail, without falling. The weight is under the table, and the centre of
gravity is exactly beneath the point of suspension.
Another simple experiment may prove amusing. Into a piece of wood
insert the points of two knives, and at the centre of the end of the
bar insert a needle between the knife handles. The wood and the knives
may then be balanced on another needle fixed in a cork at A.
[Illustration: Fig. 28.—Another experiment.]
We may conclude this chapter by summing up in a few words what the
Centre of Gravity is. We can define it as “that point in a body upon
which the body, acted on solely by the force of gravity, will balance
itself in all positions.” Such a point exists in every body, and
equally in a number of bodies fastened tightly together. The Centre of
Gravity has by some writers been denominated the _Centre of Parallel
Forces_, or the _Centre of Magnitude_, but the Centre of Gravity is the
most usual and best understood term.
FOOTNOTES:
[7] M. A. G. has written us an interesting letter on the subject of
similar experiments, which we here transcribe:—
“When a siphon of seltzer water has been opened some little time, and
the equilibrium of tension is nearly established between the escaped
gas and the dissolved gas, a vertical stream of bubbles is seen to rise
from the bottom of the apparatus, which present a very clear example
of the law of ascension of bubbles; that is to say (putting out of the
question the expansion of the bubbles in their passage upwards), it
is an inverse representation of the law of gravity affecting falling
bodies. The bubbles, in fact, detach themselves from their starting
point with perfect regularity; and as the interval varies in one
file from another, we have before us a multiplied representation of
that terrible law which Attwood’s machine made such a bugbear to the
commercial world. I believe it is possible, by counting the number
of bubbles that detach themselves in a second, in each file, and the
number which the whole stream contains at a given instant, to carry
the verification further; but I must confess that I have not done so
myself.”