by which boys raise stones will show the pressure of the atmosphere. It
consists of a piece of soft but firm leather, having a piece of string
drawn through its center. The leather is made quite wet and pliable, and
then its under part is placed on the stone and stamped down by the foot.
This pressing of the leather excludes the air from between the leather
and the stone, and by pulling the string a vacuum is left underneath its
center; consequently the weight of the air about the edges of the leather,
not being counterbalanced by any air between it and the stone, enables
the boy to lift it.
WEIGHT OF THE AIR PROVED BY A PAIR OF BELLOWS.
Shut the nozzle and valve-hole of a pair of bellows, and after having
squeezed the air out of them, if they are perfectly air-tight, we shall
find that a very great force, even some hundreds of pounds, is necessary
for separating the boards. They are kept together by the weight of
the heavy air which surrounds them in the same manner as if they were
surrounded by water.
THE PRESSURE OF THE AIR SHOWN BY A WINE-GLASS.
Place a card on a wine-glass filled with water, then invert the glass, the
water will not escape, the pressure of the atmosphere on the outside of
the card being sufficient to support the water.
[Illustration]
ANOTHER.
[Illustration]
Invert a tall glass jar in a dish of water, and place a lighted taper
under it; as the taper consumes the air in the jar, the water from the
pressure without _rises up_ to supply the place of the air removed by the
combustion. In the operation of cupping, the operator holds the flame of
a lamp under a bell-shaped glass. The air within this being rarefied and
expanded, a considerable portion is given off. In this state the glass is
placed upon the flesh, and as the air within it cools, it contracts, and
the glass adheres to the flesh by the difference of the pressure of the
internal and external air.
ELASTICITY OF THE AIR.
[Illustration]
This can be shown by a beautiful philosophical toy which may easily be
constructed. Procure a glass jar, such as is here represented. Then mould
three or four little figures in wax, and make them hollow within, and
having each a minute opening at the heel, by which water may pass in
and out. Place them in the jar, as seen in the figure, and adjust them
by the quantity of water admitted to them, so that in specific gravity
they differ a little from each other. The mouth of the jar should now be
covered with a piece of skin or India-rubber, and then, if the hand be
pressed upon the top or mouth of the jar, the figures will be seen to rise
or descend as the pressure is gentle or heavy, rising and falling, or
standing still, according to the pressure made.
REASON FOR THIS.
The reason of this is, that the pressure on the top of the jar condenses
the air between the cover and the water surface; this condensation then
presses on the water below, and influences it through its whole extent,
compressing also the air in the figures, forcing as much more water into
them as to render them heavier than water, and therefore heavy enough to
sink.
THE AIR-PUMP.
[Illustration]
The time was, and that not very long ago, when the air-pump was only
obtainable by the philosophical professor, or by persons of enlarged
means. But now, owing to our "cheap way of doing things," a small air-pump
may be obtained for about five or six dollars, and we would strongly
advise our young friend to procure one, as it will be a source of endless
amusement to him; and, supposing that he takes our advice, we give him the
following experiments.
The air-pump consists of a bell glass, called the receiver, A, and a
stand, upon which is a perforated plate B. The hole in this plate is
connected with two pistons, the rods of which are moved by a wheel handle
backwards and forwards, and thus pump the air out of the receiver. When
the air is taken out, a stop-cock is turned, and then the experiments may
be performed.
Under the receiver of an air-pump, when the air has been thoroughly
exhausted, light and heavy bodies fall with the same swiftness. Animals
quickly die for want of air, combustion ceases, gunpowder will not
explode, a bell sounds faint, magnets are powerless, and waters and other
fluids turn to vapor.
TO PROVE THAT AIR HAS WEIGHT.
Take a florence flask, fitted up with a screw and fine oiled silk valve.
Screw the flask on the plate of the air-pump, exhaust the air, take it off
the plate and weigh it. Then let in the air, and again weigh the whole,
and it will be found to have increased by several grains.
[Illustration]
TO PROVE AIR ELASTIC.
Place a bladder, out of which all the air has apparently been squeezed,
under the receiver, upon it lay a weight, exhaust the air, and it will be
seen that the small quantity of air left within the bladder will so expand
itself as to lift the weight. Put a corked bottle into the receiver,
exhaust the air, and the cork will fly out.
AIR IN THE EGG.
Take a fresh egg and cut off a little of the shell and film from its
smaller end, then put the egg under a receiver and pump out the air, upon
which all the contents of the egg will be forced out by the expansion of
the small bubble of air contained in the great end between the shell and
the film.
[Illustration: AIR IN THE EGG.]
[Illustration: DESCENDING SMOKE.]
THE DESCENDING SMOKE.
Set a lighted candle on a plate, and cover it with a tall receiver. The
candle will continue to burn while the air remains, but when exhausted,
will go out, and the smoke from the wick, instead of rising, will descend
in dense clouds towards the bottom of the glass, because the air which
would have supported it has been withdrawn.
HALF EAGLE AND FEATHER.
[Illustration]
Place a nicely-adjusted pair of forceps at the top of the receiver,
communicating with the top at the outside through a hole, so that they
may be opened by the fingers. Then place on each of the little plates
a _half-eagle_ and a _feather_. Exhaust the air from the receiver, and
having done so, detach the objects, so that they may fall. In the open air
the half-eagle will fall long before the feather, but in vacuo, as in the
receiver now exhausted of its air, they will fall both together, and reach
the bottom of the glass at same instant.
THE SOUNDLESS BELL.
Set a bell on the pump-plate, having a contrivance so as to ring it at
pleasure, and cover it with a receiver, then make the clapper sound
against the bell, and it will be heard to sound very well; now exhaust the
receiver of air, and then, when the clapper strikes against the sides of
the bell, the sound can be scarcely heard.
[Illustration: SOUNDLESS BELL.]
[Illustration: FLOATING FISH.]
THE FLOATING FISH.
If a glass vessel, containing water, in which a couple of fish are put
be placed under the receiver, upon exhausting the air, the fish will be
unable to keep at the bottom of the glass, owing to the expansion of
the air within their bodies, contained in the air bladder. They will
consequently rise and float, belly upwards, upon the surface of the water.
THE MYSTERIOUS CIRCLES.
Cut from a card two discs or circular pieces, about two inches in
diameter. In the center of one of them make a hole, into which put the
tube of a common quill, one end being even with the surface of the card.
Make the other piece a little convex, and lay its center over the end of
the quill, with the concave side of the card downwards, the center of the
upper card being from one eighth to one fourth of an inch above the end
of the quill--attempt to blow off the upper card by blowing through the
quill, and _it will be found impossible_.
If, however, the edges of the two cards be made to fit each other very
accurately, the upper card will move, and sometimes it will be thrown off;
but when the edges of the cards are, on two sides, sufficiently far apart
to permit the air to escape, the loose card will retain its position,
even when the current of air sent against it be strong. The experiment
will succeed equally well, whether the current of air be made from the
mouth or from a pair of bellows. When the quill fits the card rather
loosely, a comparatively light puff will throw both cards three or four
feet in height. When, from the humidity of the breath, the upper surface
of the perforated card has a little expanded, and the two opposite sides
are somewhat depressed, those depressed sides may be seen distinctly to
rise and approach the upper card, directly in proportion to the force of
the current of air.
Another fact to be shown with this simple apparatus appears equally
inexplicable with the former. Lay the loose card upon the hand with the
concave side up; blow forcibly through the tube, and, at the same time,
bring the two cards towards each other; when within three eighths of an
inch, if the current of air be strong, the loose card will suddenly rise,
and adhere to the perforated card. If the card through which the quill
passes has several holes made in it, the loose card may be instantly
thrown off with the least puff of air.
For the explanation of the above phenomenon, a gold medal and one hundred
guineas were offered, some years since, by the Royal Society. Such
explanation has been given by Dr. Robert Hare, late of the University of
Pennsylvania, and is as follows:
Supposing the diameters of the discs of card to be to that of the hole as
8 to 1, the area of the former to the latter must be as 64 to 1. Hence,
if the discs were to be separated (their surfaces remaining parallel)
with a velocity as great as that of the air blast, a column of air must,
meantime, be interposed, 64 times greater than that which would escape
from the tube during the interim; consequently, if all the air necessary
to preserve the balance be supplied from the tube, the discs must be
separated with a velocity as much less than that of the blast, as the
column required between them is greater than that yielded by the tube; and
yet the air cannot be supplied from any other source, unless a deficit of
pressure be created between the discs, unfavorable to their separation.
It follows, then, that, under the circumstances in question, the
discs cannot be made to move asunder with a velocity greater than one
sixty-fourth of that of the blast. Of course all the force of the current
of air through the tube will be expended on the moveable disc, and the
thin ring of air, which exists round the orifice between the discs; and
since the moveable discs can only move with one sixty-fourth the velocity
of the blast, the ring of air in the interstice must experience nearly all
the force of the jet, and must be driven outwards, the blast following it,
in various currents radiating from the common center of the tube and discs.
THE DIVING BELL.
[Illustration]
[Illustration]
The diving-bell is a pneumatic engine, by means of which persons can
descend to great depths in the sea, and recover from it valuable portions
of wrecks and other matters. Its principle may be well illustrated by the
following experiment. Take a glass tumbler, and plunge it into the water
with the mouth downwards, and it will be found that the water will not
rise much more than half way in the tumbler. This may be made very evident
if a piece of cork be suffered to float inside of the glass on the surface
of the water. The air within the tumbler does not entirely exclude the
water, because air is elastic, and consequently compressible, and hence
the air in the tumbler is what is called condensed. The diving-bell is
formed upon the above principle, but instead of being of glass, it is a
wooden or metal vessel, of very large dimensions, so as to hold three or
four persons, who are supplied with air from above by means of a tube,
having a corresponding tube to let off the breathed air, the circulation
of which is kept up by pumps, which pump the air in and draw it out of the
bell.
THE AIR BALLOON.
The art of sailing or navigating a body through the air is called
aëronautics. In remote ages, Icarus is said to have risen so high in the
air that the sun melted his wings, and he fell into the Ægean sea, and
was drowned; and there is reason to believe, from some figures that have
recently been discovered on Egyptian and Assyrian monuments, that the
ancients possessed means of rising in the air with which we are not now
acquainted.
The air-balloon, as now constructed, is a bag of silk of large dimensions,
usually cut in gores, and is, when expanded by gas, of a pear shape. It
ascends in the atmosphere because its whole bulk is much lighter than the
air would be in the space it occupies. It is, in fact, a vessel filled
with a fluid which will float on another fluid lighter than itself.
HOW TO MAKE AN AIR-BALLOON.
[Illustration]
The best shape for an air-balloon, or rather a gas-balloon, is that of a
peg-top. And in preparing the gores proceed as follows: Get some close
texture silk, and cut it into a form resembling a narrow pear with a very
thin stalk. Fourteen of these pieces will be found to be the best number;
and, of course, the breadths of each piece must be measured accordingly.
When sewing them together, it will be of advantage to coat the parts
that overlap with a layer of varnish, as this will save much trouble
afterwards, and hold the silk firmer in its place during the stitching.
The threads must be placed very regularly, or the balloon will be drawn
out of shape, and it will be found useful if the gores are covered with
an interior coating of varnish before they are finally sewn together.
Take care not to have the varnish too thick. To the upper part of the
balloon there should be a valve opening inwards, to which a string should
be fastened, passing through a hole made in a small piece of wood fixed
in the lower part of the balloon, so that the aëronaut may open the valve
when he wishes to descend; and this should be imitated on a small scale,
so that the young aëronaut may be perfectly familiar with the construction
of a balloon. The gores are to be covered with a varnish of India rubber
dissolved in a mixture of turpentine and naphtha. Over the whole of the
upper part should be a net-work, which should come down to the middle,
with various cords, proceeding from it to the circumference of a circle
about two feet below the balloon. The circle may be made of wood, or of
several pieces of slender cane bound together. The meshes should be small
at the top, against which part of the balloon the inflammable air exerts
the greatest force, and increase in size as they recede from the top.
The car is made of wicker work; it is usually covered with leather, and
is well varnished or painted. It is suspended by ropes proceeding from
the net which goes over the balloon. Balloons of this kind cannot be made
smaller than six feet in diameter, of oiled silk, as the weight of the
material is too great for the air to buoy it up. They may be made smaller
of thin slips of bladder, or other membrane glued together, or of thin
gutta-percha cloth, which is now extensively used for this purpose; with
this they may be made a foot in diameter, and will rise beautifully.
HOW TO FILL A BALLOON.
Procure a large stone bottle which will hold a gallon of water, into
this put a pound of iron filings, or granulated zinc, with two quarts of
water, and add to this by degrees one pint of sulphuric acid. Then take
a tube, either of glass or metal, and introduce one end of it through a
cork, which place in the bottle, then put the other end into the neck of
the balloon, and the gas will rise into the body of it. When quite full
withdraw the tube, and tie the neck of the balloon with strong cord very
tightly. If freed it will now rise in the air.
TO MAKE FIRE-BALLOONS.
[Illustration]
Cut the gores, according to the form already given, from well woven tissue
paper, paste the gores nicely together, and look well over the surface of
the paper for any small hole or slit, over which paste a piece of paper,
and let it dry. Pass a wire round the neck of the balloon, and have two
cross pieces at its diameter a little bent, so that a piece of soft cotton
dipped in spirits of wine may be laid on them. When all is prepared let
some one hold the balloon from its top by means of a stick, while you
dip the cotton in spirits of wine till it is thoroughly saturated, place
it under the balloon and set fire to it, but be very careful you do not
set fire to the balloon. When the air is sufficiently heated within, the
balloon will indicate a desire to rise, and when it pulls very hard, let
it go, and it will ascend to a great height in the air, and at night
present a very beautiful appearance.
PARACHUTES.
These are easily made by cutting a piece of paper in a circular form, and
placing threads round the edges, which may be made to converge to a point,
at which a cork may be placed as a balance. They ascend by the air getting
under them, and are frequently blown to a great distance.
[Illustration]
THE MYSTERIOUS BOTTLE.
Pierce a few holes with a glazier's diamond in a common black bottle;
place it in a vase or jug of water, so that the neck only is above the
surface. Then, with a funnel, fill the bottle and cork it well, and while
it is in the jug or vase. Take it out, notwithstanding the holes in the
bottom, it will not leak; wipe it dry, and give it to some person to
uncork. The moment the cork is drawn, to the party's astonishment, the
water will begin to run out of the bottom of the bottle.
CAOUTCHOUC BALLOONS.
Put a little ether into a bottle of caoutchouc, close it tightly, soak it
in hot water, and it will become inflated to a considerable size. These
globes may be made so thin as to be transparent.
A piece of caoutchouc, the size of a walnut, has thus been extended to
a ball fifteen inches in diameter; and a few years since a caoutchouc
balloon, thus made, escaped from Philadelphia, and was found one hundred
and thirty miles from that city.
THE MAGIC OF
OPTICS AND OPTICAL AMUSEMENTS.
[Illustration:
"'Seeing is believing,' so the sages say,
To prove this false, hear me, my friends, I pray,
And very soon you all will be agreeing,
That nought is so deceptive as our _seeing_."--MARTIN.
]
Optics is the science of _light_ and _vision_. Concerning the nature
of light, two theories are at present very ably maintained by their
respective advocates. One is termed the Newtonian theory, and the other
the Huygenean. The Newtonian theory considers light to consist of
inconceivably small bodies emanating from the sun, or any other luminous
body. The Huygenean conceives it to consist in the undulations of a highly
elastic and subtle fluid, propagated round luminous centers in spherical
waves, like those arising in a placid lake when a stone is dropped into
the water.
LIGHT AS AN EFFECT.
Light follows the same laws as gravity, and its intensity or degree
decreases as the square of the distance from the luminous body increases.
Thus, at the distance of two yards from a candle we shall have four times
less light than we should have, were we only one yard from it, and so on
in the same proportion.
[Illustration]
REFRACTION.
[Illustration]
Bodies which suffer the rays of light to pass through them, such as water
or glass, are called refracting media. When rays of light enter these,
they do not proceed in straight lines, but are said to be refracted,
or bent out of their course, as seen in the drawing. The ray of light
proceeding from B through the glass L G is bent from the point C, instead
of passing in the direction of the dotted line. But if the ray F C falls
perpendicularly on the glass, there is no refraction, and it proceeds
in a direct line to K; hence refraction only takes place when rays fall
obliquely or aslant on the media.
THE INVISIBLE COIN MADE VISIBLE.
[Illustration]
If a coin be placed in a basin, so that on standing at a certain distance
it be just hid from the eye of an observer by the rim or edge of the
basin, and then water be poured in by a second person, the first keeping
his position; as the water rises the coin will become visible, and will
appear to have moved from the side to the middle of the basin.
THE MULTIPLYING GLASS.
[Illustration]
The multiplying glass is a semi-circular piece of glass cut into facets or
distinct surfaces; and in looking through it we have an illustration of
the laws of refraction, for if a small object, such as a fly, be placed at
D, an eye at E will see as many flies as there are surfaces or facets on
the glass.
TRANSPARENT BODIES.
[Illustration]
Transparent bodies, such as glass, may be made of such form as to cause
all the rays which pass through them from any given point to meet in any
other given point beyond them, or which will disperse them from the given
point. These are called lenses, and have different names according to
their form. 1 is called the plano-convex lens; 2, plano-concave; 3, double
convex; 4, double concave; 5, a meniscus, so called from its resembling
the crescent moon.
THE PRISM.
[Illustration: P.]
The prism is a triangular solid of glass, and by it the young optician
may decompose a ray of light into its primitive and supplementary colors,
for a ray of light is of a compound nature. By the prism the ray A is
divided into its three primitive colors, blue, red, and yellow; and their
four supplementary ones, violet, indigo, green, and orange. The best way
to perform this experiment is to cut a small slit in a window-shutter, on
which the sun shines at some period of the day, and directly opposite the
hole place a prism P; a beam of light in passing through it will then be
decomposed, and if let fall upon a sheet of white paper, or against a
white wall, the seven colors of the rainbow will be observed.
TO MAKE A PRISM.
Provide two small pieces of window-glass and a lump of wax; soften and
mould the wax, stick the two pieces of glass upon it, so that they meet,
as in the cut, where _w_ is the wax, _g_ and _g_ the glasses stuck to it
(Fig. 1). The end view (Fig. 2) will show the angle, _a_, at which the
pieces of glass meet; into which angle put a drop of water.
[Illustration: Fig. 1.]
[Illustration: Fig. 2.]
To use the instrument thus made, make a small hole, or a narrow horizontal
slit, so that you can see the sky through it, when you stand at some
distance from it in the room; or a piece of pasteboard placed in the upper
part of the window-sash, with a slit cut in it, will serve the purpose of
the hole in the shutter. The slit should be about one tenth of an inch
wide, and an inch or two long, with even edges. Then hold the prism in
your hand, place it close to your eye, and look through the drop of water,
when you will see a beautiful train of colors, called a spectrum; at one
end red, at the other violet, and in the middle yellowish green.
[Illustration: Fig. 3.]
The annexed Figure 3 will better explain the direction in which we look:
here, _e_ is the eye of the spectator, _p_ is the prism, _h_ the hole in
the shutter or pasteboard, _s_ the spectrum. By a little practice, you
will soon become accustomed to look in the right direction, and will see
the colors very bright and distinct.
By means of this simple contrivance white light may be analyzed, and
proved to consist of colored rays, and several of its properties be
beautifully illustrated.
COMPOSITION OF LIGHT.
The beam of light passing through the prism is decomposed, and the spaces
occupied by the colors are in the following proportions: red, 6; orange,
4; yellow, 7; green, 8; blue, 8; indigo, 6; violet, 11. Now, if you paste
a sheet of white paper on a circular piece of board about six inches in
diameter, and divide it with a pencil into fifty parts, and paint colors
in them in the proportions given above, painting them dark in the center
parts, and gradually fainter at the edges, till they blend with the one
adjoining; and if the board be then fixed to an axle, and made to revolve
quickly, the colors will no longer appear separate and distinct, but
becoming gradually less visible, they will ultimately appear _white_,
giving this appearance to the whole surface of the paper.
[Illustration]
A NATURAL CAMERA OBSCURA.
The human eye is a camera obscura, for on the back of it, on the retina,
every object in a landscape is beautifully depicted in miniature. This may
be proved by the
BULLOCK'S EYE EXPERIMENT.
Procure a fresh bullock's eye from the butcher, and carefully thin the
outer coat of it behind; take care not to cut it, for if this should
be done the vitreous humor will escape, and the experiment cannot be
performed. Having so prepared the eye, if the pupil of it be directed
to any bright objects, they will appear distinctly delineated on the
back part precisely as objects appear in the instrument we are about to
describe. The effect will be heightened if the eye is viewed in a dark
room with a small hole in the shutter, but in every case the appearance
will be very striking.
THE CAMERA OBSCURA.
[Illustration]