Experiment of the Law of Motion.
In shooting at “taw,” if the marble be struck “plump,” as it is called,
it moves forward exactly in the same line of direction; but if struck
sideways, it will move in an oblique direction, and its course will be
in a line situated between the direction of its former motion and that
of the force impressed. It is called the resolution of forces.
Balancing.
The center of gravity in a body is that part about which all the other
parts equally balance each other. In balancing a stick upon the finger,
or upon the chin, it is necessary only to keep the chin or finger
exactly under the point which is called the center of gravity.
The Balanced Coin.
It seems to be an astounding statement that a quarter, or other piece
of money, can be made to spin on the point of a needle. To perform
this experiment, procure a bottle, cork it, and in the cork place a
needle. Now take another cork and cut a slit in it, so that the edge
of the coin will fit into the slit; next place two forks in the cork,
and placing the edge of the coin on the needle, it will spin around
without falling off. The reason is this: that the weight of the forks
projecting as they do so much below the coin, brings the center of
gravity of the arrangement much below the point of suspension, or the
point of the needle, and therefore the coin remains perfectly safe and
upright.
The Spanish Dancer.
The laws which govern the motion of bodies are capable of many pleasing
illustrations, and the example which we now give of causing rotary
motion is very interesting and easily performed.
Take a piece of card, and cut out a little figure, and paste or gum
it in an erect position on the inside of a watch-glass. Then procure
a black japanned waiter, or a clean plate will do, and, holding it
in an inclined position, place the figure and watch-glass on it,
and they will, of course, slide down. Next let fall a drop of water
on the waiter, place the watch-glass on it, and again incline the
waiter, and instead of the watch-glass sliding down, it will begin to
revolve. It will continue to revolve with increasing velocity, obeying
the inclination and position of the plane, as directed by the hand
of the experimentalist. The reason of this is, in the first place,
in consequence of the cohesion of the water to the two surfaces, a
new force is introduced, by which an unequal degree of resistance is
imparted to different parts of the watch-glass in contact with the
waiter, and, consequently, in its effort to slide down, it revolves.
Again, if the drop of water be observed, it will be seen that it
undergoes a change of figure; a film of water, by capillary action, is
drawn to the foremost portion of the glass, while, by the centrifugal
force, a body of water is thrown under the under part of it. The effect
of both these actions is to accelerate the motion, or, in other words,
to gradually increase the speed.
The Mechanical Bucephalus.
The illustration of the horse furnishes a very good solution of a
popular paradox in mechanics: Given, a body having a tendency to fall
by its own weight; required, how to prevent it from falling by adding
to it a weight on the same side on which it tends to fall. Take a horse
in an erect position, the center of gravity of which is somewhere about
the middle of its body. It is evident, therefore, that were it placed
on its hinder legs, on a table, the line of its direction, or center,
would fall considerably beyond its base, and the horse would fall on
the ground; but to prevent this, there is a stiff wire attached to a
weight or bullet, connected with the body of the horse, and by this
means a horse prances on a table without falling off; so that the
figure that was incapable of supporting itself, is actually prevented
from falling by adding a weight to its unsupported end. This seems
almost impossible, but when we consider that in order to have the
desired effect, the wire must be bent, and the weight be further under
the table than the horse’s feet are on it, the mystery is solved, as it
brings the total weight of bullet and horse in such a position that the
tendency is rather to make it stand up than to let it fall down.
The Revolving Image.
This little figure may be made to balance itself amusingly. Get a piece
of wood, about two inches long; cut one end of it into the form of a
man’s head and shoulders, and let the other end taper off to a fine
point. Next furnish the little gentleman with a pair of wafters, shaped
like oars, instead of arms, but they must be more than double the
length of his body; stick them in his shoulders, and he is complete.
When you place him on the tip of your finger, if you have taken care
to make the point exactly in the center, he will stand upright. By
blowing on the waiters he may be made to turn around very quickly. It
is explained by the reasons that were given in the experiment of the
“balanced coin.”
The Bridge of Knives.
Place three glasses in the form of a triangle, and arrange the handles
of three knives upon them. Nos. 1, 2, and 3, the blade of No. 1 over
that of No. 2, and that over No. 3, which rests on No. 1. The bridge so
made will be self-supported.
The Parlor Boomerang.
The boomerang is a weapon used by the savages of Australia. By them
it is made of a flat piece of hard wood. The peculiarity of this
instrument is, that in whatever direction it is thrown, it will return
to the place from whence it started, in a curve. The Australian
aborigines use it with great dexterity, making it travel around a house
and return to their feet, or they can throw it on the ground so that it
will fly into the air, form a perfect arc over their heads, and strike
them on the back. This curious instrument can be made in miniature, and
is a very amusing toy for the parlor.
Get a piece of tolerably stiff cardboard, and cut from it a figure
resembling a boomerang.
The next thing is to propel it through the air so that it will return
to your feet; to do this, lay the boomerang on a flat book, allowing
one end to project about an inch; then, holding the book to a slight
angle, strike the projecting end of the boomerang with a piece of
stick, or heavy pen-holder, when it will fly across the room and return
to your feet.
The Balanced Turk.
A decanter or bottle is first obtained, and in its cork is placed a
needle; on this is balanced a ball of wood, having a cork or wooden
figure cut out, standing on the top. From the ball project two wires,
bent semicircularly, having at their extremities two bullets. Push
the bullets, and the whole will turn around on the needle, the figure
standing upright all the while; and, twist it about from side to side
as much as you like, it will always regain its erect position. The two
bullets in this case cause the center of gravity to fall below the ball
on which the figure is placed, and, in consequence, as the center of
gravity always assumes the lowest position, it cannot do so without
making the figure stand erect, or, in other words, until the bullets
themselves are equally balanced. Any boy may whittle one of these toys
out with a jack-knife.
The Complacent Vizier.
Among the novelties which scientific investigation has added to our
toys, are several figures which will raise themselves upright when
thrown down, and regain the erect position, notwithstanding their
equilibrium is disturbed. The figures themselves are made of the pith
of elder trees, or any other very light substance. Each is placed on
half a bullet, or may be made to stand on its head, by making its cap
of lead. Their appearance is very droll when they are moved about, as
they seem every moment to be falling over, and yet continually right
themselves. The philosophy of this is, that the center of gravity being
in the base, and always trying to assume the lowest position, it keeps
the figures upright. However much the equilibrium is disturbed, it will
always try to regain its original position.