810. _Why does a humming-top make a humming noise?_
Because the hollow wood of the top vibrates, and the edges of the hole
in its sides _strike against the air as it spins_; the air is thereby
set in vibration.
811. _Why does a peg-top hum less than a humming-top?_
Because, _being a solid body of wood_, and having no _hole in its
sides_, its particles are _not so easily thrown into vibration_;
consequently it does not so readily impart vibrations to the air.
812. _Why does a peg-top sometimes hum, and at other times not?_
Because, if it is spun with _great force_, and its peg is _struck
sharply_ against the pavement, _the wood is set in vibration_, and
the surface of the top, repelling the air by its rapid motion, causes
_vibratory waves_. But if it be spun with insufficient force, _the wood
is not set in vibration_.
[Illustration: Fig. 23.--HUMMING-TOP BEFORE SPINNING.]
[Illustration: Fig. 24.--HUMMING-TOP SPINNING.]
813. _Why do we see the figures painted upon the humming-top, before it
spins, but not while it is spinning?_
Because the rapid whirling of the top brings the images of its
different parts so quickly in succession upon _the retina of the eye_,
that they _deface each other_, and _impart an impression of coloured
rings, instead of definite objects_.
[Verse: "Train up a child in the way he should go; and when he is
old, he will not depart from it."--PROVERBS XXII.]
814. _Why does a top stand erect when it spins, but fall when it stops?_
Because the top is under the influence of, and is balanced between
_opposing forces_. The rapid rotation of the top gives to all its
particles a tendency to _fly from the centre_. If the atoms of the wood
were not held together by the _attraction of cohesion_, they would fly
away in a circle outward from the top, _just as drops of water fly
off from a mop, while it is being twirled_. If you take a spoonful of
sand, salt, or dust, and drop it upon the top, it will be scattered in
a circle, just as the atoms of the top would be, _if they were free to
separate_, but not with the same force, because the atoms of the salt,
&c., not being in an active state of rotation, would only be influenced
_by momentary contact with the rotating body_. This tendency of the
particles of a rotating body to fly outward from the centre, is called
_the centrifugal force_.
_Centrifugal._--From two Latin words meaning receding from the
centre.
The other force influencing the top is _the attraction of gravitation_:
the attraction which, were the top not spinning, would draw it towards
the earth. The "spill" projecting from the bottom of the top _stands in
the line in which the top is drawn towards the earth_ and keeps it from
obeying the law of gravitation. Therefore the rotatory motion given to
the top, by the rapid unwinding of the string, and the tendency of its
atoms to fly outward, _balance the top_ upon the line in which it is
drawn to the earth, and which is occupied by the spill, which prevents
it falling to the ground.
815. _Why does a top first reel around upon the spill, then become
upright, and "sleep," and then reel again, and fall?_
[Illustration: Fig. 25.--PEG-TOP "REELING."]
Because, in being thrown from the hand, the top is delivered a little
out of the perpendicular, but the spill _is rounded off at the point_,
and when the top is rotating rapidly, the gravitative force which
attracts the top to the ground continually acting upon it, _draws the
weight of the top on to the extreme centre of the round point_. When
the rotation subsides, and the centrifugal force is weakened, then the
top _is no longer balanced upon the extreme point of the spill_, but
falls upon its sides, until the force of gravitation is exerted _beyond
the line of the spill_, upon the body of the top, and then it falls to
the ground.
[Verse: "Even a child is known by his doings, whether his work be
pure, and whether it be right."--PROVERBS XX.]
816. _Why does a top "sleep?"_
Because at that period of its spinning, which is called "sleeping," the
_centrifugal_ and the _gravitative forces_ acting upon the top, are
_nearly balanced_; and the top, obeying chiefly the _rotatory force_,
appears to be in a state of comparative rest.
817. _Why does the top cease to spin?_
Because _the friction of the air against its sides_, and the _friction
of the spill against the ground_, act in opposition to the _rotatory
force_, which is a temporary impulse applied by external means--the
hand of the person who spins it--and as soon as this _applied force_
is expended, the top yields to the law of gravitation, which is _a
permanent and ever-prevailing force_.
818. _Why does a marble revolve, as it is propelled along the ground?_
Because, in propelling the marble, _the thumb impels the upper surface
forward, and the finger draws the under surface backward_. This
gives a tendency to the upper and lower hemispheres of the marble
_to separate_, which they would do, but for the _cohesion of the
atoms_ of the marble. The upper part of the marble, therefore, rolls
forward, _drawing after it the under part_, which acquires a forward
motion by the force with which it is drawn upward, and in this way the
opposite portions of the marble act upon each other in the successive
revolutions.
When the marble strikes upon the earth, a new influence is exerted
upon it, which is _the friction of the earth_ upon the surface that
comes in contact with it; but the upper part of the marble, being free,
_overcomes the friction acting upon the lower part_, and thus the
marble continues to progress, until _the applied force which projected
it is expended_.
[Verse: "Better is a poor and a wise child, than an old and foolish
king who will no more be admonished."--ECCLESIASTES IV.]
819. _Why does a striped marble appear to have a greater number of
stripes when rolling, than when at rest?_
Because the stripes are presented in _rapid succession_ to the eye; and
as the eye receives _fresh impressions of stripes before the previous
impressions have passed away_, the stripes appear multiplied.
[Illustration: Fig. 26.--MARBLE AT REST.]
[Illustration: Fig. 27.--MARBLE ROLLING.]
820. _Why does a marble rebound when dropped upon the pavement?_
Because the force of its fall to the earth _compresses the atoms_ of
which the marble is composed; and the atoms then exert the force of
_elasticity to restore themselves to their former condition_; and by
the exercise of this force the marble is _repelled_, or _thrown upward
from the pavement_. Although a marble may be made of very hard stone,
yet that stone may be _elastic_, and possess, though in a much less
degree, _the same kind of elasticity which causes the India-rubber ball
to rebound from the earth_.
821. _Why does a marble, assuming it to be impelled with equal force,
roll further on ice than on pavement, and further on pavement than on a
pebble walk?_
Because the _friction_ is greater upon pavement than upon ice, and
greater upon a pebble walk than upon pavement.
822. _How many forces contribute to stay the progress of a rolling
marble?_
The friction of the _air_, the friction of the _earth_, and the
_attraction of gravitation_, which tends to bring all bodies to a state
of rest.
[Verse: "He shall turn the heart of the fathers towards the
children, and the heart of the children to their fathers."--MALACHI IV.]
823. _Why do the stripes upon a marble disappear when it is spun with
great velocity?_
Because, as in the case of the humming-top, the different parts of the
surface are _brought so rapidly in succession to the sight_, that they
_deface or confuse_ the impressions upon the retina.
[Illustration: Fig. 28--MARBLE SPINNING RAPIDLY.]
824. _Why are rings most perceptible at the opposite points, or poles,
of the marble?_
Because the point, or pole, _upon which the marble spins_, and that
which _corresponds to it_, on the upper surface, travel _less rapidly_
than the central portions, which being of a larger circumference, pass
through a greater amount of space, in the same period of time. The
stripes at the _poles_ of the marble, are, therefore visible, while
those at its _equator_ are imperceptible. (_See_ 522.)