CHAPTER XXXIX.
825. _Why are soap-bubbles round?_
Because they are _equally pressed upon all parts of their surface_ by the atmosphere.
826. _Why are bubbles elongated when being blown?_
Because the _unequal pressure of the current of breath_ by which they are being filled, alters the _relative pressure_ upon the outer surfaces.
827. _Why does the bubble close, and become a perfect sphere, when shaken from the pipe?_
Because the _attraction of cohesion_ draws the particles of soap together, directly the bubble is set free from the bowl.
[Verse: "Children's children are the crown of old men; and the glory of children are their fathers."--PROVERBS XVII.]
828. _Why do bubbles, blown in the sunshine, change their colours?_
Because the films of the bubbles constantly change in thickness, through the atoms from the upper part descending towards the bottom, and therefore the varying thickness of film _refracts, in different degrees, the rays of light_.
829. _Why do bubbles burst?_
Because the atoms that compose their films _fall towards the earth by gravitation_; the upper portion of the bubbles then _becomes very thin_, and as the denser air of the atmosphere _presses towards the warm breath within the bubble, it bursts the film_.
_See_ 236, 237, _etc._, 501, _etc._
830. _Why do balloons ascend in air?_
Because the air or gas which they contain is _specifically lighter than the atmosphere_; _the atmosphere, therefore, forces itself underneath the balloon_, by its own tendency towards the earth, and the balloon is thereby raised upwards. _A balloon is but a larger kind of bubble, made of stronger materials._
831. _Why does an air-balloon become inflated when the spirit set upon the sponge is lit?_
Because the _heat_ of the flame, and the _burning of the spirit_, A, create a volume of _rarefied_, or _thin air_, which inflates the balloon, and makes it _specifically lighter_ than the surrounding medium.
[Verse: "A wise son heareth his father's instruction."--PROVERBS XIII.]
832. _Why do balloons sometimes burst when they ascend very high?_
Because, as they get into the _thinner air_, which exists at _high altitudes_, the gas within them expands, and the coating of the balloon is burst asunder.
833. _Why does the gas of balloons expand in thin air?_
Because the air exerts a _less amount of pressure_ upon the air or gas contained in the balloons.
834. _Why do parachutes fall very gradually to the ground?_
Because the _air_, coming in contact with the _under surface_ of the expanded head of the parachute resists its downward progress.
835. _Why does a shuttlecock travel slowly through the air?_
Because the air acts upon the feathers of the shuttlecock, in the same manner as it does upon the parachute--it strikes against their expanded surface, and resists their progress through the air.
836. _Why does the shuttlecock spin in the air?_
Because the surfaces of the feathers fall upon the air _obliquely_, or slantingly, and therefore, as the shuttlecock descends, it turns in the air.
[Verse: "Come ye children, hearken unto me, I will teach you the fear of the Lord."--PSALM XXXV.]
837. _Why do we hear a noise when we strike the shuttlecock with the battledore?_
Because the _percussion_ of the shuttlecock upon the parchment of the battledore causes it to vibrate, and the vibrations are imparted to the air.
838. _Why is the sound a dull and short one?_
Because the vibrations of the parchment are _not very rapid_, therefore there is _little intensity_ in the vibrations of the air.
839. _Why does the exercise, afforded by playing battledore and shuttlecock, make us feel warm?_
Because it makes us breathe _more freely_, and causes the _blood to flow faster_; we, therefore, inhale more _oxygen_, which produces heat by combining with the _carbon_ of our _blood_.
840. _Why does a kite rise in the air?_
A kite rises in the air by the force of the wind, which _strikes obliquely_ upon its _under surface_. The string is attached to the "belly-band" in such a manner that it is nearer the _top_ than the _bottom_ of the band: this causes the bottom of the kite, when its surface is met by the wind, to recede in the direction of the wind: the top is accordingly _thrown forward_, and the kite is made to _lie obliquely_ upon the current of air moving against it. The kite then being _drawn by the string in one direction_, and _pressed by the air in another direction_, moves in a line which _describes a medium between the two forces acting upon it_.
[Verse: "Be ye therefore followers of God, as dear children; and walk in love, as Christ also hath loved us."--EPHESIANS V.]
841. _Why does the kite-string feel hot when running through the hand?_
Because the _rapid friction_ sets free the _latent heat_ of the _string_, attracts the heat of the _hand_ to the spot where the friction occurs, and sets free the latent heat of the _air_, which follows the _string_ through the hand, and is compressed by the friction.
842. _Why does running with the kite cause it to rise higher?_
Because it _increases the force_ with which the wind strikes upon the surface of the kite. If a person were to _run with a kite at the rate of five miles an hour, through a still air_, the effect would be _equal to a wind flying at the rate of five miles an hour_ against a kite held by a _stationary string_.
843. _Why does the flying-top rise in the air?_
Because its wings _meet the air obliquely_, just as the surface of the kite does. And the _twirling of the top_, causing the oblique surfaces of its wings to strike the air, produces _the equivalent effect of a wind from the earth blowing the top upwards_.
[Verse: "Children obey your parents in the Lord: for this is right."]
844. _Why does the flying-top return to the earth when its rotations are expended?_
Because the _reaction_ produced by its wings striking upon the air, is insufficient to counteract the _attraction of gravitation_.
845. _Why does a pea, into which a pin has been stuck, dance in suspension upon a jet of air blown through a pipe?_
Because the jet of air, being _slightly compressed_ under the _convex_ form of the pea, by the weight of the pin, forms a _concave cup of air_, in which the pea rests.
In the case put, it is supposed that the pin is _passed through the pea_ until its head comes in contact with it. The pin is dropped into the hole of the pipe, and the breath is then applied, the pipe being held upright. The pea will rise in the air, and be suspended upon the jet, while the point of the pin will rotate around the stem of the pipe. There are other methods of fixing the pin which alter the result, and require a different explanation to that given above.
LESSON XL.
846. _Why does a mouse, painted upon one side of a card, and a trap upon the other, represent to the eye a mouse in a trap when the card is rapidly twirled upon a string?_
Because the image of the mouse is brought to the retina of the eye before the image of the trap has passed away. The two impressions, therefore, _unite upon the retina_, and produce the image of a mouse in a trap.
[Verse: "Honour thy father and thy mother * * That it may be well with thee, and thou mayest be long on the earth."--EPHESIANS VI.]
847. _Why will a bow stretched out of its natural position, propel an arrow through the air?_
Because its substance, being _highly elastic_, the particles thereof seek to restore themselves to their former state, as soon as the resisting power is withdrawn. The _force_ derived from this elasticity, is communicated to the arrow by the string against which it is placed.
848. _Why is the arrow propelled forward?_
Because the elasticity of the bow, _acting equally upon its two ends_, to which the string is fastened, produce a line of force in a _diagonal direction_. It thus illustrates the law, that _when a body is acted upon by two forces at the same time, whose directions are inclined to each other, it will not follow either of them, but will describe a line between the two_.
849. _What forces tend to arrest the flight of the arrow?_
The _friction of the air_, and the _attraction of gravitation_.
[Verse: "My son, give, I pray thee, glory to the Lord God of Israel, and make confession unto him."--JOSHUA VII.]
850. _Why are feathers usually fastened to the ends of arrows?_
Because the _greater friction_ of air acting upon them, opposes the progress of that part of the arrow in a greater degree than it does the other portion. The effect is, _to keep the point of the arrow forward_, and in a straight line with its opposite extremity. If the arrow were shot the reverse way from the bow, it would _turn round_, in the course of its flight, in consequence of the friction of the air, offering greater resistance to the progress of the feathered end.
851. _Why does a Jew's harp give musical sounds?_
Because the _vibrations of the metal tongue_ are communicated to the ear.
852. _Why will not the Jew's harp produce loud sounds unless it is applied to the mouth?_
Because the vibrations are not very intense, but when it is blown upon by the breath, the air is pressed upon it, and the vibrations are thereby rendered more powerful.
853. _Why does the alteration of the arrangement of the mouth, affect the formation of the sounds?_
Because it sends the air to the tongue of the harp in _a greater or lesser degree of compression_.
[Verse: "Hear, ye children, the instruction of a father, and attend to know understanding."--PROVERBS IV.]
854. _Why does the pressure applied to the handle of an air pistol propel the cork?_
Because, between the cork A and the air-tight piston C, there is a _closed chamber of air_ B. When the handle D, which moves the piston C, is rapidly pushed in, it _compresses the air_ until it is so much condensed, that it forces out the cork A.
855. _Why must the handle be drawn out, before the cork is placed in?_
Because otherwise a partial _vacuum_ would be formed between A and C, and there would not be sufficient air to force out the cork by the return of the piston C D.
856. _Why does water rise in a syringe when the handle is drawn out?_
Because the pressure of the air on the water outside of the syringe, forces it into the space vacated by the drawing up of the handle, and where, otherwise, a _vacuum_ would be formed.
857. _Why does not the water run out when the syringe is raised?_
Because the pressure of the air upon the small orifice resists the weight of the water.
858. _Why does the water leak out, but not run?_
Because water has a tendency always to _move to the lowest point_, but as the air does not enter freely the water cannot escape. It therefore _drops_, as small portions of the air enter.
[Verse: "Remember now thy creator in the days of thy youth."--ECCLESIASTES XI.]
859. _Why cannot the handle be pressed in, if the finger is applied to the orifice?_
Because water is not _compressible_, like air; it must therefore escape before the handle can be pressed in. Air may be forced into a much smaller compass than is natural to it; but it is impossible to _compress water_ in any great degree.
860. _Why does a "sucker" raise a stone?_
Because underneath the sucker _a vacuum_ is formed and the external air, pressing on all sides _against the vacuum_, lifts the stone. The term "sucker" is founded upon the mistaken notion that the leather "sucks," or "draws" the stone. That such is not the case is evident: if, when the stone is suspended, a pin's point be passed under the leather, so as to open a small passage for the air, the stone will _drop instantly_.
861. _Why does a hoop roll, without falling to the ground?_
Because the _centrifugal force_ gives it a motion which is called the _tangent to a circle_--that is, a tendency in all its parts _to fly off in a straight line_. When a piece of clay adhering to the hoop flies off, it leaves the hoop in a line which is straight with the part of the surface from which it was propelled; this line is _the tangent to the circle of the hoop_; and the tendency of all the parts of the hoop to fly off in this manner, counteracts the attraction of the earth, so long as the hoop is kept in motion.
[Verse: "Children obey your parents in all things: for this is well-pleasing unto the Lord."--COLOSSIANS III.]
862. _Why does the hoop, in falling, make several side revolutions?_
Because its onward movement, not being quite expended, influences the _centre of gravity of the hoop_, and changes its line of direction. The hoop is also elastic, and when its sides strike the earth, they spring up again, and continue turning until the opposing forces are overcome by the _attraction of gravitation_.
863. _Why will a little boy balance a large boy on a see-saw?_
Because the "see-saw" may be placed so that its ends are at _unequal distances from the centre_. This gives the little boy the power of _leverage_, by which is meant the increase of power, or weight, by _mechanical means_.
864. _Why does the little boy sink to the ground when the larger boy slightly kicks the earth?_
Because the larger boy, by kicking against the earth, opposes by mechanical force the _attraction of gravitation_ acting upon him, and he becomes _temporarily_ less attracted to the earth than the little boy.
865. _Why can the little boy, if he choose, keep the big boy up, when once he is up?_
Because, as the big boy is then on _an inclined plane_ with the _fulcrum_, or centre upon which the see-saw moves, the arm of _the lever_, upon which the big boy sits, is _relatively shortened_, and he has then _less mechanical power_. Also, a portion of the weight of the larger boy is transmitted along the lever _to the arm upon which the little boy sits_.
[Verse: "Little children, let no man deceive you: he that doeth righteousness is righteous, even as he is righteous."--1 JOHN III.]
866. _Why is the ball propelled upward, in the game of trap and ball, when the trigger is struck?_
Because, when the trigger is struck at A, it is forced downwards, turning upon the fulcrum B, the opposite end, forming the spoon, is thereby forced upwards, describing a small _arc_, or curved line; but directly the ball is set free from the spoon, it rises in a _right line_ with the _direction it was taking, at the moment it was set free_.
867. _What principles of natural philosophy are illustrated by the results of bat and ball?_
_Percussion_, when the bat strikes the ball; _rotatory motion_, when the ball is sent whirling away; _momentum_, which it acquires by velocity; _elasticity_, when it rebounds from an object against which it strikes; _reflected motion_, when it is turned by a body upon which it impinges; _friction_, as it rolls along the ground; the _communication of force_, when it sets another body in motion against which it strikes; _gravitation_, when it falls to the earth; and _inertia_, when it lies in a state of rest.
[Verse: "A wise son makes a glad father: but a foolish son is the heaviness of his mother."--PROVERBS X.]
868. _Why do pith-tumblers always pitch upon one end?_
Because the _lead_ B is _specifically heavier_ than the _pith_ to which it is attached; it therefore always falls undermost; and as the lead is rounded off, just like the spill of a top, after the head has oscillated a little, and expended the force of the momentum of its fall, it will settle upon its _centre of gravity_, or the point through which it is _attracted to the earth_.
869. _Why do the figures upon the "Thaumatrope" appear to dance, when they are made to revolve before a mirror?_
Because the eye, in looking through the holes in the card, towards the reflections in the mirror, receives a _rapid succession of impressions_. As the figures upon the card are represented in a graduated series of positions--the _first_ one standing upright, the _second_ with his knees a little bent, the _third_ a little more bent, as in the act of springing, and so on, the _figure_ being in each case _the same_, but the position _slightly altered_, imparts an impression to the mind, through the eye, that _one figure_ is passing through a _series of motions_.
_Thaumatrope._--From two Greek words, meaning _wonder_ and _to turn_.
We have said enough, we hope, to show that even the play-hours of children may be made instructive to them; and that the simplest toys may be used to illustrate some of the grandest laws of nature. Nor may this kind of instruction be confined to children alone. Grown-up people, whether participators in the sports of youth, or simple observers of their games, may gain instruction for themselves, and be the better teachers of their children, by taking an interest in their enjoyments, and giving to their minds, through the attractiveness of pastime, a taste for observing and estimating the varied phenomena which present themselves.
[Verse: "Jesus said, Suffer little children, and forbid them not, to come unto me; for of such is the kingdom of heaven."--MATTHEW XIX.]
Moreover, we think that parental government acquires a greater power when it leans towards the natural desires of childhood, and wins those desires into a proper direction. Love existing between parent and child is the best tie to home, and the strongest incentive to duty. There is also something in the gentleness of childish nature which may influence for good the sterner mould of man, too often warped and clouded by the cares of life.
In Kay's "Life of Sir John Malcolm," we find an admirable and apt passage. Sir John says:--"I have been employed these last few hours with John Elliot, and other boys, in trying how long we could keep up two cricket-balls. Lord Minto caught us. He says he must send me on a commission to some very young monarch, for that I shall never have the gravity of an ambassador for a prince turned of twelve. He, however, added the well-known and admirable story of Henry IV. of France, who, when caught on all fours carrying one of his children, by the Spanish envoy, looked up and said, 'Is your excellency married?' 'I am, and have a family,' was the reply. 'Well, then,' said the monarch, 'I am satisfied, and shall take another turn round the room,' and off he galloped, with his son on his back flogging and spurring him. I have sometimes thought of breaking myself of what are termed boyish habits; but reflection has satisfied me that it would be very foolish, and that I should esteem it a blessing that I can find amusement in everything, from tossing a cricket-ball, to negotiating a treaty with the Emperor of China. Men who will give themselves entirely to business, and despise (which is the term) trifles, are very able, in their general conception of the great outlines of a plan, but they feel a want of knowledge, which is only to be gained by mixing with all classes in the world, when they come to those lesser points upon which its successful execution may depend."
[Verse: "Whether therefore ye eat, or drink, or whatsoever ye do, do all to the glory of God."--CORINTH. X.]