Reason Why — Chapter 40 Mouse and Trap Spinner

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.

] [Illustration: Fig. 36. --CARD WITH MOUSE-TRAP.

] [Illustration: Fig. 37. --REVERSE OF CARD WITH MOUSE.

] 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elastic_, the particles thereof seek to restore themselves to their former state, as soon as the resisting power is withdrawn.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.

[Illustration: Fig. 38. --BOW AND ARROW.

] [Illustration: Fig. 39. --JEW'S HARP.

] 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. [Illustration: Fig. 40. --AIR PISTOL, OR "POP-GUN."

] 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.

[Illustration: Fig. 41. --SYRINGE, WITH JET OF WATER.

] 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.

[Illustration: Fig. 42. --"SUCKER."

] [Illustration: Fig. 43. --HOOP.

] 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_.

[Illustration: Fig. 44. --BOYS AND "SEE-SAW."

] 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.

] [Illustration: Fig. 45. --TRAP AND BALL.

] 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_.

[Illustration: Fig. 46. --BAT AND BALL.

] 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_.

[Illustration: Fig. 47. PITH-TUMBLER.

] 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.

[Illustration: Fig. 48. --THAUMATROPE, OR "WONDER-TURNER."

] 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.

] CHAPTER XLI.