Reason Why — Chapter 39 Why are Soap Bubbles Round

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.

] [Illustration: Fig. 29. --BLOWING SOAP BUBBLES.

] 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.

[Illustration: Fig. 30. --AIR-BALLOON.

] 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.

[Illustration: Fig. 31. --PAPER PARACHUTE.

] 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.

] [Illustration: Fig. 32. --BATTLEDORE AND SHUTTLECOCK.

] 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.

] [Illustration: Fig. 33. --DIAGRAM EXPLAINING THE FLIGHT OF A KITE.

] 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_.

[Illustration: Fig. 34. --FLYING-TOP.

] [Illustration: Fig. 35. --PEA AND PIPE.

] 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.