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<h2>CHAPTER XXXIX.</h2> 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? [Pg 188] Because the attraction of cohesion draws the particles of soap together, directly the bubble is set free from the bowl. <hr class="bible-verse" /> "Children's children are the crown of old men; and the glory of children are their fathers."—Proverbs xvii. <hr class="bible-verse" /> <div class="figcenter"> <ANTIMG src="images/i-194.jpg" width-obs="500" height-obs="245" alt="" /> <div class="caption">Fig. 29.—BLOWING SOAP BUBBLES.</div> </div> 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? [Pg 189] 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. <hr class="bible-verse" /> "A wise son heareth his father's instruction."—Proverbs xiii. <hr class="bible-verse" /> 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. <div class="figcenter"> <div class="figsub"> <ANTIMG src="images/i-195a.jpg" width-obs="156" height-obs="214" alt="" /> <div class="caption">Fig. 30.—AIR-BALLOON.</div> </div> <div class="figsub"> <ANTIMG src="images/i-195b.jpg" width-obs="200" height-obs="214" alt="" /> <div class="caption">Fig. 31.—PAPER PARACHUTE.</div> </div></div> 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? [Pg 190] Because the surfaces of the feathers fall upon the air obliquely, or slantingly, and therefore, as the shuttlecock descends, it turns in the air. <hr class="bible-verse" /> "Come ye children, hearken unto me, I will teach you the fear of the Lord."—Psalm xxxv. <hr class="bible-verse" /> <div class="figcenter"> <ANTIMG src="images/i-196.jpg" width-obs="500" height-obs="251" alt="" /> <div class="caption">Fig. 32.—BATTLEDORE AND SHUTTLECOCK.</div> </div> 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: [Pg 191] 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. <hr class="bible-verse" /> "Be ye therefore followers of God, as dear children; and walk in love, as Christ also hath loved us."—Ephesians v. <hr class="bible-verse" /> <div class="figcenter"> <ANTIMG src="images/i-197.jpg" width-obs="500" height-obs="275" alt="" /> <div class="caption">Fig. 33.—DIAGRAM EXPLAINING THE FLIGHT OF A KITE.</div> </div> 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 [Pg 192] surfaces of its wings to strike the air, produces the equivalent effect of a wind from the earth blowing the top upwards. <hr class="bible-verse" /> "Children obey your parents in the Lord: for this is right." <hr class="bible-verse" /> 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. <div class="figcenter"> <div class="figsub"> <ANTIMG src="images/i-198a.jpg" width-obs="200" height-obs="320" alt="" /> <div class="caption">Fig. 34.—FLYING-TOP.</div> </div> <div class="figsub"> <ANTIMG src="images/i-198b.jpg" width-obs="303" height-obs="320" alt="" /> <div class="caption">Fig. 35.—PEA AND PIPE.</div> </div></div> 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. <hr class="chap" /> <div style="break-after:column;"></div>

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