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.] [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?_