When the marble strikes upon the earth, a new influence is exerted upon it, which is _the friction of the earth_ upon the surface that comes in contact with it; but the upper part of the marble, being free, _overcomes the friction acting upon the lower part_, and thus the marble continues to progress, until _the applied force which projected it is expended_. [Verse: "Better is a poor and a wise child, than an old and foolish king who will no more be admonished."--ECCLESIASTES IV.] 819. _Why does a striped marble appear to have a greater number of stripes when rolling, than when at rest?_ Because the stripes are presented in _rapid succession_ to the eye; and as the eye receives _fresh impressions of stripes before the previous impressions have passed away_, the stripes appear multiplied. [Illustration: Fig. 26.--MARBLE AT REST.] [Illustration: Fig. 27.--MARBLE ROLLING.] 820. _Why does a marble rebound when dropped upon the pavement?_ Because the force of its fall to the earth _compresses the atoms_ of which the marble is composed; and the atoms then exert the force of _elasticity to restore themselves to their former condition_; and by the exercise of this force the marble is _repelled_, or _thrown upward from the pavement_. Although a marble may be made of very hard stone, yet that stone may be _elastic_, and possess, though in a much less degree, _the same kind of elasticity which causes the India-rubber ball to rebound from the earth_. 821. _Why does a marble, assuming it to be impelled with equal force, roll further on ice than on pavement, and further on pavement than on a pebble walk?_ Because the _friction_ is greater upon pavement than upon ice, and greater upon a pebble walk than upon pavement. 822. _How many forces contribute to stay the progress of a rolling marble?_ The friction of the _air_, the friction of the _earth_, and the _attraction of gravitation_, which tends to bring all bodies to a state of rest. [Verse: "He shall turn the heart of the fathers towards the children, and the heart of the children to their fathers."--MALACHI IV.] 823. _Why do the stripes upon a marble disappear when it is spun with great velocity?_ Because, as in the case of the humming-top, the different parts of the surface are _brought so rapidly in succession to the sight_, that they _deface or confuse_ the impressions upon the retina. [Illustration: Fig. 28--MARBLE SPINNING RAPIDLY.] 824. _Why are rings most perceptible at the opposite points, or poles, of the marble?_ Because the point, or pole, _upon which the marble spins_, and that which _corresponds to it_, on the upper surface, travel _less rapidly_ than the central portions, which being of a larger circumference, pass through a greater amount of space, in the same period of time. The stripes at the _poles_ of the marble, are, therefore visible, while those at its _equator_ are imperceptible. (_See_ 522.) CHAPTER XXXIX. 825. _Why are soap-bubbles round?_