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<h2>CHAPTER XXXVIII.</h2> 810. Why does a humming-top make a humming noise? Because the hollow wood of the top vibrates, and the edges of the hole in its sides strike against the air as it spins; the air is thereby set in vibration. 811. Why does a peg-top hum less than a humming-top? Because, being a solid body of wood, and having no hole in its sides, its particles are not so easily thrown into vibration; consequently it does not so readily impart vibrations to the air. 812. Why does a peg-top sometimes hum, and at other times not? Because, if it is spun with great force, and its peg is struck sharply against the pavement, the wood is set in vibration, and the surface of the top, repelling the air by its rapid motion, causes vibratory waves. But if it be spun with insufficient force, the wood is not set in vibration. <div class="figcenter"> <div class="figsub"> <ANTIMG src="images/i-183a.jpg" width-obs="300" height-obs="268" alt="" /> <div class="caption">Fig. 23.—HUMMING-TOP BEFORE SPINNING.</div> </div> <div class="figsub"> <ANTIMG src="images/i-183b.jpg" width-obs="206" height-obs="268" alt="" /> <div class="caption">Fig. 24.—HUMMING-TOP SPINNING.</div> </div></div> 813. Why do we see the figures painted upon the humming-top, before it spins, but not while it is spinning? Because the rapid whirling of the top brings the images of its different parts so quickly in succession upon the retina of the eye, [Pg 184] that they deface each other, and impart an impression of coloured rings, instead of definite objects. <hr class="bible-verse" /> "Train up a child in the way he should go; and when he is old, he will not depart from it."—Proverbs xxii. <hr class="bible-verse" /> 814. Why does a top stand erect when it spins, but fall when it stops? Because the top is under the influence of, and is balanced between opposing forces. The rapid rotation of the top gives to all its particles a tendency to fly from the centre. If the atoms of the wood were not held together by the attraction of cohesion, they would fly away in a circle outward from the top, just as drops of water fly off from a mop, while it is being twirled. If you take a spoonful of sand, salt, or dust, and drop it upon the top, it will be scattered in a circle, just as the atoms of the top would be, if they were free to separate, but not with the same force, because the atoms of the salt, &c., not being in an active state of rotation, would only be influenced by momentary contact with the rotating body. This tendency of the particles of a rotating body to fly outward from the centre, is called the centrifugal force. Centrifugal.—From two Latin words meaning receding from the centre. The other force influencing the top is the attraction of gravitation: the attraction which, were the top not spinning, would draw it towards the earth. The "spill" projecting from the bottom of the top stands in the line in which the top is drawn towards the earth and keeps it from obeying the law of gravitation. Therefore the rotatory motion given to the top, by the rapid unwinding of the string, and the tendency of its atoms to fly outward, balance the top upon the line in which it is drawn to the earth, and which is occupied by the spill, which prevents it falling to the ground. 815. Why does a top first reel around upon the spill, then become upright, and "sleep," and then reel again, and fall? <div class="figcenter"> <ANTIMG src="images/i-190.jpg" width-obs="250" height-obs="243" alt="" /> <div class="caption">Fig. 25.—PEG-TOP "REELING."</div> </div> Because, in being thrown from the hand, the top is delivered a little out of the perpendicular, but the spill is rounded off at the point, and when the top is rotating rapidly, the gravitative force which attracts the top to the ground continually acting upon it, draws the weight [Pg 185] of the top on to the extreme centre of the round point. When the rotation subsides, and the centrifugal force is weakened, then the top is no longer balanced upon the extreme point of the spill, but falls upon its sides, until the force of gravitation is exerted beyond the line of the spill, upon the body of the top, and then it falls to the ground. <hr class="bible-verse" /> "Even a child is known by his doings, whether his work be pure, and whether it be right."—Proverbs xx. <hr class="bible-verse" /> 816. Why does a top "sleep?" Because at that period of its spinning, which is called "sleeping," the centrifugal and the gravitative forces acting upon the top, are nearly balanced; and the top, obeying chiefly the rotatory force, appears to be in a state of comparative rest. 817. Why does the top cease to spin? Because the friction of the air against its sides, and the friction of the spill against the ground, act in opposition to the rotatory force, which is a temporary impulse applied by external means—the hand of the person who spins it—and as soon as this applied force is expended, the top yields to the law of gravitation, which is a permanent and ever-prevailing force. 818. Why does a marble revolve, as it is propelled along the ground? Because, in propelling the marble, the thumb impels the upper surface forward, and the finger draws the under surface backward. This gives a tendency to the upper and lower hemispheres of the marble to separate, which they would do, but for the cohesion of the atoms of the marble. The upper part of the marble, therefore, rolls forward, drawing after it the under part, which acquires a forward motion by the force with which it is drawn upward, and in this way the opposite portions of the marble act upon each other in the successive revolutions. 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. [Pg 186] <hr class="bible-verse" /> "Better is a poor and a wise child, than an old and foolish king who will no more be admonished."—Ecclesiastes iv. <hr class="bible-verse" /> 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. <div class="figcenter"> <div class="figsub"> <ANTIMG src="images/i-192a.jpg" width-obs="184" height-obs="180" alt="" /> <div class="caption">Fig. 26.—MARBLE AT REST.</div> </div> <div class="figsub"> <ANTIMG src="images/i-192b.jpg" width-obs="238" height-obs="180" alt="" /> <div class="caption">Fig. 27.—MARBLE ROLLING.</div> </div></div> 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? [Pg 187] 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. <hr class="bible-verse" /> "He shall turn the heart of the fathers towards the children, and the heart of the children to their fathers."—Malachi iv. <hr class="bible-verse" /> 823. Why do the stripes upon a marble disappear when it is spun with great velocity? <div class="figleft"> <ANTIMG src="images/i-193.jpg" width-obs="250" height-obs="189" alt="" /> <div class="caption">Fig. 28—MARBLE SPINNING RAPIDLY.</div> </div> 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. 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.) <hr class="chap" /> <div style="break-after:column;"></div>

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