who understand Nature's works, are likely to receive her best protection. [Verse: "The father of the righteous shall greatly rejoice; and he that begetteth a wise child shall have joy of him."--PROVERBS XXIII.] CHAPTER XXXVIII. 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_. [Illustration: Fig. 23.--HUMMING-TOP BEFORE SPINNING.] [Illustration: Fig. 24.--HUMMING-TOP SPINNING.] 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_, that they _deface each other_, and _impart an impression of coloured rings, instead of definite objects_. [Verse: "Train up a child in the way he should go; and when he is old, he will not depart from it."--PROVERBS XXII.] 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.