THE BEASOX WHY. 183 



" The father of the righteous shall greatly rejoice ; and he that begetteth a wise 

 child shall have joy of him." PEOVEEBS xxm. 



CHAPTER XXXVIIL 



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. 



Fig. 23. HTTMMING-TOP BEFOBE Fig. ^.-HUMMING-TCP 



SPINNING. 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 quiokly in succession upon the retina of the eye, 



