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.—HUMMING-TOP BEFORE SPINNING.
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.