This instrument illustrates the law, that the axis of rotation is
preserved, in any fixed direction, immovable, while the particles
surrounding it are in rapid rotary motion. Hence the humming and the
peg-top stand erect, the axis of rotation—the spike or the peg—being
kept in a vertical position while in motion; it falls as soon as this
motion sinks below a certain rate. This power may be illustrated by
placing a disc of wood, or of metal, upon one end of a weighing beam,
from which one of the scales has been removed. The disc being equipoised
by weights in the opposite scale, place the beam at an angle of 45—the
disc being the lowest end—then, by striking the disc, get it into rapid
rotation. It will be found that, while spinning, the beam is preserved
rigidly in its position; but, as the disc comes to rest, the beam is
restored to its horizontal position. Several small weights may be placed
in the scale, while the disc is rotating, without disturbing the
position of the beam.
[Illustration]