1.
Then slightly taper the end marked B until it is nicely rounded as
shown in Fig. 2. Next make an arm of a two-arm windmill such as
boys make. Make a hole through the center or this one arm. Enlarge
the hole slightly, enough to allow a common pin to hold the arm to
the end B and not interfere with the revolving arm. Two or three
of these arms may have to be made before one is secured that is of
the exact proportions to catch the vibrations right.
To operate the trick, grip the stick firmly in one hand, and with
the forward and backward motion of the other allow the first
finger to slide along the top edge, the second finger along the
side, and the thumb nail will then vibrate along the notches, thus
making the arm revolve in one direction. To make the arm revolve
in the opposite direction--keep the hand moving all the time, so
the observer will not detect the change which the hand makes
--allow the first finger to slide along the top, as in the other
movement, the thumb and second finger changing places: e. g., In
the first movement you scratch the notches with the thumb nail
while the hand is going from the body, and in the second movement
you scratch the notches with the nail of the second finger when
the hand is coming toward the body, thus producing two different
vibrations.
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