Next make a dent with a lead pencil at the exact centre on the
vane side, and revolve the pencil until the dent is well polished.
[Illustration: FIG. 146.]
Hold a pin, point upwards, in the right hand, and with the left centre the
mill, vanes pointing downwards, on the pin (Fig. 146). The mill will
immediately commence to revolve at a steady pace, and will continue to do
so indefinitely; though, if the head of the pin be stuck in, say, a piece
of bread, no motion will occur. The secret is that the heat of the hand
causes a very slight upward current of warmed air, which is sufficient to
make the very delicately poised windmill revolve.
A Pneumatic Puzzle.--For the very simple apparatus illustrated by Fig.
147 one needs only half a cotton reel, three pins, and a piece of glass or
metal tubing which fits the hole in the reel. Adjust a halfpenny centrally
over the hole and stick the pins into the reel at three equidistant points,
so that they do not quite touch the coin, and with their ends sloping
slightly outwards to allow the halfpenny to fall away.
[Illustration: FIG.
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