eighth of an inch away draw dotted parallel lines, all on the same side of
their fellow lines in order of rotation. Cut out along the large circle,
and then with a. sharp knife follow the lines shown double in Fig. 145.
This gives eight little vanes, each of which must be bent upwards to
approximately the same angle round a flat ruler held with an edge on the
dotted line. 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. 147.--Apparatus for illustrating an apparent
scientific paradox.]

Press the coin against the reel and blow hard through the tube. One would
expect the coin to fall; but, on the contrary, the harder you blow the
tighter will it stick, even if the reel be pointed downwards. Only when you
stop blowing will it fall to the floor.

This is a very interesting experiment, and will mystify onlookers who do
not understand the reason for the apparent paradox, which is this. The air
blown through the reel strikes a very limited part of the nearer side of
the halfpenny. In order to escape, it has to make a right-angle turn and
pass between coin and reel, and, while travelling in this direction, loses
most of its repulsive force. The result is that the total pressure on the
underside of the coin, plus the effect of gravity, is exactly balanced by
the atmospheric pressure on the outside, and the coin remains at that
distance from the reel which gives equilibrium of forces. When one stops
blow