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10 HOW TO MAKE MAGIC TOYS.
nearly filled with quicksilver, the aperture through which
the metal is passed in is closed, the arms and legs are fast-
ened to the pinions, ‘a hollow head of papier mache is fixed
on the body, and the mannikin is clothed in any kind of
fancy costume. :
This dene, place the Tumbler on his legs on the top step.
The quicksilver having descended into the receptacle, and
being placed on the left of the axis of rotation of the legs
will tend to place itself in the vertical plane which passes
through this axis,
There will now be a motion from left to right in the bot-
tom of the mannikin and consequently from right to left
in the top. The Tumbler will then turn over backwards,
but its arms remain vertical, and when they rest on the
step, as they are shorter than the legs, the quicksilver
flows from one receptacle into the other. The motions are
immediately repeated, and one leg oscillates from the left
to the right and produces a complete revolution, at the
conclusion of which the Tumbler rests on the second step,
in exactly-the same position he was in on the first.
For this toy to work well several conditions must be ob-
served. In the first place, the weight of the lower portion
of the body must be inconsiderable compared with the
weight of the mercury, without this condition being ful-
filled the mercury would not have sufficient power, in the
second position, to overcome the inertia of the mass it
must lift up. Then as a certain difference of length must
exist between the arms and legs, the steps must also be
regulated to a certain minimum height, so that the chan-
nels through which the quicksilver flows from one recep-
tacle to the other be sufficiently inclined. If this height
were exactly equal to the difference in length between the
arms and legs, the channel through which the mereury
flows would be horizontal when the Tumbler attained his
third position, For the Tumbler to take in this position,
an inclination equal to that which it takes in the second,
the height of the steps must be just double the difference ~
of length between the arms and legs.
There are also little details of construction which must
-be attended to. In the first place the legs must encounter
a check which will prevent them turning any further when
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