[Illustration: FIG. 1.]
[Illustration: FIG. 2.]
Fig. 1 is an illustration of centrifugal force, or the tendency of a
body revolving rapidly around a fixed centre to fly off from that
centre. A tumbler is placed upon a round piece of card-board, to which
strings are attached so that they hold the glass firmly in place. Some
water is poured into the glass, and it can then be swung round the head
without the water being spilled, even when the glass is upside down. For
the experiment shown in Fig. 2 a wine-glass, a piece of cork, a plate,
and some water will be needed. Pour the water on the plate, light a
piece of paper resting on the cork, and cover the flame with the glass
turned upside down. What follows? The water rises in the glass. The
reason is that the burning of the paper having consumed a part of the
oxygen in the air, its volume is diminished, and the pressure of the
outside atmosphere forces the water into the glass.
[Illustration: BEFORE DAYLIGHT--CHRISTMAS MORNING.
"Merry Christmas, Grandpa! What you going to give us?]