Make two wheels out of tin.  They may be of any size, but wheel A must be larger than wheel B. On wheel A fasten two pieces of wood, C, to cross in the center, and place a bell on the four ends, as shown.  The smaller wheel, B, must be separated from the other with a round piece of wood or an old spool.  Tie four buttons with split rings to the smaller wheel, B. The blades on the wheels should be bent opposite on one wheel from the others so as to make the wheels turn in different directions.  When turning, the buttons will strike the bells and make them ring constantly.

[Illustration:  Musical Windmill]

** Optical Illusions [74]

By giving the page a revolving or rinsing motion the three circular figures printed on the next page appear to rotate.  The best effect will be produced by laying the book down flat on the desk or table and revolving, first

[Illustration:  Move These Figures Rapidly with a Rinsing Motion]

in one direction and then in the opposite direction, in such a way that any given point on the page will describe a circle of about 1/2 in. diameter.  Fig. 1 then appears to rotate in the same direction as the revolution; Fig. 2 appears to revolve in the opposite direction, and Fig. 3 appears to revolve sometimes in the same direction and at other times in the opposite direction.

A curious effect can be produced with Fig. 1 by covering up Figs. 2 and 3 with a piece of plain paper and laying a coin or other small object on the paper.  If the vision is then concentrated on the coin or other object while same is being revolved, Fig. 1 will be seen to rotate.

** Barrel-Stave Hammock [75]

A hammock made of barrel staves is more comfortable than one would think, considering the nature of the material employed in making it.  Good smooth staves should be selected for this purpose, and if one cares to go to little trouble a thorough sandpapering will make a great improvement.  Cut half circles out of each stave, as shown at AA, and pass ropes around

[Illustration:  Cheap and Comfortable]

the ends as shown at B. When finished the weight will then be supported by four ropes at each end, which allows the use of small sized ropes, such as clothes lines.  A hammock of this kind may be left out in the rain without injury.  —­Contributed by H.G.M., St. Louis, Mo.

** A Singing Telephone [75]

Those who have not already tried the experiment may be interested to know that a telephone may be made to sing by holding the receiver about 1/16 in. from the transmitter, as shown in the illustration.  The experiment will

[Illustration:  To Make a Telephone Sing]

work well on most telephones, but not on all.

When the receiver is placed in the position shown it acts like an ordinary buzzer, and the function of the transmitter will then be that of an interrupter.  The slightest movement of the transmitter diaphragm will cause an increased movement of the receiver diaphragm.  This in turn will act on the transmitter, thus setting up sympathetic vibrations between the two, which accounts for the sound.