[Illustration:  Toy Cannon]

noise.  Seven or eight inches is about the right length for a 1-in. bore.  Screw the plug and pipe up tightly and then drill a 1/16-in. fuse hole at D.

If desired the cannon may be mounted on a block of wood, F, by means of a U-bolt or large staple, E.  —­Contributed by Carson Birkhead, Moorhead, Miss.

** Controller for a Small Motor [42]

An easy way of making a controlling and reversing device for small motors is as follows: 

Cut a piece of wood (A) about 6 in. by 4-1/2 in., and 1/4 in. thick, and another piece (B) 6 in. by 1 in., and 1/4 in. thick.  Drive a nail through this near the center for a pivot (C).  To the under side of one end nail a copper brush (D) to extend out about an inch.  On the upper side, at the same end, nail another brush (E) so that it projects at both sides and is bent down to the level of the end brush.  Then on the board put

[Illustration:  Reverse for Motor]

a semi-circle of brass-headed tacks as shown at F, leaving a small space at the middle and placing five tacks on either side, so that the end brush will come in contact with each one.  Connect these tacks on the under side of the board with coils of German-silver wire, using about 8 in. of wire to each coil.  Fix these by soldering or bending over the ends of the tacks.  Then nail two strips of copper (G) in such position that the side brush will remain on the one as long as the end brush remains on the tacks on that side.

Put sides about 1-1/2 in. high around this apparatus, raising the board a little from the bottom to allow room for the coil.  A lid may be added if desired.  Connect up as shown.  —­Contributed by Chas. H. Boyd, Philadelphia.

** How to Make a Simple Water Rheostat [43]

[Illustration:  Wiring Plan for Water Rheostat]

The materials necessary are:  One 5-point wood-base switch, 4 jars, some sheet copper or brass for plates, about 5 ft. of rubber-covered wire, and some No. 18 gauge wire for the wiring.

The size of the jars depends on the voltage.  If you are going to use a current of low tension, as from batteries, the jars need not be very large, but if you intend to use the electric light current of 110 voltage it will be necessary to use large jars or wooden boxes made watertight, which will hold about 6 or 7 gal.  Each jar to be filled with 20 parts water to 1 part sulphuric acid.  Jars are set in a row in some convenient place out of the way.

Next cut out eight copper or brass disks, two for each jar.  Their size also depends on the voltage.  The disks that are placed in the lower part of the jars are connected with a rubber covered wire extending a little above the top of the jar.

To wire the apparatus, refer to the sketch and you will see that jar No. 1 is connected to point No. 1 on switch; No. 2, on No. 2, and so on until all is complete and we have one remaining point on switch.  Above the jars place a wire to suspend the other or top disks in the solution.  This wire is also connected to one terminal on the motor and to remaining point on switch.  The arm of the switch is connected to one terminal of battery, or source of current, and the other terminal connected direct to remaining terminal of motor.