is fitted with a piece of copper wire provided with a small brass spring tip.  These springs lie in the plane of the pendulum, which serves to swing the central tip first against one and then against the other of the side tips, thereby closing the circuit of first one magnet and then the other.  Each magnet attracts the pendulum until its circuit is broken by release of the center tip, and on the return swing of the pendulum the circuit of the other magnet is similarly closed.  Thus the pendulum is kept in motion by the alternate magnetic impulses.  The clock train is taken from a standard clock and the motion of the pendulum is imparted to the escape wheel by means of a pawl, bearing on the latter, which is lifted at each forward stroke of the pendulum by an arm projecting forward from the pivotal end of the pendulum rod.

** Method of Joining Boards [268]

The amateur wood-worker often has trouble in joining two boards together so that they will fit square and tight.  The accompanying sketch shows a simple and effective method of doing this.  Secure a board, A, about 12 in. wide that is perfectly flat.  Fasten another board, B, about 6 in. wide, to the first one with screws or glue.  Now place the board to be joined, C, on the board B, letting it extend over the inside edge about 1 in. and fastening it to the others with clamps at each end.  Lay the plane on its side and plane the edge straight.  Place the second board in the clamps in the same manner as the first, only have the opposite side up.  If the cutting edge of the blade is not vertical, the boards planed in this manner will fit as shown in the upper sketch.  In using this method, first-class joints can be made without much trouble.  —­Contributed by V. Metzech, Chicago.

[Illustration:  Joining Boards]

** Toy Gun for Throwing Cardboard Squares [269]

The parts of the gun are attached to a thin piece of wood 1 in. wide and 5 in. long.  It is best to use a piece of wood cut from the side or cover of a cigar box.  A rectangular hole 3/16 in. wide and 1 in. long is cut in the wood longitudinally along its axis and 1-3/8 in. from one end, as shown at A, Fig. 1.  A small notch is made with the point of a knife blade at B and notches are cut in the end of the wood as shown at C. Rubber bands are fastened in these notches as shown in Fig. 2.  The trigger, whose dimensions are given in Fig. 3, is fastened in the hole A, Fig. 1, by driving a pin through the wood.  The assembled parts are shown in Fig. 4.

Place the cardboard square in the nick B, attach the rubber bands and pull the trigger.  The top rubber band will fly off and drive the cardboard

[Illustration:  Details of Toy Gun]

square 75 ft. or more.  The cardboard should be about 1/2 in. square.  These can be cut from any old pasteboard box.  —­Contributed by Elmer A. Vanderslice, Phoenixville, Pa.

** Photographic Developing Tray [269]