if it is
passed over D the circuit will be opened. Pulley D is fastened to
a piece of spring steel, E, which in operation is bent, as shown
by the dotted lines, thus causing the switch to snap open quickly
and prevent forming an arc.
--Contributed by Douglas Royer, Roanoke, Va.

[Illustration: Will Open or Close Circuit as Desired]

** How to Make a Rotary Pump [81]

[Illustration: Details of Rotary Pump]

A simple rotary pump is constructed on the principle of creating a
vacuum in a rubber tube and so causing water to rise to fill the
vacuum. Figs. 3, 4 and 5 show all the parts needed, excepting the
crank and tubing. The dimensions and description given are for a
minimum pump, but a larger one could be built in proportion.

Through the center of a block of wood 4 in. square and 7/8 in.
thick (A, Figs. 1, 2 and 3) saw a circular opening 2-7/8 in. in
diameter. On each side of this block cut a larger circle 3-1/4 in.
in diameter, having the same center as the first circle (Fig. 3).
Cut the last circles only 1/4 in. deep, leaving the first circle
in the form of a ridge or track 3/8 in. wide, against which the
rubber tubing, E, is compressed by wheels. Bore two 1/4 in. holes
(HH, Fig. 1) from the outside of the block to the edge of the
inner circle. Put the rubber tube, E, through one of these holes,
pass it around the track and out through the other hole. Notice
the break (S) in the track; this is necessary in order to place in
position the piece holding the wheels.

Fig. 4 shows the wheel-holder, B. Make it of hard wood 3-1/8 in.
long, 1 in. wide and a little less than 7/8 in. thick, so that it
will run freely between the sides (Fig. 5) when they are placed.
Cut two grooves, one in each end, 1 in. deep and 1/2 in. wide. In
these grooves place wheels, CC, to turn on pins of stout wire.
These wheels should be 3/4 in. in diameter. When placed in the
holder their centers must be exactly 2 in. apart, or so arranged
that the distance between the edge of the wheels and the track (K,
Fig. 1) is equal to the thickness of the tubing when pressed flat.
If the wheels fit too tightly, they will bind; it too loose, they
will let the air through. Bore a hole through the middle of the
wheel-holder and insert the crankpin, D, which should be about 1/2
in. in diameter. The crankpin should fit tightly; if necessary
drive a brad through to keep it from slipping.

In the sides (Fig. 5) bore a hole in the center of the crankpin