the wooden axle and is wrapped one or two
turns around it, so that when

[Illustration: The Hygrometer]

the thread is pulled the pointer will move on the scale. It will
be noticed that the thread B is not perfectly straight, but bends
toward D. For this reason a very small shrinkage of B, such as
occurs when the atmosphere is dry, will cause an increased
movement of C, which will be further increased in the movement of
the pointer. An instrument of this kind is very interesting and
costs nothing to make.
--Contributed by Reader, Denver.

** The Protection of a Spring Lock [72]

After shutting the front door and hearing the spring lock snap
into its socket, most people go off with a childlike faith in the
safety of their goods and chattels. But the cold fact is that
there is scarcely any locking device which affords less protection
than the ordinary spring lock. It is the simplest thing in the
world for a sneak thief to slip a thin knife between the
door-casing and the strip, push back the bolt, and walk in.

Fortunately, it is equally easy to block that trick. Take a narrow
piece of tin 3 or 4 in. long, bend it at right angles throughout
its length, and tack it firmly in the angle between the casing and
strip, so as to make it impossible to reach the bolt without
tearing off the strip.

Another way is to drive nails through the strip at intervals of
half an inch, enough to protect the bolt from being meddled with.

** A Controller and Reverse for a Battery Motor [72]

Secure a cigar or starch box and use to make the base, B. Two
wood-base switches, S S, are cut off a little past the center and
fastened to the base with a piece of wood between them. The upper
switch, S, is connected to different equal points on a coil of
wire, W, while the lower switch, S, is connected each point to a
battery, as shown. The reverse switch, R, is made from two brass
or copper strips fastened at the top to the base with screws and
joined together by a piece of hard rubber or wood with a small
handle attached. Connect wires A to the armature and wires F to
the field of the motor. By this arrangement one, two or three and
so on up until all the battery cells are used and different points
of resistance secured on the coil of wire. The reverse lever when
moved from right to left, or left to right, changes the direction
of the armature in the motor from one way to the other.
--Contributed by J. Fremont Hilscher, Jr., West S