which interrupts these selected rays of light will show upon it a picture of the object, only inverted.  If that screen happens to be a photographically sensitive plate, which is protected from all other light by being in a dark box, upon it will be imprinted a photographic image which can be made visible by the application of certain chemicals, when it becomes a negative, from which may be printed positives.  This camera is not a theoretical possibility, but an actual fact.  I have made and used one successfully, as a demonstration of pin-hole photography.

** Use for an Old Clock [367]

Remove the hair spring of the clock, and fasten a spring to one end of the pawl and a small wire to the other end.  Make a slit in the case of the clock opposite the pawl.  Fasten the spring on the outside in any convenient way and pass the wire through the slit to an eccentric or other oscillating body.  To make the dial, paste a piece of paper over the old dial, pull the wire back and forth one hundred times, and make a mark where the minute hand stops.  Using this for a unit divide up the whole dial.  The hour hand has an inner circle of its own.  Put the alarm hand at a little before twelve and wind the alarm.  When the alarm is

[Illustration:  Revolution Recorder]

unwound the hour hand starts on a new trip.  The clock I used was put on an amateur windmill and when the hour hand went around once 86,400 revolutions or jerks on the wire were made, while the minute hand recorded one-twelfth of this number, or 7,200.  —­Contributed by Richard H. Ranger, Indianapolis, Ind.

** Renewing Dry Batteries [367]

Dry batteries, if not too far gone, can be renewed by simply boring a small hole through the composition on top of each carbon and pouring some strong salt water or sal ammoniac solution into the holes.  This kink is sent us by a reader who says that the process will make the battery nearly as good as new if it is not too far gone beforehand.

** Saving a Brush [367]

If a round brush spreads too much, slip a rubber band over the upper part of the bristles.

** How to Make a Simple Burglar Alarm [368]

Take a piece of any wood about 6 by 8 in. for the base.  This may be finished in any way desired.  For the contact points use brass or any sheet metal

[Illustration:  Simple Burglar Alarm]

which will be satisfactory.  Take a piece about 2-1/2 or 3 in. in length and bend the ends up about 1/2 in. in a vertical position as shown.  Fasten this to the top of the board using screws or nails.  Under this strip of metal fasten a copper wire which can be connected to a binding-post on the board if desired.  Take another piece of metal about 4-1/2 in. in length and make a lever of it in the shape shown in the diagram.  Fasten this so that one end of it will swing freely, but not loosely between the ends of the other piece marked C-C.  Near the end fasten a spiral spring, S, which can be obtained almost anywhere.  Fasten the end of this to the screw marked X. Also fasten to this screw a copper wire leading to the binding-post.  In the lower end of the lever make a small hole to fasten a string through.