ry point of the object, from
light reflected from these points. Certain of these rays strike
the pin hole in the front of the camera, represented here by RRRR.
These rays pass through the pin hole, and as light travels only in
straight lines, reach the plate AA, forming an inverted image of
the object, in this case a candle in a candlestick. Millions of
rays are given off by every point in every object which is lighted
by either direct or reflected light. To all practical purposes
only one of these rays from each point in an object can pass
through a minute opening like a pin hole. This being so, any
screen 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 b