5 deg. into these cylinders. Corresponding
mirrors, EE, are put in the base parallel with those in those
cylinders. An opening extends downward from D of each cylinder so
that light entering at one end of the

[Illustration: Details of X-Ray Machine]

cylinder is reflected down at right angles by the first mirror to
the second, from the second to the third, from the third to the
fourth which reflects the light to the eye. Thus the light never
passes through the cylinders and the observer does not see
through, but around any object inserted at X between the
cylinders.

** How to Make a a Non-Polarizing Battery [151]

Bichromate batteries are very expensive to maintain and dry cells
do not furnish enough amperage for some kinds of experimental
work. A cell of a battery that will run 10 hours with an output of
over 1 ampere can be made as follows: Secure a jar about 4 in. in
diameter and 8 in. high and place in the bottom of this jar the
lower half of a tin baking powder can, to which a wire has been
soldered for connections. Place in the can a mixture of 2 oz.
black oxide of copper, 1 oz. black oxide of manganese and some
iron filings.

Purchase a small crowfoot zinc and hang it about 1 in. above the
half can. Prepare a 10 per cent solution of caustic soda and fill
the jar within 1 in. of the top. Place on top the solution a thin
layer of kerosene or paraffin. The cell will only cost about 50
cents to make and 25 cents for each renewal. When renewing, always
remove the oil with a siphon.
--Contributed by Robert Canfield, University Park, Colo.

** A Home-Made Barometer [151]

Take 1/4 oz. of pulverized campor, 62 gr. of pulverized nitrate of
potassium, 31 gr. nitrate of ammonia and dissolve in 2 oz.
alcohol. Put the solution in a long, slender bottle, closed at the
top with a piece of bladder' containing a pinhole to admit air,
says Metal Worker. When rain is coming the solid particles will
tend gradually to mount, little crystals forming in the liquid,
which otherwise remains clear; if high winds are approaching the
liquid will become as if fermenting, while a film of solid
particles forms on the surface; during fair weather the liquid
will remain clear and the solid particles will rest at the bottom.

** Lock Lubricant [151]

A door lock may be lubricated by using some lead scraped from the
lead in a pencil and put in the lock. This may be done by putting
the scrapings on a piece of paper and b