[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 blowing them into the lock through the keyhole.

** Rust Proofing Bolts [151]

Where bolts are subject to rust, the threads should be painted with pure white lead; then they will not rust fast.

** Painting Yellow Pine [151]

When painting yellow pine exposed to the weather add a little pine tar with the priming coat.

** Revolving a Wheel with Boat Sails [152]

A novel windmill or revolving wheel can be made by placing a light wheel so it will turn freely on the end

[Illustration:  An Unusual Type of Windmill]

of a post, and placing four small sailing boats at equal points on the rim of the wheel.  It makes no difference which way the wind blows, the wheel will revolve in one direction.  In Fig. 1 the direction of the wind is shown by the arrows, and how the sails catch the wind and cause the wheel to revolve.  Figure 2 shows how the wheel will appear when complete.  This device makes an attractive advertising sign.