he position of the hinge joint on the arm of
the washing machine, as shown in Fig. 2. The pressure at the
nozzle is about 20 lb. per square inch, and is sufficient to drive
the waterwheel under all ordinary circumstances.
--Contributed by P. J. O'Gara, Auburn, Cal,

** An Optical Illusion [130]

When looking at the accompanying sketch you will say that the
letters are alternately inclined to the right and left. They are
not so and can be proved by measuring the distance of the top and
bottom of any vertical strokes from the edge of the entire block.
They will be found to be exactly the same distance. Or take any of
the horizontal strokes of the four letters and see how far their
extremities are from the top and bottom of the entire block. It
will be found that a line joining the extremities of the strokes
are strictly parallel to the top or bottom and that they are not
on a slant at all. It is the slant of the numerous short lines
that go to make up the letter as a whole that deceives the eye.

[Illustration: Illusion]

** Home-Made Micrometer [130]

It often becomes necessary to find the thickness of material so
thin, or inconvenient to measure, that a rule or other measuring
device will not serve the purpose. A simple, fairly accurate, and
easily made apparatus of the micrometer form may be constructed as
shown by the accompanying sketch. Secure a common iron or brass
bolt about 1/4-in. in diameter and about 2-1/2 in. long, with as
fine a thread as possible, and the thread cut to within a short
distance of the head of the bolt. The head of the bolts should
have a slot cut for the use of a screwdriver. Clamp together two
blocks of wood with square corners which are about 1 in. wide, 3/4
in. thick and 2-1/2 in. long and fasten them together with small
pieces nailed across the ends. The width of the blocks will then
be about 2 in. Bore a 1/4-in. hole through the center of the
blocks in the 2 in. direction. Remove the clamp and set the nut
into one of the blocks, so that the hole will be continuous with
the hole in the wood. Cut out a piece from the block combination,
leaving it shaped like a bench, and glue the bottoms of the legs
to a piece of thin board about 2-1/2 in. square for a support.

Solder one end of a stiff wire that is about 2 in. long to the
head of the bolt at right angles to the shaft, and fix a disc of
heavy pasteboard with a radius equal to the length of the wire,
and with its circumfer