cury for about
one-half of the stroke. Cover the mercury over with a little
alcohol. A No. 14 gauge iron wire is bent and put into the side of
the bottle with the end extending to the bottom. The other end of
this wire is attached to one binding-post placed at the end of the
bottom block. The other binding-post is connected to a small brass
brush attached to the side of the vertical piece, which is placed
with some pressure on the moving wire. The motor can be run with a
current from a separate course or connected as shown on the same
batteries with the coil. The proper height of the mercury can be
regulated for best results. The motor must run continuous if the
coil is used for writing code signals, wireless, etc.
--Contributed by Haraden Pratt, San Francisco, Cal.

** How to Make a Barometer [188]

Atmospheric pressure is measured by the barometer. The weight of
the air in round numbers is 15 lb. to the square inch and will
support a column of water 1 in. square, 34 ft. high, or a column
of mercury (density 13.6) 1 in. square, 30 in. high. The parts
necessary to make a simple barometer are, a glass tube 1/8 in.
internal diameter and about 34 in. long, a bottle 1 in. inside
diameter and 2 in. high. Seal one end of the tube by holding it in
the flame of a gas burner, which will soon soften the glass so it
can be pinched together with pliers. Put a little paraffin in the
bottle and melt it by holding the bottle over a small flame. When
cool the paraffin should cover the bottom about 1/16 in. thick.
The tube is now to be filled with mercury. This may be
accomplished with a paper funnel, but before attempting to put in
the mercury, place a large dish or tray beneath the tube to catch
any mercury that may accidentally be spilled. Only redistilled
mercury should be used, and the tube should be perfectly clean
before filling. When the tube is filled to

[Illustration: Barometer]

within 1 in. of the open end place the forefinger over the hole
and tilt the tube up and down so all the air will gather at the
finger end. The filling is continued until the tube is full of
mercury. The glass bottle containing the wax covered bottom is now
placed over the end of the tube and pressed firmly to insure an
airtight fit with the tube. The bottle and tube are inverted and
after a few ounces of mercury are put in the bottle the tube may
be raised out of the wax, but be careful not to bring its edge
above the surface of the mercu