per
sq. inch. This gives a very convenient standard of measurement, as every
inch rise above the zero mark indicates 1 lb. of pressure.
CONSTRUCTION.
The mercury tube should be made first. Take a piece of glass tubing 20
inches long, and bend it at a point 9 inches from one end after heating in
a spirit flame. The legs should be kept as parallel as possible. Lay the
tube, while the heated part is still pliant, on a flat surface, the bend
projecting over the edge, So that the two legs shall be in line. When the
glass has cooled, bend over two inches of the longer leg to an angle of
about 45 degrees.
A standard for the tube is now made out of one-inch wood. Hollow out a bed
in which the tube shall lie and be completely protected. To the right of
the tube the standard is notched to take a small bottle. The notch should
be slightly narrower than the diameter of the bottle, and have its sides
hollowed out to fit.
Halfway up the tube draw a zero mark across the standards, and above this a
scale of inches in fractions on both sides. Each inch represents 1 lb.
pressure.
The cork of the bottle must be pierced with a red-hot wire for two glass
tubes, one of which is bent over for the blowing tube.
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