tly under 15 lbs. per square
inch.

This fact is utilized in the pressure recorder shown in Fig. 167, a
U-shaped glass tube half filled with mercury. A rubber tube is attached to
the bent-over end of one of the legs, so that the effects of blowing or
suction may be communicated to the mercury in that leg. Normally the
mercury stands level in both tubes at what may be called the zero mark. Any
change of level in one leg is accompanied by an equal change in the
opposite direction in the other. Therefore, if by blowing the mercury is
made to rise an inch in the left leg, the pressure exerted is obviously
that required to support a two-inch column of mercury--that is, 1 lb. 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. Both tubes should be
pointed at the bottle end so that they may enter the cork easily. Make the
top of the cork air tight with sealing-wax. The purpose of the bottle is to
catch any mercury that might be sucked out of the tube; one does not wish
mercurial poisoning to result from the experiments. Also it prevents any
saliva entering the mercury tube.

When the latter has been secured to the standard by a couple of slips of
tin nailed to the front, connect it up with the bottle, and fill it up to
the zero mark with mercury poured in through a small paper fu