This flange may obviously be made sufficiently stout
to give the requisite additional weighting. As the flange is constantly
immersed, its weight must not be added to that of the sides in computing
the value of _w_ for making the correction of pressure in respect of
the immersion of the bell. Its effective weight in giving pressure to the
contained gas is its actual weight less its actual weight divided by its
specific gravity (say 7.2 for cast iron, 7.78 for wrought iron or mild
steel, or 11.4 for lead). Thus if _x_ lb. of steel is added to the
rim its weight in computing the value of W in the formula _p_ =
35.333W / _d_^2 should be taken as x - x / 7.78. If the actual
weight is 7.78 lb., the weight taken for computing W is 7.78 - 1 = 6.78
lb.
THE PRESSURE GAUGE.--The measurement of gas pressure is effected by means
of a simple instrument known as a pressure gauge. It comprises a glass U-
tube filled to about half its height with water. The vacant upper half of
one limb is put in communication with the gas-supply of which the
pressure is to be determined, while the other limb remains open to the
atmosphere. The difference then observed, when the U-tube is held
vertical, between the levels of the water in the two limbs of the tube
indicates the difference between the pressure of the gas-supply and the
atmospheric pressure.
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