JAMES WATT AND HIS INVENTIONS. 117
plicable at high pressure. The scale, A, is marked with
numbers indicating the pressure, which numbers are indi-
cated by the head of a rod floating up with the mercury.
A similar gange was used to determine the degree of
perfection of vacuum attained in the condenser, the mer-
cury falling in the outer leg as the vacuum became more
complete. A perfect vacuum would cause a depression of
level in that leg to 30 inches below the level of the mercury
in the leg connected with the condenser. In a more usual
form, it consisted of a simple glass tube having its lower
end immersed in a cistern of mercury, as in the ordinary
barometer, the top of the tube being connected with a pipe
leading to the condenser. With a perfect vacuum in the
condenser, the mercury would rise in the tube very nearly
30 inches. Ordinarily, the vacuum is not nearly perfect,
and, a back pressure remaining in the condenser of one or
two pounds per square inch, the atmospheric pressure re-
maining unbalanced is only sufficient to raise the mercury
26 or 28 inches above the level of the liquid metal in the
cistern.
FIG. 30.
Mercury Steam-Gange. Glass Water-Gange.
To determine the height of water in his boiler, Watt added to the gauge-cocks already long in use the "glass water-gauge," which is still seen in nearly every well-ar-
