XXXVIII
for ascertaining the height of any place by means of the baro
meter. The height of the atmosphere has been calculated at
from 30-40 miles, but the existence of air has been proved for
a height of 198-212 miles.
§ 86. An instrument for measuring the elastic force of gases
or vapours is called a Manometer (see Q. 353). For the same
temperature the density of a gas is proportional to its pressure.
A bent glass-tube, partly filled with mercury, about 20 feet high
and open at the top, is connected with the vessel in which the
gas is contained or with the boiler of an engine. The elastic
force of the vapour being equal to the atmospheric pressure the
mercury will stand at the same level in the tube and in the bulb,
and the number 1 is marked on the board to which the tube is
fastened; 30 inches above (the force of one atmosphere) we mark
2, etc. etc. Thus for example the mercury forced up to 5 we
see that the vapour in the boiler has an elastic force equal to 5
atmospheres, this means each square inch of the boiler has to
sustain a pressure of 5 x 15 or 75 pounds.
§ 87. A very delicate and portable barometer exists in the
Aneroid barometer constructed by Casella, in which without
mercury the differing pressure of the air is communicated to a
thin corrugated metal on the top of a metal box exhausted of
air. The motions of this metal are transmitted by multiplying
levers to an index, which moves on a scale. This barometer
may also be modified in such a way as to act as a manometer,
and occupies less place than the instrument described above.
§ 88. 2) There are plenty of instances from which the pres
sure of air may be seen: Water rises in pumps 26 feet (the
theoretical limit would be 34′), 189; a bottle full of water invert
ed and placed vertically in water will not be emptied, 190; if a
tumbler full of water be covered with a sheet of paper and in
