XXXVII
atmosphere. 187. Use of the barometer see: 188. In conse
quence of this coincidence between the state of the weather and
the height of mercury in the barometer, the following indications
have been marked om a barometer:
31 inches = Very dry.
30 ⅔ " = Settled weather.
30 ⅓ " = Fine weather.
30 " = Variable.
29 ⅔ " = Rain or wind.
29 ⅓ " = Much rain.
29 " = Storm.
As a change of weather takes place not only in consequence
of a decrease or increase of the air's weight, the instrument does
not always prove reliable. There are different kinds of baro
meters: Our ordinary barometers are either cistern or syphon
barometers; in the former the tube is immersed in a spherical
well filled with mercury, in the latter the tube is only bent and
the shorter branch serves as a cistern.— Mercury is used for
barometers, because it stands at the least height and does not
moisten the glass. Why is it necessary that the upper part
of a barometer contains no air? How do we know that it is
so? Is it necessary that the tube should have the same width?
Why is this desirable for the syphon barometer? If the baro
meter is removed from its vertical position the column of mer
cury will be longer, why?
§ 85. The barometer may also be used for measuring the
heights of places above the sea-level, as we see that a diminu
tion of 1 inch in the height of the barometer corresponds to an
ascent of about 875 feet. But as the density of air decreases
as we ascend, the fall of mercury in the barometer does not go
on in the same proportion, and complete tables are necessary
