ers (in _outer_ shape and size like aneroids) have not
yet been tested adequately in very moist, hot, or cold air for a
sufficient time. They, as well as sympiesometers, are likewise dependent
or secondary instruments, and liable to deterioration. For limited
employment, when sufficiently compared, they may be very useful,
especially in a few cases of electrical changes not foretold or shown by
mercury.

The SYMPIESOMETER is considered to be more sensitive than the marine
barometer, falling sooner, and rising earlier: but this is partly in
consequence of the marine barometer tube being contracted, to prevent
oscillation or "pumping." In the sympiesometer a gas is used, which
presses on the confined surface of the liquid with an uniform pressure
at an equal state of temperature. The liquid is raised or depressed by
an increase or diminution in the density of the atmosphere, and change
of temperature is allowed for, by the sliding scale of the instrument
being always set to agree with the height of the mercury in the attached
thermometer, bringing the _pointer_ on the sliding scale of the
sympiesometer to the same degree on the inverted scale (over which it
slides) as is indicated by the thermometer. The height of the fluid, as
then shown by the sliding scale, indicates the pressure of the
atmosphere.

As the instrument is delicate, great care should be taken, in carrying
or handling, to keep the top always upwards, and to exclude casual rays
of the sun, or a fire, or lamp.

Oil sympiesometers seem to be affected more than mercurial, or others,
and much more than the barometer, by lightning or electricity. That
they, and the hermetically sealed "STORM GLASSES," are influenced by
causes besides pressure and temperature, appears now to be certain.

* * *

The daily movement of the barometer may be noted (in a form or table of
double entry) at the time of each observation, by a dot at the place
corresponding to its altitude, and the time of observing; which dot
should be connected with the previous one by a line. The resulting free
curve (or zig-zag) will show at a glance what have been the movements
during the days immediately previous, by which, and not merely by the
last observation, a judgment may be formed of the weather to be
expected.

Such a diagram may be filled up by _uncorrected_ observations, its
object being to serve as a weather guide for immediate use, rather than
for future investig