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_Thunder_, indicated by the _fall_ of the mercury in _sultry
weather_.
_Cold_, indicated by the _rise_ of the mercury in spring, autumn,
and winter.
_Heat_, by the _fall_ of the mercury in summer and autumn.
_Frost_, indicated by the _rise_ of the mercury in winter.
_Thaw_, by the _fall_ of the mercury during a frost.
_Continued bad weather_, when the _fall_ of the mercury has been
_gradual_ through several fine days.
_Continued fine weather_, when the _rise_ of the mercury has been
_gradual_ through several foul days.
_Bad weather of short duration_, when it sets in quickly.
_Fine weather of short duration_, when it sets in quickly.
_Changeable weather_, when an _extreme_ change has _suddenly_ set
in.
_Wind_, indicated by a rapid _rise_ or _fall unattended by a
change of temperature_.
The mercury _rising_, and the air becoming _cooler_, promises
_fine weather_; but the mercury _rising_, and the air becoming
_warmer_, the weather will _be changeable_.
If the top of the column of mercury appears _convex_, or curved
upwards, it is an additional proof that the mercury is _rising_.
Expect _fine_ weather.
If the top of the column is _concave_, or curved downwards, it is
an additional proof that the mercury is _falling_. Expect _bad_
weather.
CHAPTER XXXIII.
709. _What is the thermometer?_
The thermometer is an instrument in which _mercury_ is employed to
indicate _degrees of heat_. Its name is derived from two Greek words,
meaning _heat measurer_.
710. _Why does mercury indicate degrees of heat?_
Because it _expands_ readily with _heat_, and _contracts_ with _cold_;
and as it passes freely through small tubes, it is the most convenient
medium for indicating _changes of temperature_.
711. _Why are there Reaumur's Thermometers and Fahrenheit's
Thermometers?_
Because their inventors, after whom they are named, adopted a different
system of _notation_, or _thermometrical marks_; and as their
thermometers have been adopted by various countries and authors, it is
now difficult to dispense with either of them.
[Verse: "When ye see a cloud rise out of the west straightway ye
say, There cometh a shower; and so it is. And when ye see the south
wind blow, ye say there will be heat; and it cometh to pass."--LUKE
XIII.]
[Illustration: Fig. 22.--THE THERMOMETERS OF REAUMUR AND FAHRENHEIT
COMPARED.]