meaning understood before this is likely to
happen.

There are certain simple and easily understood
facts and observations on which modern
meteorology rests. First, the air has weight and
is highly elastic, and its weight or the pressure
that it exercises constantly varies. A happy
thought suggested itself to Torricelli, to measure
this pressure by balancing against it a column
of mercury in an empty tube. He took a tube
of strong glass, of considerable length, and filled
it with clean mercury, and then inverted it in a
basin of mercury. He was surprised to find that
the column of mercury always sank till it remained
stationary thirty inches or thereabouts above
the level of the mercury in the basin. This
nearly uniform column of thirty inches, always
held up in the tube, he concluded must be
exactly equivalent in weight or pressure to the
whole amount of the pressure of the air, since
there was no air whatever left in the top of the
tube, and the only thing that could keep the
fluid metal from falling into the basin must be
a counteracting and equal weight arising from
the body of the air from the earth to its upper
limit. But hardly was the experiment made,
when it was observed that, in times of wind and
rain, the mercury did not stand so high as in
time of continued fine weather. Thus the
barometer or weight-measurer came to be regarded
as a weather-glass; not that it ever can do more
than measure the pressure of the air, but
because bad weather generally follows the fall,
and fair weather the rise of the mercury in
the tube.

Used with the anemometer or wind-measurer,
which is only a carefully made weather-cock of
which the indications are registered, the direction
of the wind and the force with which it
blows, two other air meteors may be compared,
and are found to have much relation to each
other. The mercury, indeed, quite as invariably
sinks when high winds are prevalent as when
rainy weather is at hand.

Vapour of water is present in the air at all times,
but more in proportion as the air is warmer.
Air at all temperatures holds vapour in solution
without appearing damp or depositing water, but,
as the quantity varies with the heat, there is often
a change involving the getting rid of a certain
part of the water. To measure the moisture of
the air, requires an instrument specially
contrived. Such is the hygrometer, or wet-measurer;
but that instrument is of no use without
observations of temperature, and for this purpose
we must have a thermometer or heat-measurer.
The old construction of the former instrument
was very picturesque, but not very precise.
Who does not remember the monk on the
mantel-piece, with a cowl which covered his
head in the damp, but moved off from it when
the air was drier? A more accurate contrivance,
based on the same principle, is still occasionally
used. The heat-measurer is, as all know, a small
tube of glass partly filled with mercury or
coloured spirit, of which there is a supply in a
bulb at the extremity. When heat is applied
the fluid in the bulb expands, and to occupy a
larger space is obliged to force itself somewhat
higher in the tube. Cold produces a contrary
effect.

There is another equally simple and
effectual mode of making observations of the
state of the air with regard to moisture. A
glass of cold spring-water, or of ice-water,
brought into the air on a warm day is soon
clouded with pearly drops, which have been
obtained from the sudden chilling of the adjacent
air by the glass and its contents. The more
moist the air the less need is there of having the
water extremely cold to produce this effect, and
by noticing the exact temperature by the
thermometer at which the dew begins to be thrown
down, we obtain what is called the dew-point,
which in fact marks the condition of the air for
moisture. There are other ingenious modes of
arriving at the same result with equal or greater
accuracy.

Some other uses are made of the thermometer,
and it is found that the temperature of
the air is constantly varying, not only from hour
to hour, but at the same time in different strata
of air near the earth. Wherever the sun's rays
can reach, the cause is manifest; but it is found
by experiment that, in shade and at night, the
same thing happens. Heat is radiated through
the air, and radiation takes place much more
rapidly through a clear than in a cloudy atmosphere.
The earth receives heat during the day and parts
with it at night. Thus there is another constant
source of disturbance in the atmosphere.

Electricity, again, acts a most important part
in all that is going on around us. The effect of
this agent is generally obscure, always strange,
unlike other agents, and sometimes very
terrible. Incessantly developed by every change that
takes place by evaporation, by all phenomena of
life, and by the action of light, its presence and
state can generally only be detected by very
delicate instruments. The flappings of a small
piece of leaf gold become a means to this end;
but the telegraph wires afford another almost
equal though very disagreeable means of
detecting the electric excitement.

When the aurora is seen between the observer
and the pole to which he is nearest, there is an
amount of magnetic disturbance in the earth and
air which is even more widely extended than the
fiercest electric storm. Contrivances are not
wanting by which this also can be measured and
recorded, and the finger of the storm, whether
electric or magnetic, now traces its own path on
the sheet placed to receive the mark.

The compass-needle is disturbed when the
aurora appears, and this connexion of two
phenomena apparently so little connected, is one of
those discoveries for which we have to thank
the modest philosopher whose name we have
already connected with this article.

The curious phenomena of snow and hail
involve considerations not fitted for discussion here,
and explanations that would rather confuse than
enlighten the general reader. They are by no
means so simple as some of us are in the habit