The Sun.
London, England, 1831-04-23 00:00:00

DIRECTIONS ENGRAVED UPON THE
COMMON WEATHER GLASS ABSURD.

The barometer has been called a weather glass.
It'iSes are attempted to be established, by which,
from the height of the mercury, the coming
etate of the weather may be predicted, and we
.accordingly find the words " Rain," " Fair,"
" Changeable," " Frost," &c., engraved on the
scale attached to common domestic barometers,
as if, when the mercury stands at the height
marked by these words, the weather is always
subject to the vicissitudes expressed by them.
These marks are, however, entitled to no atten-
tion ; and it is only surprising to find their use
continued in the present times, when knowledge
is so widely diffused. They are, in fact, to
ranked scarcely above the vox stellaruin, or as-
trological almanack.

Two barometers, one near the level of the
river Thames, and the other on the heights of
Hampstead, will differ by half an inch ; the latter
being always half an inch lower than the former.
If the words, therefore, engraved upon the plates
are to be relied on, similar changes of weather
could never happen at these two situations. But
what is even more absurd, such a scale would
inform us that the weather at the foot of a high
building, such as St. Paul's, must always be dif-
ferent from the weather at the top of it.
It is observed that the changes of weather are
indicated, not by the actual heat of the mercury,
but by its change of height. One of the most
general, though not absolutely invariable, rules
is, that when the mercury is very low, and there-
fore the atmosphere very light, high winds and
storms may be expected.
The following rules may geruerallv be relied
upon, at least to a certain extent

1. Generally the risilig of the mercury indi-
cates the approach of fair weather ; the falling of
it shews the approach of foul weather.
2. In sultry weather the fall of the mercury
indicates coming thunder. In winter, the rise
of the mercury indicates frost. In frost, its fall
indicates thaw; and its rise indicates snow.

8. Whatever change of weather suddenly fol•
lows a change in the barometer, may be expected
to last but a short time. Thus, if fair weather
follow immediately the rise of the mercury, there
will be very little of it ; and, in the same way, if
foul weather follow the fall of the mercury, it will
last but a short time.

4. If fair weather continue for several days,
during which the mercury continually falls, a
long continuance of foul weather will probably
ensue ; and again, if foul weather continue for
several days, while mercury continually rises, a
long succession of fair weather will Probably
succeed.

5. A fluctuating and unsettled state in the
!mercurial column indicates changeable weather.
The domestic barometer would become a
much more useful instrument, if, instead of the
words usually engraved on the plate, a short list
of the best established rules, such as the above.
accompanied it, which might be either engraved
on the .plate, or printed on a card. It would be
right, however, to express the rules only' with
that degree of probability which observation of
past phenomena has justified. There is no rule
respecting these effects which will hold good
with perfect certainty in every case.—Dr.
Lardner's Cabinet Cyclopcedia, Vol. XVII
Hydrostatics and Pneumatics, by Dr. Lardner'