These marks are, however, entitled to no attention; 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 be ranked
scarcely above the _vox stellarum_, or astrological almanac.
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 different from the weather at
the top of it.
It is observed that the changes of weather are indicated, not by the
actual height 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 therefore the atmosphere very light, high winds
and storms may be expected.
The following rules may generally be relied upon, at least to a certain
extent:
1. _Generally_ the rising of the mercury indicates the approach of fair
weather; the falling of it shows the approach of foul weather.
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