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.

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.

3. Whatever change of weather suddenly follows 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 the mercury