e other one, when the atmosphere
is extremely damp, or moist; but lower at other times,--in proportion to
the dryness of air, and consequent evaporation,--as far as twelve or
fifteen degrees in this climate; twenty or even more elsewhere. From
four to eight degrees of difference is usual in England; and about seven
is considered healthy for living rooms.

The thermometer fixed to a barometer intended to be used only as a
weather-glass shows the temperature of air about it nearly--but does not
show the temperature of mercury within exactly. It does so however near
enough for ordinary practical purposes--provided that no sun, nor fire,
nor lamp heat is allowed to act on the instrument partially.

The mercury in the cistern and tube being affected by cold or heat,
makes it advisable to consider this when endeavouring to foretell coming
weather by the length of the column.

* * *

Briefly, the barometer shows weight or pressure of the air; the
thermometer--heat and cold, or temperature; and the wet thermometer,
compared with a dry one, the degree of moisture or dampness.[10]

It should be remembered that the state of the air _foretells_, rather
than shows present weather (an invaluable fact too often overlooked);
that the longer the time between the signs and the change foretold by
them, the longer such altered weather will last; and, on the contrary,
the less the time between a warning and a change, the shorter will be
the continuance of such foretold weather.

* * *

If the barometer has been about its ordinary height, say near thirty
inches, at the sea level,[11] and is steady, or rising--while the
thermometer falls, and dampness becomes less--North-westerly, Northerly,
or North-easterly wind--or less wind--may be expected.

On the contrary--if a fall takes place, with a rising thermometer and
increased dampness, wind and rain may be expected from the
South-eastward, Southward, or South-westward.

A fall, with a low thermometer, foretells snow.

Exceptions to these rules occur when a North-easterly wind, with wet
(rain or snow) is impending, before which the barometer often rises (on
account of the _direction_ of the coming wind alone), and deceives
persons who, from that sign only, expect fair weather.

When the barometer is rather below its ordinary height, say, below
twenty-nine inches and nine-tenths (at the sea level _only_), a rise
foretells less wind, or a change