A. Because the air is becoming more _dry_, and therefore its _pressure_ is greater.

Q. _Why does the mercury SINK at the approach of FOUL weather?_

A. Because the air is _laden with vapour_, or _disturbed by wind_.

Q. _Why does VAPOUR in the air make the mercury SINK?_

A. Because vaporized air is _lighter than dry air_, and therefore its _pressure is less_ on the mercury of the barometer.

Q. _What is the 10TH SPECIAL RULE in regard to the barometer?_

A. If (in frosty weather) it _begins to snow_, the barometer generally rises to 32, where it remains as long as the snow continues to fall; if, after this, the weather _clear up_, you may expect _very severe cold_.

Q. _How can you know if the MERCURY of the barometer be RISING?_

A. If it be _convex_ (i. e. higher in the _middle_ than at the _sides_;) it is in a _rising state_.

Q. _How can you tell if the MERCURY of the barometer be about to FALL?_

A. If it be _concave_ (i. e. _hollow_ in the _middle_) it is in a _falling state_.

Q. _Why is the mercury CONVEX when it is RISING?_

A. The sides of the mercury _rub against the gla.s.s tube_, and are _delayed_ by it, so that the _middle_ part _rises faster_ than the _sides_.

Q. _Why is the mercury CONCAVE when it is FALLING?_

A. The sides of the mercury _rub against the gla.s.s tube_, and are _delayed_ by it, so that the _middle_ part _sinks faster_ than the _sides_.

Q. _What effect does a THUNDER-STORM produce on the weather?_

A. Thunder is generally _preceded by hot_ weather, and _followed by cold_ and showery weather.

Q. _What effect does a SUDDEN CHANGE produce on the weather?_

A. A great and sudden change (either from hot to cold, or from cold to hot) is generally followed _by rain within 24 hours_.

Q. _Why is a sudden CHANGE from HOT to COLD followed by RAIN?_

A. The cold _condenses the air_ and its vapour; which, being condensed and squeezed out, _falls in rain_.

Q. _Why is a sudden CHANGE from COLD to HOT followed by RAIN?_

A. Because the air is _quickly saturated with moisture_; and as soon as _night_ comes on, the temperature is _lowered again_, and some of the abundant moisture falls in rain.

Q. _Why is the air quickly SATURATED with MOISTURE, when HEAT succeeds rapidly from COLD?_

A. Because the evaporation (which was checked by the cold) is _carried on very rapidly_, in consequence of the _diminished pressure_ of the air.

(N. B. The _less the pressure_ of the air, the more _rapidly it evaporates_ moisture.)

Q. _When does the barometer VARY MOST?_

A. In winter time.

Q. _Why does the barometer vary MORE in WINTER than in SUMMER time?_

A. Because the _difference of temperature_ between the torrid and temperate zones is _so great_, that the state of the air is perpetually _disturbed_ by their mixing together.

Q. _When does the barometer VARY LEAST?_

A. In summer time.

Q. _Why does the barometer vary LESS in SUMMER than in WINTER time?_

A. Because the temperature of our island is _so nearly equal_ to that of the torrid zone, that its state is _not much disturbed_ by interchange of currents.

Q. _What effect has WIND on the barometer?_

A. NORTH and EAST winds make the mercury _rise_; all _other_ winds make it _sink_; but SOUTH and WEST winds make it _sink lower_ than any other winds.

Q. _Have HEAT and COLD any effect on the barometer?_

A. No, not of _themselves_; but because _cold_ weather is generally either _dry_, or _rough with north-east winds_, therefore the mercury _rises_ in cold weather; and because warm weather is often _moist_ or _fanned by south-west winds_, therefore, the mercury sinks.

Q. _Why is the mercury of a barometer LOWER in the TORRID than in the FRIGID zones?_

A. Because the warm air of the torrid zone contains much more _vapour_ than the condensed air of the frigid zone; and the _moister_ the air, the _less is its pressure_.