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 glass 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 glass 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?_