A. If the air be very _cold_ it is _so much condensed_, that it has already parted with as much moisture as it can spare. Q. _What is the 4TH SPECIAL RULE in regard to the barometer?_ A. When the barometer stands _very low_ indeed, there is never _much_ rain, although a _fine day_ will seldom occur at such times. Q. _What kind of WEATHER will it be when the barometer is UNUSUALLY LOW?_ A. There will be _short heavy showers_, with sudden _squalls of wind_ from the _west_. Q. _Why will there be VERY LITTLE RAIN if the barometer be UNUSUALLY LOW?_ A. Because the air must be very _warm_, or very _moist_, or perhaps _both_. Q. _Why will there be little or no rain, if the_ AIR _be very_ WARM? A. If the air be very warm it will have a tendency to _imbibe more moisture_, and not to part with what it has. Q. _Why will there be little or no rain if the air be MOIST, and the barometer remains very LOW?_ A. If the air be ever so moist, rain will never fall till _cold air_ has been introduced to _condense the vapour_; and the moment that the _cold_ air is introduced, the barometer will _rise_. Q. _What is the 5TH SPECIAL RULE in regard to the barometer?_ A. In summer-time (after a long continuance of fair weather) the barometer will _fall gradually_ for 2 or 3 days before _rain_ comes; but if the fall of the mercury be very _sudden_, a _thunder-storm_ is at hand. Q. _What is the 6TH SPECIAL RULE in regard to the barometer?_ A. When the sky is cloudless, and seems to promise fair weather, if the barometer be _low_, the face of the sky will soon be suddenly _overcast_. Q. _What is the 7TH SPECIAL RULE in regard to the barometer?_ A. Dark dense clouds will pass over _without rain_, when the barometer is _high_; but if the barometer be low, it will often rain _without any gathering of clouds_. Q. _What is the 8TH SPECIAL RULE in regard to the barometer?_ A. The _higher_ the barometer, the greater is the probability of _fair weather_. Q. _Why is the barometer HIGH in FINE weather?_ A. Because the air contains but _very little vapour_. The _drier_ the air, the _higher_ does the mercury of the barometer rise. Q. _What is the 9TH SPECIAL RULE in regard to the barometer?_ A. When the mercury is in a _rising_ state, _fine_ weather is at hand; but when the mercury is in a _sinking_ state, _foul_ weather is near. Q. _Why does the mercury RISE at the approach of FINE weather?_ 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?_