A. Because the mercury in the tube _rises and falls_, as the air becomes lighter or heavier: and we can generally tell by the _weight_ of the air, what kind of weather to expect. Q. _Does the WEIGHT of the air VARY MUCH?_ A. Yes; the atmosphere in England varies as much as _one-tenth part_ more or less. Q. _What is the chief USE of a BAROMETER?_ A. To warn _sailors_ how to _regulate their ships_, before squalls come on. Q. _How can a BAROMETER warn SAILORS to regulate their SHIPS?_ A. As the barometer will tell when _wind_, _rain_, or _storm_ is at hand, the sailor can make his ship trim before it overtakes him. Q. _Are there any RULES which can be depended on?_ A. Yes; there are _ten special rules_ to direct us how to know the changes of weather, by marking the mercury of a barometer. Q. _What is the 1ST SPECIAL RULE in regard to the barometer?_ A. The barometer is _highest of all_ during a _long frost_; and it generally rises with a _north-east wind_. Q. _Why is the barometer HIGHEST of all during a long FROST?_ A. Because long frost _condenses the air very greatly_; and the more air is _condensed_, the greater is its _pressure_ on the mercury of the barometer. Q. _Why does the barometer generally RISE with a NORTH-EAST wind?_ A. Because NORTH-EAST winds make the air both _cold and dry_: the air, therefore, is both _condensed_, and _without vapour_. Q. _What is the 2ND SPECIAL RULE in regard to the barometer?_ A. The barometer is _lowest of all_ during a _thaw which follows a long frost_: it generally falls with SOUTH and WESTERN winds. Q. _Why does the barometer fall LOWEST of all at the BREAKING UP of a long FROST?_ A. 1st--Because the air (which had been much _dried_ by the frost) _absorbs the moisture_ of the fresh warm current of wind from the south or south-west: and 2ndly--The air (which had been much _condensed_ by the frost) is suddenly _expanded_ by the warm wind which is introduced. Q. _Why does the barometer fall very low with SOUTH and WEST winds?_ A. Because SOUTH and WEST winds come heavily _laden with vapour_; and _vaporized_ air is lighter than _dry air_. Q. _What effect has WIND on the mercury?_ A. _All_ winds make the barometer _drop_, except EASTERN winds: those winds which blow from the SOUTH, and SOUTH-WEST make it _drop the lowest_. Q. _Why do WINDS generally make the mercury of a barometer DROP?_ A. Wind is caused by a _partial vacuum_ in some parts of the globe; and as the air _rushes in_ to supply this deficiency, _its general pressure is lessened_, and the barometer falls. Q. _What is the 3RD SPECIAL RULE in regard to the barometer?_ A. While the barometer stands above 30°, the air must be very _dry_ or very _cold_, or perhaps _both_, and _no rain_ may be expected. Q. _Why will there be NO RAIN if the AIR be very DRY?_ A. If the air be very _dry_ it will _absorb moisture_, and not part with what it has _in rain_. Q. _Why will there be NO RAIN if the AIR be very COLD?_