A Guide to the Scientific Knowledge of Things Familiar
CHAPTER XXIII.
BAROMETER.
Q. _What is a BAROMETER?_
A. A weather-glass, or instrument to show the changes of the weather, by marking the _variations in the weight of air_.
Q. _What is a THERMOMETER?_
A. An instrument to show how _hot or cold_ anything is.
Q. _What is the DIFFERENCE between a THERMOMETER and a BAROMETER?_
A. In a THERMOMETER the mercury is _sealed up from the air_:
In a BAROMETER the mercury is left _exposed_ (or open) to the air.
Q. _If the mercury of the thermometer be SEALED UP from the air, how can the air AFFECT it?_
A. The heat of the air passing _through the glass tube_ into the mercury, causes it to _expand more or less_, and rise in the tube accordingly.
Q. _Why is the TUBE of a BAROMETER left OPEN?_
A. That the air may _press upon it_ freely; and as this pressure is _more_ or _less_, the mercury _rises or falls_ in the tube.
Q. _How can weather be affected by the WEIGHT of the air?_
A. When air is warm or moist, it is _lighter_ than usual:
When it is cold or dry, it is _heavier_: and as a barometer marks whether the air be _light_ or _heavy_, it indicates these _changes_.
Q. _How can you tell (by looking at a BAROMETER) what KIND of WEATHER it will be?_
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?_
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?_
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?_
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_.
Q. _In what MONTHS is the barometer HIGHEST?_
A. In May and August; next to these, in June, March, September, and April.
Q. _In what MONTHS is the barometer LOWEST?_
A. In November and February; then in October, July, December, and January.
Q. _What are the DRIEST months?_
A. March and June; then May and August; then April and November.
Q. _What are the WETTEST months?_
A. October and February; then July and September; then January and December.
Q. _Why is there LESS wet from MARCH to AUGUST, than there is from August to March?_
A. Because the _heat is constantly increasing_; and the capacity of the air to absorb and retain moisture increases likewise.
Q. _Why is there MORE wet from AUGUST to MARCH, than there is from March to August?_
A. Because the _heat is constantly decreasing_, and the capacity of the air to retain moisture decreases also; so that (although it often rains) yet the air is always on the point of saturation.
Q. _Why does the mercury of a barometer RISE in a FROST?_
A. Because frost _condenses the air_; and condensed air is heavier than _rarefied_ air.
Q. _Why does the mercury of a barometer FALL in a THAW?_
A. Because the air is both _warmer_ (or more rarefied), and also filled with _vapour_.
Q. _What does a SUDDEN rise or fall of the barometer indicate?_
A. If the _rise_ be sudden, fine weather will not continue long:
If the _fall_ be sudden, foul weather will not continue long.
Q. _What sort of weather may we expect if the barometer be very FLUCTUATING?_
A. If the mercury fluctuates much, the weather will be very _changeable and unsettled_.
The FALL of the barometer.
In very _hot_ weather, the fall of the mercury denotes _thunder_.
Except in very hot weather, the sudden falling of the barometer denotes high wind.
In _frosty_ weather, the fall of the barometer denotes _thaw_.
If _wet_ weather happens _soon_ after the fall of the barometer, expect but _little_ of it.
In _wet_ weather if the barometer falls, expect much wet.
In _fair_ weather, if the barometer falls much and _remains_ low, expect much wet in a few days, and probably _wind_.
N. B. The barometer sinks lowest of all for wind and rain together, next to that for wind (except it be an east or north-east wind).
* * * * *
The RISE of the barometer.
In _winter_ the rise of the barometer presages _frost_.
In _frosty_ weather, the rise of the barometer presages _snow_.
If _fair_ weather happens _soon_ after the rise of the barometer, expect but _little_ of it.
In _wet_ weather, if the mercury rises high and _remains_ so, expect continued _fine_ weather in a day or two.
In wet weather, if the mercury rises suddenly very high, fine weather will not last long.
N. B. The barometer rises highest of all for north and east winds; for all _other_ winds it sinks.
* * * * *
If the barometer be UNSETTLED.
If the motion of the mercury be _unsettled_, expect unsettled weather.
If it stand at "MUCH RAIN" and rise to "CHANGEABLE," expects _fair weather of short continuance_.
If it stand at "FAIR" and fall to "CHANGEABLE," expect _foul_ weather.
N. B. Its motion _upwards_ indicates the approach of fine weather: its motion _downwards_ indicates the approach of foul weather.