A Guide To The Scientific Knowledge Of Things Familiar Part 71

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 pa.s.s 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?_