A Guide To The Scientific Knowledge Of Things Familiar Part 2

Q. _Why is THUNDER sometimes ONE VAST CRASH?_

A. When the lightning-cloud is near the earth, as the flash is _straight_,--the whole volume of air (through which it pa.s.ses) _collapses at once_; and produces one unbroken sudden _crash_.

Q. _What is meant by the air collapsing?_

A. When the rent air _closes again_, it is said to collapse.

Q. _Why is the PEAL sometimes an IRREGULAR mangling broken ROAR?_

A. When the lightning-cloud is a long way off, as the flash is zigzag, the air does not collapse _all at once_; and as we hear the concussion of one part after another, the peal is broken, protracted, and irregular.

Q. _Which part of the collapsing air do we hear first?_

A. That part _nearest_ the _earth_; then the strata above; and last of all, _that_ in the immediate vicinity of the cloud.

Q. _What is meant by_ "STRATA _of air?_"

A. If a board were laid upon the earth, and several other boards were piled upon it, this pile would represent strata of wood.

Q. _How does this ill.u.s.tration apply to the air?_

A. A layer of air covers the earth; another layer rests upon _it_; and thus layer is piled upon layer, for 50 miles in height. Each layer is a "stratum" of air; and the _plural_ of stratum is strata.

Q. _Why do we hear the collapsing of the air NEAREST the earth FIRST?_

A. Because sound takes a whole _second of time_ to travel 380 yards; but the air is ripped from top to bottom instantaneously: if, therefore, the cloud were 1000 yards off, we should hear the collapsing of the lowest strata nearly _three seconds_, before we heard that in the immediate vicinity of the cloud.

Q. _Why is the THUNDER sometimes like a deep GROWL?_

A. When the storm is _far distant_, the thunder sounds like a deep growl.

Q. _Does not SCENERY affect the sound of thunder?_

A. Yes; the _flatter_ the country, the more unbroken the peal: _Mountain_ scenery _breaks_ the peal, and makes it harsh and irregular.

Q. _What is the cause of ROLLING THUNDER?_

A. The rolling is produced by the _reverberation_ of the thunder along the ma.s.sive clouds.

Q. _What is meant by the reverberation?_

A. The echo.

Q. _Why is a flash of lightning generally followed by a POURING RAIN?_

A. The cloud _collapses_, as soon as the electric fluid has left it; and the water it contained is squeezed out.

Q. _Why is a flash of lightning generally followed by a GUST of WIND?_

A. The flash _rent the air asunder_ through which it darted; and when the two parts collapse, a rapid motion is produced, which we call _wind_: the _vibration_ of the thunder contributes also to agitate the air.

Q. _What is meant by the_ "VIBRATION _of the thunder_?"

A. The quivering motion it gives to the air, by its loud sound.

Q. _Why is there NO THUNDER to what is called SUMMER LIGHTNING?_

A. Because the lightning-clouds are _so far off_, that the sound of the thunder is _lost_, before it reaches the earth.

Q. _Do_ THUNDER-BOLTS _ever drop from the clouds?_

A. No; the notion of _thunder-bolts_ falling from the clouds, arises from the _globular_ form, that is sometimes a.s.sumed by a flash of lightning.

Q. _Why is the_ THUNDER _often several moments_ AFTER _the FLASH?_[1]

A. The flash travels nearly _a million_ times faster than the thunder; if, therefore, the thunder has _far to come_, it will not reach the earth till a considerable time _after the flash_.

[1] The speed of lightning is so great, that it would go 480 times round the earth in one minute: whereas, thunder would go scarcely 13 miles in the same s.p.a.ce of time.

Q. _Can we not tell the DISTANCE of a thunder-cloud, by observing the interval which elapses between the flash and the peal?_

A. Yes; the flash is _instantaneous_, but the thunder will take a whole _second of time_ to travel 380 yards: hence, if the flash is 5 seconds before the thunder, the cloud is 1900 yards off.

(i. e. 380 5 = 1900 yards.)

Q. _What PLACES are most DANGEROUS to be in, during a STORM?_

A. It is very dangerous to be near a tree, or lofty building; it is dangerous also, to be near a river, or any running water.