A Guide To The Scientific Knowledge Of Things Familiar Part 46

Q. _How can a thin covering of Ba.s.s or even MUSLIN protect trees from FROST?_

A. Because _any covering_ prevents the _radiation of heat from the tree_; and if the tree be _not cooled down by radiation_, the vapour of the air will _not be frozen_ as it comes in contact with it.

Q. _Why is the Ba.s.s or CANVa.s.s itself (which covers the tree) always DRENCHED with DEW?_

A. The ba.s.s or canva.s.s covering _radiates heat_ both _upwards and downwards_; and is, therefore, _so cooled down_, that it readily _chills all the vapour of the air_ (which pa.s.ses over it) _into dew_.

Q. _Why does SNOW at the foot of a HEDGE or WALL melt sooner, than in an open field?_

A. Because the hedge or wall _radiates heat into the snow beneath_, which melts it.

Q. _Why is there NO DEW after a WINDY NIGHT?_

A. 1st--Because the wind _evaporates the moisture_, as fast as it is deposited; and

2ndly--It _disturbs the radiation of heat_, and diminishes the deposition of dew thereby.

Q. _Why are VALLEYS & HOLLOWS often thickly covered with DEW, although they are sheltered?_

A. The surrounding hills prevent the _repose of air_ (in the valleys) _from being disturbed_; but do not _overhang_ and _screen_ them, so as to _arrest their radiation_.

Q. _Why does DEW FALL more ABUNDANTLY on SOME THINGS than upon OTHERS?_

A. Because some things _radiate heat more freely_ than others, and therefore become _much cooler_ in the night.

Q. _Why are things which RADIATE HEAT MOST FREELY, always the most THICKLY COVERED with DEW?_

A. Because the vapour of the air is _chilled into dew_, the moment it comes in contact with them.

Q. _What kind of things RADIATE HEAT most FREELY?_

A. Gra.s.s, wood, and the leaves of plants, radiate heat _very freely_: but polished metal, smooth stones, and woollen cloth, part with their heat _very tardily_.

Q. _Do the leaves of ALL plants radiate heat EQUALLY WELL?_

A. No. Rough _woolly leaves_ (like those of a holly-hock) radiate heat much _more freely_, than the _hard smooth polished leaves_ of a common laurel.

Q. _Shew the WISDOM of G.o.d in making gra.s.s, the leaves of trees, and ALL VEGETABLES, EXCELLENT RADIATORS of heat._

A. As vegetables _require much moisture_, and would often perish without a plentiful deposit of dew, G.o.d wisely made them to _radiate heat freely_, so as to _chill the vapour_ (which touches them) _into dew_.

Q. _Will polished METAL, smooth STONES, and woollen CLOTH, readily collect DEW?_

A. No. While gra.s.s and the leaves of plants _are completely drenched with dew_, a piece of _polished metal_, or of _woollen cloth_ (lying on the same spot) will be _almost dry_.

Q. _Why would POLISHED METAL and WOOLLEN CLOTH be DRY, while gra.s.s and leaves are drenched with DEW?_

A. Because the polished metal and woollen cloth _part with their heat so slowly_, that the vapour of the air is _not chilled into dew_ as it pa.s.ses over them.

Q. _Why is a GRAVEL WALK almost DRY, when a gra.s.s plat is covered thick with DEW?_

A. _Gra.s.s_, (_being a good radiator_) throws off its heat very _freely_; but _gravel (being a very bad radiator)_ parts with its heat very _reluctantly_.

Q. _Is that the reason why GRa.s.s is SATURATED with DEW, and the GRAVEL is NOT?_

A. Yes. When the vapour of warm air comes in contact with the _cold gra.s.s_, it is instantly chilled into dew; but (as the gravel is _not so cold as the gra.s.s_) the vapour of air is _not so freely condensed_ as it pa.s.ses over the gravel.

Q. _Why does DEW rarely fall upon hard ROCKS and BARREN lands?_

A. Rocks and barren lands are so _compact_ and _hard_, that they can neither _absorb nor radiate much heat_; and (as their _temperature varies but very little_) very little _dew_ distils upon them.

Q. _Why does DEW fall more abundantly on CULTIVATED soils, than on BARREN lands?_

A. Because cultivated soils (being _loose and porous_) _absorb_ heat freely during the day, and _radiate it_ by night; and (being _much cooled by the rapid radiation of heat_) as the vapour of the air pa.s.ses over them, it is plentifully _condensed into dew_.

Q. _Shew the WISDOM of G.o.d in this arrangement._

A. Every plant and inch of land which _needs the moisture of dew_, is adapted to _collect it_; but _not a single drop even of dew is wasted_, where its refres.h.i.+ng moisture is _not required_.

Q. _Shew the WISDOM of G.o.d in making polished METAL and woollen CLOTH BAD RADIATORS of heat._

A. If polished metal collected dew as easily as gra.s.s, it could _never be kept dry_, and _free from rust_. Again, if woollen garments collected dew as readily as the leaves of trees, we should be _often soaking wet_, and subject to _constant colds_.

Q. _Shew how this affords a beautiful ill.u.s.tration of GIDEON'S MIRACLE, recorded in the book of Judges, VI. 37, 38._

A. The _fleece of wool_ (which is a _very bad radiator_ of heat) was _soaking wet_ with dew: when the _gra.s.s_ (which is a most _excellent radiator_) was _quite dry_.

Q. _Was not this CONTRARY to the laws of NATURE?_