A Guide To The Scientific Knowledge Of Things Familiar Part 48

Q. _How does RETAINING their COLD account for their being so WET?_

A. As the vapour of the warm air _touches the cold wall_, it is _chilled_ and _condensed into water_, which _sticks to the wall_, and sometimes trickles down in little streams.

Q. _Why does a thick WELL-BUILT HOUSE contract more DAMP of this kind, than an ORDINARY one?_

A. Because the walls are much _thicker_; and (if the frost has penetrated _far into the bricks_) it takes a long time to reduce them to the _same temperature as the air_.

Q. _Why are BANISTERS, &c. DAMP after a THAW?_

A. The wooden banister (being made of some very close-grained, varnished wood) cannot _change its temperature so fast_ as the air; and, therefore, _remains cold_ some time after the thaw has set in.

Q. _How does THIS account for the BANISTERS being DAMP?_

A. The vapour of the warm air (_coming in contact with the cold banister_) is _chilled_, and condensed into _water upon it_.

Q. _Why is our BREATH VISIBLE in WINTER and NOT in SUMMER?_

A. In _winter_ the coldness of the air condenses our breath into _visible vapour_; but in _summer_ the air is _not cold enough_ to condense it into visible vapour.

Q. _Why are our HAIR and the BRIM of our HAT often covered with little drops of pearly DEW in winter-time?_

A. The breath (issuing from our mouth and nose) _is condensed into drops_, as it comes in contact with our cold hair or hat; and (being condensed) hangs there in little dew-drops.

Q. _Why does the STEAM of a RAILWAY BOILER often pour down, like fine rain, when the steam is "let off?"_

A. The steam from the steam-pipe (when the air is cold) _is condensed by contact with the chill air_, and falls like fine rain.

Q. _Why is there LESS DEW when the WIND is EASTERLY, than when the wind is WESTERLY?_

A. _Easterly_ winds cross the _continent of Europe_, and, (as they pa.s.s over _land_) are _dry_ and _arid_; but _westerly_ winds cross the _Atlantic Ocean_; and (as they pa.s.s over _water_) are _moist_ and _full of vapour_.

Q. _How does the DRYNESS of an eastern wind PREVENT DEW-FALLS?_

A. As the easterly winds are _dry_, they _imbibe_ the moisture of the air; and, therefore, there _is very little_ left to be condensed into _dew_.

Q. _How does the MOISTNESS of a western wind PROMOTE dew-falls?_

A. As the westerly winds are _saturated with vapour_, they require a _very little reduction of heat_ to cause a _copious deposition of dew_.

Q. _When is DEW most COPIOUSLY distilled?_

A. After a hot day in summer or autumn, with the _wind in the west_.

Q. _Why is DEW distilled most COPIOUSLY after a HOT day?_

A. Because the surface of the earth _radiates_ heat very freely at sunset; and (becoming thus _much colder than the air_) _chills its vapour_, and condenses it into dew.

Q. _Does not AIR radiate heat, as well as the EARTH and its various plants?_

A. No. The air _never radiates heat_, nor is the air itself _made hot_ by the _rays of the sun_.

Q. _How is the AIR made HOT or COLD?_

A. By _convection of hot or cold currents_.

Q. _What is meant by "CONVECTION of hot and cold currents?"_

A. The air (which is heated by the surface of the earth) _ascends, warming the air_ through which it pa.s.ses. _Other_ air (being warmed in a similar way) _also ascends, carrying heat_; till _all the air_ is made hot.

Q. _Is the AIR made COLD in a similar way?_

A. Yes. The air resting on the earth is _made cold by contact_: this cold air makes the _air above it cold_; and cold currents or winds _shake the whole together_, till all becomes of one temperature.

Q. _Why is MEAT very subject to TAINT on a MOON-LIGHT night?_

A. In a bright moon-light night, _meat radiates heat very freely_; and is, therefore, soon _covered with dew_, which produces _rapid decomposition_.

Q. _Why do PLANTS GROW RAPIDLY in MOON-LIGHT nights?_

A. In bright moon-light nights _rapid radiation is carried on_, and _dew is plentifully deposited_ on young plants, which conduces much to their growth and vigour.

Q. _Why is evening DEW INJURIOUS to HEALTH?_

A. Because the condensed vapours are always laden with _noxious exhalations from the earth_: this is especially the case in _marshy_ countries.

Q. _Is HONEY-DEW a similar thing to DEW?_

A. No. Honey-dew is a sweet liquid _shed by a very small insect_ (called the aphis), and deposited in autumn _on the under surface_ of favourite leaves.