A Guide to the Scientific Knowledge of Things Familiar
CHAPTER XXIV.
SNOW. HAIL. RAIN.
Q. _What is SNOW?_
A. The condensed vapour of the air _frozen_, and precipitated to the earth.
Q. _What is the CAUSE of SNOW?_
A. When the air is nearly saturated with vapour, and condensed by a current of air _below freezing point_, some of the vapour is squeezed out, and frozen into snow.
A few years ago, some fishermen (who wintered at Nova-Zembla), after they had been shut up in a hut for several days, _opened the window_, and the cold external air rushing in, instantly condensed the air of the hut, and the vapour (which was squeezed out) fell on the floor _in a shower of snow_.
Q. _Why does SNOW fall in WINTER time?_
A. Because the sun's rays are too _oblique_ to heat the surface of the earth; and (as the _earth has no heat_ to radiate into the air) the air is very cold.
Q. _What is SLEET?_
A. When flakes of snow (in their descent) pass through a bed of air _above freezing point_, they melt; and fall to the earth as half-melted snow or sleet.
Q. _What is the USE of SNOW?_
A. To keep the _earth warm_, and to _nourish_ it.
Q. _How can snow keep the EARTH WARM?_
A. Because it is a very _bad conductor_; in consequence of which, the earth which is covered with snow, very rarely descends _below freezing point_, even when the air is 15 or 20 degrees colder.
Q. _Why is SNOW a BAD CONDUCTOR of heat and cold?_
A. Because _air_ is confined and entangled between the crystals, and _air_ is a very _bad conductor_; when, therefore, the earth is covered with snow, it cannot throw off its heat by radiation.
Q. _Tell me the words of the PSALMIST_ (cxlvii. 16.) _respecting snow, and explain what he means._
A. The Psalmist says--"The Lord giveth snow like wool:" and he means not only that snow is as _white as wool_, but that it is also as _warm as wool_.
Q. _Why is WOOL WARM?_
A. Because _air_ is entangled between the fibres of the wool, and air is a _bad conductor_.
Q. _Why is SNOW WARM?_
A. Because _air_ is entangled between the crystals of the snow, and air is a _bad conductor_.
Q. _Why does SNOW NOURISH the earth?_
A. Because it supplies it with _moisture_ for a considerable time; which penetrates slowly into the soil, and insinuates itself through every clod, ridge, and furrow.
Q. _Why is there NO SNOW in SUMMER time?_
A. No snow reaches the general surface of the earth in summer time, because the _heat of the earth_ melts it in its descent.
Q. _Why are some MOUNTAINS ALWAYS COVERED with SNOW?_
A. 1st--Because the _air is more rarefied_; and rarefied air _abstracts heat_ which it holds in a _latent state_:
2ndly--As the mountain top is _not surrounded by earth_ to radiate heat into the air; therefore, the snow is _not melted_ in its descent, but falls on the mountain, and lies there.
Q. _Why is SNOW WHITE?_
A. Snow is formed of an infinite number of very minute crystals and prisms, which reflect all the colours of the rays of light; and these colours _uniting_ before they meet the eye, cause snow to appear white.
Q. _What is HAIL?_
A. Rain, which has passed in its descent _through some cold bed of air_, and has been frozen into drops of ice.
Q. _Why is ONE bed of air COLDER than another?_
A. This is frequently caused by _electricity_ in the air, _unequally distributed_.
Q. _Why is HAIL frequently accompanied with THUNDER and LIGHTNING?_
A. 1st--Because the _congelation of water into hail_ disturbs the electricity of the air: and
2ndly--The _friction_ (produced by the fall of hail) excites it still more.
Q. _Why does HAIL fall generally in SUMMER and AUTUMN?_
A. 1st--Because the _air is more highly electrified_ in summer and autumn: and
2ndly--The vapours (being rarefied) ascend to the more elevated regions, where the _cold is greater_ than it is nearer the earth.
Q. _What TWO things are essential to cause HAIL?_
A. Two _strata of clouds_ having _opposite electricities_, and _two currents of wind_. The _lower cloud_ (being negative) is the one _precipitated_.
Q. _What is RAIN?_
A. The vapour of the clouds or air _condensed_, and precipitated to the earth.
Q. _Why is the vapour of the air or clouds PRECIPITATED?_
A. When the air is _saturated with vapour_, if a cold current _condenses_ it, it is no longer _able to hold all its vapour_ in solution, and some of it is squeezed out, and falls as rain.
Q. _Why does RAIN fall in DROPS?_
A. The vapoury particles in their descent _attract each other_; and those which are sufficiently near, _unite_ and form into a drop.
Q. _Why does not the COLD of NIGHT ALWAYS cause rain?_
A. When the air is not _near saturation_ (although condensed by the chill of evening), it will still be able to hold its vapour in solution.
Q. _Why does a PASSING CLOUD often drop RAIN?_
A. Because the cloud (travelling about on the wind) comes into contact with _something that chills it_; and its vapour being squeezed out, _falls to the earth as rain_.
Q. _Why are RAIN-DROPS sometimes much LARGER than at OTHER times?_
A. When the rain-cloud is floating _near the earth_, the drops are large, because such a cloud is _much more dense_ than one which is more elevated.
The size of the rain-drop is increased according to the _rapidity_ with which the vapours are condensed.
Q. _Does not WIND sometimes INCREASE the SIZE of rain-drops?_
A. Yes; by blowing two or more drops into one.
Q. _Why do CLOUDS FALL in RAINY weather?_
A. 1st--Because the _clouds are heavy_ with abundant vapour: and
2ndly--As the density of the air is _diminished_, it is less able to buoy the clouds up.
Q. _How do you KNOW that the DENSITY of the air is DIMINISHED in RAINY weather?_
A. Because the _mercury of a barometer falls_.
Q. _Why is RAIN-water more FERTILIZING than PUMP-water?_
A. Because it contains a compound of hydrogen and nitrogen (called _ammonia_), which is a very excellent food for young plants.
Q. _Why is NOVEMBER made by God to be a RAINY MONTH?_
A. Because rain _hastens the putrefaction of the fallen leaves_ by causing fermentation.
Q. _Why does RAIN PURIFY the AIR?_
A. 1st--Because it _beats down the noxious exhalations_ collected in the air, and _dissolves_ them:
2ndly--It mixes the air of the _upper_ regions with that of the _lower regions_: and
3rdly--It _washes the earth_, and sets in motion the stagnant _sewers and ditches_.
Q. _Why are MOUNTAINOUS countries more RAINY than flat ones?_
A. The air (striking against the side of the mountains) is _carried up the inclined plane_, and brought in contact with the _cold air of the higher regions_, by which it is _condensed_, and its _vapour squeezed out_.
Q. _Why does a SPONGE SWELL when it is WETTED?_
A. Because the water _penetrates the pores_ of the sponge, and drives the particles of the sponge _further from each other_; in consequence of which, the _bulk_ of the sponge is greatly _increased_.
Q. _Why do FIDDLE-strings SNAP in WET weather?_
A. Because the moisture of the air (penetrating the string) _causes it to swell_; and (as the cord _thickens_) its _tension is increased_, and the string snaps.
Q. _Why does PAPER PUCKER when it is WETTED?_
A. Because the moisture (penetrating the paper) _drives its particles further apart_; and (as the moisture is absorbed _unequally_ by the paper) some parts are more enlarged than others; in consequence of which, the paper _blisters_ or _puckers_.
Q. _Why do the weather toys called CAPU'CHINS lift the cowl over the figures in wet weather, and remove it in dry?_
A. The cowl of the capu'chin is _fastened to a piece of cat-gut_. When the weather is _wet_, the moisture _swells the cat-gut_ and it is _shortened_, by which means the _cowl is pulled up_; but in _dry_ weather, the _string is loosened_, and the cowl falls down.
Q. _In another weather toy, the MAN comes out in WET weather, and the LADY in FINE:--Why is this?_
A. The two figures are attached to a piece of _cat-gut_ in such a manner, that when the _cat-gut is shortened by moisture_, it pulls the _man out_; but when it is _loose_, the woman _falls out by her own weight_.
Q. _Why are WET STOCKINGS DIFFICULT to PULL ON?_
A. The moisture (by penetrating the threads of the stockings) causes them to _shrink in size_.
Q. _What is the MOST RAINY spot in ENGLAND?_
A. Keswick (in Cumberland); and then Kendal (a market town in Westmoreland).
(In Keswick, about 63 inches of rain fall in a year. In Kendal, 58; Manchester, 38; Liverpool, 34; Dublin and Cambridge, 25; Lincoln, 24; London, 21; and in Paris, only 18.)
Q. _In which PART of the DAY does the MOST RAIN fall?_
A. More rain falls by _night_ than by day; because the cold night _condenses the air_, and diminishes its capacity for holding vapour in solution.
Q. _Does more rain fall in SUMMER or in WINTER time?_
A. There are _more rainy days_ from September to March; but _heavier_ rains between March and September.
Q. _Why are there MORE RAINY DAYS from September to March, than from March to September?_
A. Because the temperature of the air is _constantly decreasing_, and its capacity for vapour decreases also; in consequence of which, it is perpetually obliged to _part with some of its vapour_ in rain.
Q. _In what PART of the WORLD does RAIN fall MOST ABUNDANTLY?_
A. Near the _equator_; and the quantity of rain _decreases_ as we approach the _poles_.
Q. _Why does MORE RAIN fall at the EQUATOR than at the POLES?_
A. Because the _contrast_ between the _night and day_ is very great. The hot air _absorbs moisture very abundantly_ during the day; and when the cold night _condenses_ the air, it is unable to _retain the moisture imbibed_, and some of it falls in rain.