Fig. 3.—ILLUSTRATING THE FORMATION OF DEW.
Fig. 3.—ILLUSTRATING THE FORMATION OF DEW.
If plates of glass be laid over grass-beds, as in the engravingFig. 3, no dew will be deposited on the grass underneath the glass plates, although all around the grass will be completely wetted. The explanation is that the glasses, being radiators of heat, act in the same manner as the clouds, returning the heat to the bodies underneath them, and preventing the formation of dew thereon.
349.Why does dew form most abundantly on cloudless nights?
Because the heat which is radiated by the earth does not return to it. The temperature of the earth, and the air immediately upon its surface, is therefore lowered, and dew is formed.
It has been observed that sheep that have lain on the grass during the formation of dew have their backs completely saturated with it, but that underneath the line where their bodies turn to the earth, their coats will be dry. In the same manner glass globes suspended in the air, on dew forming nights, will be found loaded with globules of dew upon the top, but there will be no appearance of moisture underneath.
"Dost thou know the balancings of the clouds, the wondrous works of him which is perfect in knowledge."—Job xxxvii.
350.Why are star-lit nights usually colder than cloudy nights?
Because heat isradiatedfrom the earth, and passes away into the utmost regions of the atmosphere.
351.Why is there little dew under branches of thick foliage?
Because the foliageacts as a screen, which prevents the radiated heat of the earth from passing away.
352.Why is there no dew formed on windy nights?
Because, as winds generally consist of dry air, theyabsorb and bear awaythe atmospheric moisture.
353.Why are valleys and low places chiefly subject to dew?
Because the elevated lands around themprevent the disturbance of the airin which the moisture is held.
354.What bodies are most likely to be covered with dew?
All bodies that aregood radiators of heat, such as wool, swansdown, grass, leaves of plants, wood, &c.
355.What bodies are likely to receive little dew?
Allbad radiators of heat, such as polished metal surfaces, smooth stones, and polished surfaces generally. Dew will be found to lie more abundantly upon rough and woolly leaves than upon smooth ones.
356.At what period of the night is the largest amount of dew usually formed?
It is generally supposed that dew is formedmost copiouslyin themorningsandevenings. Butsuch is not the case. It is deposited at all hours of the night, butmost plentifully after midnight.
357.Why is dew formed most plentifully after midnight?
Because, asradiationhas been going on for some time, the temperature of the earth, and of various bodies upon it, has beenconsiderably reduced.
"Out of whose womb came the ice? and the hoary frost of heaven, who hath gendered it?"—Job xxxviii.
358.In what parts of the world is the maximum of dew formed?
In warm lands near the sea, or in the vicinity of rivers or lakes, as the localities of the Red Sea, the Persian Gulf the coast of Coromandel, in Alexandria, and Chili.
359.In what parts of the world is the minimum of dew formed?
It is quite absent in arid regions, in the interior of continents, such as Central Brazil, the Sahara, and Nubia.
360.Why is dew seldom formed at sea?
Because of the defectiveradiatingquality of the surface ofwater.
361.Why is a heavy dew regarded as the precursor of rain?
Because a heavy formation of dew indicates that the air issaturated with moisture.
362.What is hoar-frost?
Hoar-frost is frozen dew.
363.Why is hoar-frost said to foretell rain?
Because it shows that the air is saturated with moisture, and the temperature of the air being low, the vapours arelikely to condense, and produceshowers.
364.What is honey-dew?
Honey-dew is the name applied to asweet and sticky moistureoccasionally deposited upon the leaves of plants. It is, however, an error to call itdew, as it is procured by a class ofinsectstermedaphides.
365.What are fogs?
Fogs arecloudsformed near the earth's surface; but London fogs are distinguished from clouds by the fact that they embrace in their vaporous folds thesmokeandvolatile mattersimparted to the air by the operations of man. This is also the case with fogs generally that arise near large towns.
"Hath the rain a father? or who hath begotten the drops of dew?"—Job xxxviii.
366.Why are certain coasts liable to almost perpetual fogs?
Because of local or geographical agencies which contribute to their production. The coasts of California are almost constantly wrapped in fog; and, almost as constantly, the western coast of the American continent, as far south as Peru. Newfoundland, Nova Scotia, and Hudson's Bay, are all subject to dense and frequent fogs arising from the condensation of vapour from the water flowing from the hot Gulf-stream, coming in contact with the colder air.
367.What are dry fogs?
Dry fogs are characterised by a dull opaque appearance of the atmosphere. They are most common in certain parts of North America, though they sometimes occur in Germany and in England. They are generally referred to theelectrical state of the atmosphere, but the theory of them is still a matter of doubt.
368.What is a mist?
The termmistis generally applied to vapours that rise overmarshy places, or the surfaces ofwater, and roll or move over the land.
369.What is the difference between a mist and a fog?
Fogs, as they are known to us, generally arise over theland, and are usually mingled with the smoke of large towns. Mists generally arise over water, or wet surfaces.
370.Why do mists and fogs disappear at sunrise?
Because the condensed vapours are againexpandedanddispersedby the heat of the sun's rays.
371.Why do fogs frequently rise in the morning and fall again in the evening?
Because, warmed by the sun's rays, they become more rarefied, and fly away at an altitude where theyappearto be altogether dispelled; but at night, when the earthcools by radiation, the vapours near the earthagain condense, and settle in theform of fog.
372.Why do fogs sometimes rest upon a given locality for several days together, and then disappear?
They are probably kept near to the surface of the earth by a superstratum of cold air. A cold air lyingabove, or a cold air lyingbelow, might equally contribute to keep a fog near the surface of a particular part of the earth, until aflow of wind, or afall of rain, altered the atmospheric condition.
"He bindeth up the waters in his thick clouds; and the cloud is not rent under them."—Job xxvi.
There are many interesting facts connected with the history of dew. It has attracted the attention of natural philosophers in all ages. But its true theory was never understood until recently. The ancients imagined that dews were shed from the stars; and the alchemists and physicians of the middle ages believed that the dew distilled by night possessed penetrating and wonder-working powers. The ladies of those times sought to preserve their beauty by washing in dew, which they regarded as a "celestial wash." They collected it by placing upon the grass heaps of wool, upon the threads of which the magic drops clustered.
373.What are clouds?
Clouds are volumes ofvapour, usually elevated to a considerable height.
Fig. 4—CIRRO-CUMULUS, OR SONDER CLOUD.
Fig. 4—CIRRO-CUMULUS, OR SONDER CLOUD.
374.Whence do clouds arise?
From theevaporation of waterat the earth's surface.
375.Why do we not see them ascend?
We do, sometimes, in the form of what we callmists, but generally the vapours that rise and contribute to the formation of clouds are so thin that they areinvisible.
"With clouds he covereth the light, and commandeth it not to shine by the cloud that cometh betwixt."—Job xxxvi.
376.Why, if they are invisible when they rise, do they became visible when they have ascended?
Because the vapours becomecooledin passing through the air, and form a denser body.
377.Why, when they are condensed, do they not follow the course of gravitation, and descend?
Because the vapours form intominute vesicles, which we may callvapour bubbles, and these, being warmed by the sun, are specificallylighter than the air.
Because, also, the lower parts of cloudsdo partiallydescend, but again becoming morerarefiedby meeting with awarmer atmosphere, they again ascend, and are thuspoisedupon the air.
Because, also, there is always a degree of atmospheric motionupward, caused by theconvection of heatfrom the earth's surface. And, although there must also be downward movements of the air to supply the place of that which has ascended, stillthe heatof the ascending air,combined with its upward movement, expands and floats the vapour of the clouds.
378.At what height do clouds usually fly?
They fly at every degree of altitude; but clouds ofspecific characterare said to fly at given altitudes, or to occupy certainranges of altitude. We will give their probable altitudes when speaking of the specific clouds.
Fig. 5.—CIRRUS, OR CURL CLOUD.
Fig. 5.—CIRRUS, OR CURL CLOUD.
"Who giveth rain upon the earth, and sendeth waters upon the fields."—Job v.
379.How many descriptions of clouds are there?
There areseven.
1. TheCirrus(Fig. 5), estimated range of altitude from 10,000 to 24,000 feet.
2. TheCumulus(Fig. 7), from 3,000 to 10,000 feet.
3. TheStratus, an extended continuous level sheet of cloud, increasing from beneath. They fly very low.
4. TheNimbus(Fig. 10), 1,500 to 5,000 feet.
5. TheCirro-cumulus(Fig. 4), from 3,000 to 20,000 feet.
6. TheCirro-stratus(Fig. 6), from 5,000 to 10,000 feet.
7. TheCumulo-stratus(Fig. 9), from 3,000 to 10,000 feet.
Fig. 6—CIRRO-STRATUS, OR WANE CLOUD.
Fig. 6—CIRRO-STRATUS, OR WANE CLOUD.
The estimated heights given must be looked upon as very conjectural, although they have been derived from the best existing authorities. It is sufficient to know that the range of the altitude of the various clouds is from that of theNimbus, orthunder cloud, 1,500 feet, to that of theCirrus, 24,000 feet, the others being intermediate. The first three of the clouds above enumerated constitute what are called theprimary forms. The remaining four are calledsecondary forms, because they arise, as their names generally indicate, out of combinations of theprimary forms. Although, from the frequent mingling of clouds, it is not always practicable to identify them by the adopted classification, still, as there is generally a prevalence of one type of cloud over another, the observer would be able to distinguish a"Cirrus sky,"or"Cirro-cumulus sky,"&c. Upon some occasions the typical characters of the clouds are beautifully defined; and the contemplation of their forms, and the laws of their formation, affords infinite pleasure to the observer. The advantages of scientific knowledge are such, that whether you look downwards, to the earth, or upwards to the sky, you have still the writing of God to read.
380.What produces the various shapes of clouds?
1. The state of theatmosphere.
2. Theelectricalcondition of the clouds.
3. Themovementsof the atmosphere.
4. Theseason of the year.
"Behold, he withholdeth the waters, and they dry up; also he sendeth them out, and they overturn the earth."—Job xii.
381.What are the dimensions of clouds?
A single cloud has been estimated to have as many astwenty square miles of surface, and to beabove a mile in thickness, while others are no larger than ahouse, or a man'shand.
Fig. 7.—CUMULUS, OR PILE CLOUD.
Fig. 7.—CUMULUS, OR PILE CLOUD.
382.How are clouds affected by winds?
Ifcold windsblow upon the clouds, the cold condenses the vapour, turning the clouds intorain. But ifwarm dry windsblow upon the clouds, theyrarefy the vapourto a greater degree, and temporarilydisperse the clouds.
383.How do winds affect the shapes of clouds?
When winds aremild and gentle, the clouds break intosmall patches, and rise to a considerable height. But when the winds are cold and blustering, the clouds fly low, and roll along inheavy masses.
384.Why are east winds usually dry?
Because in coming towards England they pass over vast continents of land, and comparatively little ocean. Hence they are not loaded withvapours.
385.Why do west winds generally bring rain?
Because they come across theAtlantic, and are heavily charged withvapour.
386.Why are north winds generally cold and dry?
Because they come from the arctic ocean, over vast areas ofice and snow.
"Terrors are turned upon me: they pursue my soul as the wind; and my welfare passeth away as a cloud."—Job xxx.
387.Why are south winds warm and rainy?
Because they come from the southern regions, heated by thehot earth and sands, and as they cross the sea theyabsorb a large amount of vapour.
Fig. 9.—CUMULO-STRATUS, OR TWAIN CLOUD.
Fig. 9.—CUMULO-STRATUS, OR TWAIN CLOUD.
388.Why are clouds said to indicate the changes of the weather?
Because, as it is thestate of the cloudsthat, to a great extent, determines thestate of the weather, the formation of the clouds must predicate approaching changes.
389.What do cirrus clouds foretell?
Cirrusclouds foretellfineweather, when they fly high, and are thin and light.
They foretelllight rainwhen, after a long continuance of fine weather, they form fleecy lines stretched across the sky.
They foretell agale of windwhen, for some successive days, they gather in the same quarter of the heavens, as if denoting the point from which to expect the coming gale. (Fig. 5).
390.What do cumulus clouds foretell?
Cumulusclouds, when they are well defined, and advance with the wind, foretellfine weather.
When they are thin and dull, and float against the wind, or in opposition to the lower currents, theyforetell rain.
When they increase in size, and becomedull and grey at sunset, they predict athunder-storm. (Fig. 7.)
"When he made a decree for the rain, and a way for the lightning and the thunder."—Job xxviii.
391.What do stratus clouds foretell?
Stratusclouds foretelldamp and cheerless weather.
392.What do nimbus clouds foretell?
Nimbusclouds foretellrain,storm, andthunder. (Fig. 10.)
393.What do cirro-cumulus clouds foretell?
Cirro-cumulusclouds, in summer, foretellincreasing heatattended bymild rain, and asouth wind; but in winter they commonly precede thebreaking up of a frost, and the setting in offoggy and wet weather. (Fig. 4.)
394.What do cirro-stratus clouds foretell?
Cirro-stratusclouds foretellrainorsnow, according to the season of the year.
These clouds extend in long horizontal streaks, thinning away at their base, and in parts becoming wavy or patchy.
When they are thus defined in the heavens they are a certain indication ofbad weather. (Fig. 6.)
395.What do cumulo-stratus clouds foretell?
Cumulo-stratusclouds usually foretell achange of weather—from rain to fine, or from fine to rain. (Fig. 9.)
Fig. 10.—NIMBUS, OR STORM CLOUD.
Fig. 10.—NIMBUS, OR STORM CLOUD.
"Behold, I will put a fleece of wool in the floor; and if the dew be on the fleece only, and it be dry upon all the earth beside, then shall I know that thou wilt save Israel." * * *
396.Why are cloudy days colder than sunny days?
Because the clouds intercept thesolar raysin their course towards the earth.
397.Why are cloudy nights warmer than cloudless nights?
Because the cloudsradiate back to the earththe heat which the earth evolves?
Because, also, the clouds radiate to the earth the heat they havederived from the solarrays during a cloudy day.
398.Why is the earth warmer than the air during sunshine?
Because the earth freelyabsorbs the heat of the solar rays; but the air derivescomparatively little heatfrom the same source.
399.Why does the earth become colder than the air after sunset?
Because the earthparts with its heat freely by radiation; but the air does not.
400.Why do glasses, mats, or screens, prevent the frost from hitting plants?
Because they prevent theradiation of heat from the plants, and also from the earth beneath them.
401.Why are the screens frequently covered with dew on their exposed sides?
Because they radiate heat fromboth their surfaces. A piece of glass, laid horizontally over the earth, would radiate heat bothupwardsanddownwards. But on its lower surface it wouldreceivethe radiated heat of the earth, while from its upper surface it wouldthrow off its own heatand become cool. Therefore dew would be deposited upon theupper, but not on theundersurface.
402.Why does dew rest upon the upper surfaces of leaves?
Because the under surfaces receive theradiated warmth of the earth.
"And it was so: for he rose up early on the morrow, and thrust the fleece together, and wringed the dew out of the fleece, a bowl full of water."
403.Why are cultivated lands subject to heavier dews than those that are uncultivated?
Because cultivation breaks up the hard surface of the earth, and thusits radiating power is increased.
404.Why is the gravel walk through a lawn comparatively dry while the grass of the lawn is wet with dew?
Because gravel is abad radiator, but grass is agood radiator.
405.What benefit results from this arrangement?
In cultivated lands, where moisture is required, it isinducedby the very necessity which demands it; while in rocky and barren places, where it would be of no good, dewdoes not form.
406.Why does little dew form at the base of hedges and walls, and around the trunks of trees?
Because those bodies in some degreecounteract the radiationof heat from the earth; and they alsoradiate heatfrom their own substances.
407.Why do heavy morning dews and mists usually come together?
Because they both have their origin in thehumidity of the atmosphere. The temperature of the earth having fallen, dew has been deposited; but, at the same time, the condensation of the vapour in the airhas formed a screen over the surface of the earth, which has checkedthe further radiation of heat, and, consequently,the further formation of dew. The sun rises, therefore, upon an atmosphere charged with visible vapour at the earth's surface, and his first sloping rays,having little power to warm the atmosphere, the mistcontinues visible for some time.
408.What effect have winds upon the formation of dew?
Winds, generally, and especially when rapid, prevent the formation of dew. But those winds that are moist, andcontribute to the formation of clouds, indirectly aid the formation of dewthrough theformation of clouds, and also by themoisture they impart to the air.
"And Gideon said unto God, * * * Let it now be dry only upon the fleece, and upon all the ground let there be dew."
409.Why does the humidity of the atmosphere sometimes form clouds, and at others form fogs, mists, dews, &c.?
The result depends upon the varyingtemperature,motion, anddirectionof the atmosphere.
Awarm light atmosphere, of a few day's duration, will elevate the vapours to the region where they are formed intoclouds.
Achill air, lying upon the surface of the warmer earth, will occasionmistsorfogs.
Acold earth, acting upon the vapours contained in awarmer atmosphere, will condense them and occasiondews.
410.Why are frosty mornings usually clear?
Because, in the cold atmosphere which preceded the frost,there was but little evaporation; and now that the frost has set in, the vapours that existed have becomefrozenin the form ofhoar-frost.
411.Why are clear nights usually cold?
Because the "screen" afforded by the clouds does not exist; therefore the heat of the earth escapes, while the vapours of the air are abstracted from it by condensation into dew, thereby imparting greatclearness to the nights.
412.Why are hoar-frosts, or, as they are termed, "white frosts," so frequent, and "black frosts" so unusual?
Because white, orhoar frosts, result from thecoldness of the earth, which, from its great radiating power, is always varying. Butblack-frostsresult from thecoldness of the air, which is liable to less variation of temperature than the earth.
413.What is a black-frost?
Ablack-frostresults from thecoldness of the atmosphere, which is at the time overshadowed by a dull cloud, giving a darkness to everything, and a leaden appearance to thefrozen surface of water.
414.Why are black-frosts said to last?
Because as they result from the temperature of the air, which is less likely to vary than that of the earth, there is a probability that the coldness thereofwill last for some time.
"And God did so that night: for it was dry upon the fleece only, and there was dew on all the ground,"—Judges vi.
415.What benefits result from the radiation of heat, &c.?
But for theradiation of heat, we should be subjected to the most unequal temperatures. The setting of the sun would be likethe going out of a mighty fire. The earth would becomesuddenly cold, and its inhabitants would have to bury themselves in warm covering, to wait the return of day. By theradiationof heat, anequilibrium of temperatureis provided for, without which we should require a new order of existence.
The amount of heat which our earthreceives from the sun, and the economy of that heat by the laws ofradiation,reflection,absorption, andconvection, are exactly proportionate to the necessities of our planet, and the living things that inhabit it. It is held by philosophers that any change in the orbit of our earth, which would either increase or decrease the amount of heat falling upon it, would, of necessity, be followed by theannihilation of all the existing races. The planets Mercury and Venus, which are distant respectively 37 millions of miles, and 63 millions of miles, from the great source of solar heat, possess a temperature which wouldmelt our solid rocks; while Uranus (1,800 millions of miles), and Neptune (whose distance from the sun has not been determined), must receive so small an amount of heat, that water, such as ours, would become as solid as the hardest rock, and our atmosphere would be resolved into a liquid! Yet, poised in the mysterious balance of opposing forces, our orb flies unerringly on its course, at the rate of 63,000 miles an hour; preserving, in its wonderful flight, that precise relation to the sun, which takes from his life-inspiring rays the exact degree of heat, which, being shared by every atom of matter, and every form of organic existence,is just the amount needed to constitute the heat-life of the world!
416.What is rain?
Rain is thevapour of the cloudswhich, being condensed by a fall of temperature, forms drops of water that descend to the earth.
It is thereturn to the earthin the form ofwater, of the moistureabsorbed by the airin the form ofvapour.
417.Does rain ever occur without clouds?
It sometimes, but rarely happens, that a sudden transition fromwarmth to cold willprecipitate the moisture of the air, without the formation ofvisible clouds.
"Canst thou lift up thy voice to the clouds, that abundance of waters may cover thee?"—Job xxxviii.
418.Why are drops of rain sometimes large and at other times small?
Because the drops, in falling,meet and unite, and also gathermoisturein their descent. The greater the height from which a rain drop has descended,the larger it is, provided that its whole course lay through arainy atmosphere.
The size of the drops is also influenced by theamount of moisture in the atmosphere, thedegree of cold, and therapidityof thechange of temperature, by which the drops are produced.
419.In what seasons of the year are rains most prevalent?
ThroughoutCentral Europerains are most prevalent insummer, but inSouthern Europethe preponderance is on the side ofwinter rains.
420.In what months of the year does it rain most frequently in this country?
It rains more frequentlyfrom September to March, than fromMarch to September; but theheaviest rainsoccur fromMarch to September.
421.Why are there more rainy days from September to March?
Because the temperature of the air is more frequently lowered to that degree whichprecipitates its vapours.
Months in the order of their comparative wetness:—1. October. 2. February. 3. July. 4. September. 5. January. 6. December.
Months in the order of their comparative dryness:—1. March. 2. January. 3. May. 4. August. 5. April. 6. November.
422.In what part of the world does the greatest quantity of rain fall?
The greatestquantityof rain falls near theequator, and the amountdecreases towards the poles.
"Who can number the clouds in wisdom? or who can stay the bottles of heaven."—Job xxxviii.
423.In what part of the world do the heaviest rains occur?
Theheaviestrains occur in thetropics, during the hot season. The drops of rain in the tropical regions are so large, and the force with which they descend so great, that their splash upon the skin causes asmarting sensation.
424.In what parts of the world do the least rains occur?
There are some parts of the earth which arerainless, such as Egypt, the desert of Sahara, the table lands of Persia and Montgolia, the rocky flat of Arabia Petræ, &c.
425.How many rainy days are there in a year?
The frequency of rainy days is greatest in countries near the sea, and their number decreases the further we journey from the sea-border towards the inland. In England it rains on an average 152 to 155 days in the year.
426.In what part of England does the greatest amount of rain fall?
In the town ofKeswick, in Cumberland, where 63 inches of rain fall in a year; Kendal, in Westmoreland, 58 inches; Liverpool, 34 inches; Dublin, 25 inches; Lincoln, 24 inches; London, 21 inches.
427.Why do the heaviest rains occur at the tropics?
Because thehot airabsorbs a large amount of vapour, and rises into the higher regions of the atmosphere, where the vapours aresuddenly condensed into heavy rains, by cold currents from the poles.
428.Why does the greatest quantity of rain fall at the equator?
Because thehot airabsorbs a large amount of vapour, and as the atmosphere is usually calm, there is an absence of currents, by which the saturated air would be removed. In this, which is called "the Region of Calms," rain falls almost daily.
429.Why are some parts of the earth rainless?
Because, being situated in tropical or torrid latitudes, and at a distance from the ocean, the atmosphere above them is always in adry state.
"Thou, O God, didst send a plentiful rain, whereby thou didst confirm thine inheritance, when it was weary."—Psalm lxviii.
430.When is air said to be saturated with vapour?
When it cannot take upa larger quantitythan that which it already holds.
When common salt is dissolved in water, until the water can take up no more, the water is then said to besaturated with salt.
431.What proportion of water is air capable of sustaining in the form of vapour?
The amount of water held in suspension by the air averages the following proportion: one thousandcubic feet of aircontain as much vapour as, were it condensed to water, would yield abouttwo fifths of a pint.
Butone thousand cubic feet of airare capable of holdinghalf-a-pint of water; and this may be regarded as thepoint of saturation.
Thus, in a room ten feet square and ten feet high, the air,at the point of saturation, would hold in the form of vapour,half-a-pint of water. It must not be forgotten, however, that the point of saturation necessarily varies with thetemperature of the air.
432.Why are cloudy days and nights not always wet?
Because the air has not reached the state ofsaturation.
433.Why does rain purify the air?
Because it produces motion in the particles of the air, by which they areintermixed. And it precipitates noxiousvapours, and cleanses the face of the earth fromunhealthy accumulations.
434.Why are mountainous localities more rainy than flat ones?
Because the mountainsattract the clouds; and because the clouds that are flying low are borne against the sides of the mountains and directed upwards, where they meet withcold currents of air.
435.Why does more rain fall by night than by day?
Because by night the temperature of the air, heated during the day, falls to that degree which condensesits vapours into rain.
"As the hart panteth after the water brooks, so panteth my soul after thee O God."—Psalm xlii.
436.Why do bunches of dried sea-weed indicate the probability of coming rain?
Because they readily imbibe moisture, and when they become soft and damp they show that the air isapproaching the point of saturation.
437.Why does the weather-toy, called the "weather-cock," foretell the probability of rain?
Because it is made with a piece of cat-gut which swells with moisture, and as it swells,shrinks. The cat-gut is so applied that when itshrinks, it turns a rod which sends themanout of the house, and when itdriesit sends thewoman out. Therefore, when themanappears, it is a sign ofwet, and when thewomanappears it is a sign ofdry weather.
There is another toy, called the Capuchin, which is made upon the same principle. The figure lifts a hood over its head when wet is approaching, and takes it off when the weather is becoming dry. In this case, a piece of cat-gut is also employed. Various weather-toys may be made upon this principle—among others, a little umbrella, which will open on the approach of wet, and close on the return of fine weather.
A gentleman once made a wooden horse, which he declared should of itself walk across a room, without machinery of any kind. The assertion was discredited; but the horse was placed in a room close to the wall on one side. The room was locked, and otherwise fastened, so that no one could interfere with the experiment. After a time the door was opened, and it was found that the horse had actually crossed the floor, and stood on the opposite side. The horse was made from wood of a peculiar kind, liable to great expansion in wet weather, and cut in a manner to produce the greatest elongation. The fore hoofs were so made that where they were set they would remain, so that the contracting parts should draw up from behind. It is easy to understand how, in this way, the wooden horse crossed the apartment.
438.Why does ladies' hair drop out of curl upon the approach of damp weather?
Because the hairabsorbs moisture, which causes its spirals to relax and unfold.
439.Why is it said in mountainous countries that rain iscoming, because the mountains are "putting their night-caps on?"
Because the clouds descend when they areheavy with vapour, and being attracted to the mountain tops they are said to "cap the mountains."
"Hast thou entered into the treasures of the snow; or hast thou seen the treasures of the hail."—Job xxxviii.
440.What is snow?
Snow iscongealed vapour, which would have formedrain; but, through the coldness of the air, has beenfrozenin its descent intocrystalline forms. (Fig. 1.)
441.Why is snow white?
Because it reflects all the component rays oflight.
442.Why is snow said to be warm, while white garments are worn for coolness?
Snow iswarmby virtue of its light and woolly texture. But it is also warm on account of itswhiteness; for, had it beenblack, it would haveabsorbed the heat of the sun, which would havethawed the snow. Instead of which, itreflects heat; and the reflected heatfalls uponbodies above the snow, while thewarmth of the earthis preservedbeneath it.White clothing is cool, because it reflectsfromthe body of the wearer the heat of the sun.White snowiswarm, because itreflects the sun's heat upon bodies.
There are few persons but have felt the effect of the sun's raysreflectedby the white snow on a clear wintry day. And, as regards the warmth of snow towards the earth, by preventing the radiation of heat, it has been found that a thermometer buried four inches deep in snow has shown a temperature ofnine degreeshigher than at the surface.
443.Why are lofty mountains always covered with snow?
Because theupper regionsof the atmosphere areintensely cold.
444.Why are the upper regions of the atmosphere intensely cold?
Because theatmosphereretains butlittle of the heat of the sun'sraysas they pass to the earth. Because at high altitudes the air isgreatly rarefied. And because theradiation of heat from the earthdoes not materially affect suchhigh regions.
"He causeth the vapours to ascend from the ends of the earth: he maketh lightnings for the rain: he bringeth the wind out of his treasuries."—Ps. xxxv.
445.What is meant by the snow line?
Thesnow lineis the estimated altitude inall countrieswheresnow would be formed. Even at the equator, at an altitude of 15,000 to 16,000 feet from the level of the sea, snow is found upon the mountain summits, where it perpetually lies. As we proceed north or south from the equator thesnow line lessens in altitude. Had we in England a mountain 6,000 feet high, it would be perpetuallycrowned with snow.
446.Why do we hear of red snow?
Red snow is the name given to the snow in the arctic regions upon which a minute vegetable (probably theProtoccus nivalis) grows, imparting to the snow a red colour. Recent microscopic investigations have shown it to consist of a minute vegetable cell, which secretes a red colouring matter.
Snow is found to be of greater importance to man than is generally supposed. But, although in this country we are enabled to recognise the hand of Providence in the gift, there are latitudes wherein the blessing thus conferred is more deeply felt. In such countries as Canada, Sweden, and Russia, the falling of snow is looked for with glad anticipations, quite equalling those which herald the "harvest-home" of England, or the "vintage" of France. No sooner is the ground covered with snow, than cranky old vehicles that had been jolting over rough roads, and sticking fast in deep ruts of mud, are wheeled aside, and swift sledges take their place. Towns distant from each other find an easy mode of communication; the markets are enlivened, and trade thrives. Snow supplies a kind of railroad, covering the entire face of the country, and sledges glide over it, almost with the speed of the locomotive.
447.What is sleet?
Sleetis snow which, in falling, has met with awarmer current of airthan that in which it congealed. It therefore partially melts and forms a kind ofwet snow.
448.What is hail?
Hailis also thefrozen moisture of the clouds. It is probably formed byrain dropsin their descent to the earth, meeting with anexceedingly cold current of airby which they becomesuddenly frozen into hard masses.
It is also supposed that theelectricalstate of the air and of the clouds influences the formation ofhail.
"If the clouds be full of rain, they shall empty themselves upon the earth."—Eccles. xi.
449.Why is it supposed that the electrical state of the air and the clouds affects the formation of hail?
Because hail is more common in thesummerthan at other seasons, and is frequently attended by storms ofthunder and lightning.
450.Why do hail-storms most frequently occur by day?
Because the clouds, being charged with vapour to saturation, favour the formation of hail bysuddenelectrical or atmospheric changes. In the gradual cooling of night, the clouds would expend themselves in rain.
Astonishing facts respecting hail-storms are upon record. In 1719 there fell at Kremo, hailstones weighing six pounds. In 1828 there was a fall of ice at Horsley, in Staffordshire, some of the pieces of which were three inches long, by one inch broad; and other solid pieces were about three inches in circumference. Hail storms are most frequent in June and July, and least frequent in April and October. Hail clouds float much lower in the sky than other clouds; their edges are marked by frequent heavy folds; and their lower edges are streaked with white, the other portions being massive and black. (Fig. 10.)
451.What is light?
Light, according to Newton, is the effect of luminous particles which dart from the surfaces of bodies in all directions. According to this theory, the solar light which we receive woulddepart from the sun and travel to the earth.
According to Huyghens, light is caused by aninfinitely elastic ether, diffused through all space. This ether, existing everywhere, isexcited into waves, or vibrations, by the luminous body.
The theory of light is so undetermined that neither the views of Newton, nor those of Huyghens, can be said to be exclusively adopted. Writers upon natural philosophy seize hold of either or both of those theories, as they present themselves more or less favourably in the explanation of natural phenomena. In "The Reason Why,"as we have to speak of theeffectsof light rather than of itscause, we shall avoid, as far as possible, the doubtful points. But let noone be discouraged by the fact that the theory of light, as, indeed, of all the imponderable agents, is imperfectly understood. Rather let us rejoice that there are vast fields of discovery yet to be explored; and that light, the most glorious and inspiring element in nature, invites us from the sun, the moon, and the stars, and from the face of every green leaf and variegated flower, to search out the wonders of its nature, and further to exemplify the goodness and wisdom of God.
"And God said, Let there be light: and there was light."
452.What is the distance of the sun from the earth?
Ninety five millions of miles.
453.At what rate of velocity does light travel?
At the rate of 192,000 miles in asecond, through ouratmosphere; and 192,500 miles in asecondthrough avacuum.
454.How long does light take to travel from the sun to the earth?
Eight minutes and thirteen seconds.
455.What is the constitution of the sun?
It is a spherical body, 1,384,472 times larger than the earth.
456.From what does the luminosity of the sun arise?
From a luminous atmosphere, or, as M. Arago named it,photosphere, which completely surrounds the body of the sun, and which is probablyburning with great intensity.
457.What are the minor sources of light?
Light may be produced bychemical action, byelectricity, and byphosphoresence, in the latter of which various agencies unite.
458.What is a ray of light?
Arayof light is thesmallest portionof light which we can recognise.
459.What is a medium?
Amediumis a body which affordsa passage for the raysof light.
460.What is a beam of light?
Abeamof light is agroup of parallel rays.
461.What is a pencil of light?
Apencilof light is a body of rays whichcome from or move towards a point.
"And God saw the light, that it was good: and God divided the light from the darkness."—Gen. i.
462.What is the radiant point?
Theradiant pointis thatfrom which diverging rays of light are emitted.
463.What is the focus?
Thefocusis the point to whichconverging rays are directed.
Diverging, starting from a point, and separating.Converging, drawing together towards a point.
464.What is the constitution of a ray of light?
A ray ofwhite light, as we receive it from the sun, is composed ofa number of elementary rays, which, with the aid of a triangular piece of glass, called aprism, may be separated, and will produce under refraction the following colours:—
1. Anextreme redray—a mixture ofredandblue, the redpredominating.
2.Red.
3.Orange—red passing into and combining with yellow.
4.Yellow—the most luminous of all the rays.
5.Green—yellow passing into and combining with the blue.
6.Blue.
7.Indigo—a dark and intense blue.
8.Violet—blue mingled with red.
9.Lavender grey—a neutral tint.
10. Rays calledfluorescent, which are either of apure silvery blue, or adelicate green.
465.Why is a ray of light, which contains these elementary rays, white?
Because the colour of light is governed by therapidity of the vibrations of the ether-waves. When a ray of light is refracted by, or transmitted through a body, itsvibrations are frequently disturbed and altered, and thus adifferent impressionis made upon theeye.
Light which gives 37,640 vibrations inan inch, or 458,000,000,000,000 in asecond of time, produces that sensationupon the eye which makes the object that directs the vibrations appearred.Yellowlight requires 44,000 vibrationsin an inch, and 535,000,000,000,000 in asecond of time. And the other colours enumerated (see464) all require differentvelocities of vibrationto produce the colours by which they are distinguished.
"The light of the body is the eye: if therefore thine eye be single, thy whole body shall be full of light."—Matt. v.
Accepting the theory of vibrations, and applying it to the elucidation of the phenomena of light—it is unnecessary, we think, to believe that a ray ofwhitelightcontainsrays ina state of colour. It is said that if we divide a circular surface into parts, and paint the various colours in the order and proportions in which they occur in the refracted ray, and then spin the circle with great velocity, the colours will blend and appearwhite. But such is not the case; the result is in some degree an illusion, arising out of the sudden removal of the impression made upon the eye by the colours; and if a piece of white paper be held by the side of the coloured circle in motion, the latter will be found to begrey. When it is remembered that in colouring a white surface with thin colours, the white materially qualifies the colours, it must be admitted that the experiment fails to support the assertion that the colours of the spectrum produce white. But there can be no difficulty in understanding that a ray of light undergoingrefraction, becomes divided into minor rays, whichdiffering in their degrees of refrangibility, vary also in thevelocity of their vibrations, and produce the several sensations of colour.
466.Why is a substance white?
Because it reflects the light that falls upon itwithout altering its vibrations.
467.Why is a substance black?
Because itabsorbs the lightandputs an end to the vibrations.
468.Why is the rose red?
Because it imparts to the light that falls upon it thatchange in its vibratory condition, which produces on our eyes thesensation of redness.
469.Why is the lily white?
Because it reflects the light without altering its vibrations.
470.Why is the primrose yellow?
Because, though it receives white light, it alters its vibrations to 44,000 in an inch, and 535,000,000,000,000 in a second, and this is the velocity of vibration which produces upon the eye asensationofyellow.
"But if thine eye be evil, thy whole body shall be full of darkness. If therefore the light that is in thee be darkness, how great is that darkness."—Matt. v.
471.Why are there so many varieties of colour and tint in the various objects in nature?
Because every surface has a peculiar constitution, or atomic condition,by which the light falling upon it is influenced. In tropical climates, where the brightness of the sun is the most intense, there the colours of natural objects are the richest; the foliage is of the darkest green; the flowers and fruits present the brightest hues; and the plumage of the birds is of the most gaudy description. In the temperate climates these features are more subdued, still bearing relation to the degree of light. And at a certain depth of the ocean, where light penetrates only in a slight degree, the objects that abound are nearly colourless.
It has been held by many philosophers (and the theory is so far conclusive that it cannot be dispensed with) that there is an analogy between the vibratory causes ofsound, and the vibratory causes ofcolour. Any one who has seen an Æolian harp, and listened to the wild notes of its music, will be aware that the wires of the harp are swept by accidental currents of air; that when those currents have been strong, the notes of the harp have been raised to the highest pitch, and as the intensity of the currents has fallen, the musical sounds have deepened and softened, until, with melodious sighing, they have died away. No finger has touched the strings; no musical genius has presided at the harp to wake its inspiring sounds; but the vibration imparted to the air, as it swept the wires, has alone produced the chromatic sounds that have charmed the listener. If, then, the varied vibrations of theairare capable of imparting dissimilar sensations ofsoundsto theear, is it not only possible, but probable, that the different vibrations oflightmay impart the various sensations ofcoloursto theeye?