If plates of glass be laid over grass-beds, as in the engraving Fig. 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 is radiated from 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 foliage acts 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, they absorb and bear away the atmospheric moisture.
353. Why are valleys and low places chiefly subject to dew?
Because the elevated lands around them prevent the disturbance of the air in which the moisture is held.
354. What bodies are most likely to be covered with dew?
All bodies that are good radiators of heat, such as wool, swansdown, grass, leaves of plants, wood, &c.
355. What bodies are likely to receive little dew?
All bad 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 formed most copiously in the mornings and evenings. But such is not the case. It is deposited at all hours of the night, but most plentifully after midnight.
357. Why is dew formed most plentifully after midnight?
Because, as radiation has been going on for some time, the temperature of the earth, and of various bodies upon it, has been considerably 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 defective radiating quality of the surface of water.
361. Why is a heavy dew regarded as the precursor of rain?
Because a heavy formation of dew indicates that the air is saturated with moisture.
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 are likely to condense, and produce showers.
Honey-dew is the name applied to a sweet and sticky moisture occasionally deposited upon the leaves of plants. It is, however, an error to call it dew, as it is procured by a class of insects termed aphides.
Fogs are clouds formed near the earth's surface; but London fogs are distinguished from clouds by the fact that they embrace in their vaporous folds the smoke and volatile matters imparted 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.
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 the electrical state of the atmosphere, but the theory of them is still a matter of doubt.
The term mist is generally applied to vapours that rise over marshy places, or the surfaces of water, 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 the land, 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 again expanded and dispersed by 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 they appear to be altogether dispelled; but at night, when the earth cools by radiation, the vapours near the earth again condense, and settle in the form 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 lying above, or a cold air lying below, might equally contribute to keep a fog near the surface of a particular part of the earth, until a flow of wind, or a fall 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.
CHAPTER XVIII.
Clouds are volumes of vapour, usually elevated to a considerable height.
From the evaporation of water at the earth's surface.
375. Why do we not see them ascend?
We do, sometimes, in the form of what we call mists, but generally the vapours that rise and contribute to the formation of clouds are so thin that they are invisible.
"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 become cooled in 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 into minute vesicles, which we may call vapour bubbles, and these, being warmed by the sun, are specifically lighter than the air.
Because, also, the lower parts of clouds do partially descend, but again becoming more rarefied by meeting with a warmer atmosphere, they again ascend, and are thus poised upon the air.
Because, also, there is always a degree of atmospheric motion upward, caused by the convection of heat from the earth's surface. And, although there must also be downward movements of the air to supply the place of that which has ascended, still the heat of 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 of specific character are said to fly at given altitudes, or to occupy certain ranges of altitude. We will give their probable altitudes when speaking of the specific clouds.
"Who giveth rain upon the earth, and sendeth waters upon the fields."—Job v.
379. How many descriptions of clouds are there?
There are seven.
1. The Cirrus (Fig. 5), estimated range of altitude from 10,000 to 24,000 feet.
2. The Cumulus (Fig. 7), from 3,000 to 10,000 feet.
3. The Stratus, an extended continuous level sheet of cloud, increasing from beneath. They fly very low.
4. The Nimbus (Fig. 10), 1,500 to 5,000 feet.
5. The Cirro-cumulus (Fig. 4), from 3,000 to 20,000 feet.
6. The Cirro-stratus (Fig. 6), from 5,000 to 10,000 feet.
7. The Cumulo-stratus (Fig. 9), from 3,000 to 10,000 feet.
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 the Nimbus, or thunder cloud, 1,500 feet, to that of the Cirrus, 24,000 feet, the others being intermediate. The first three of the clouds above enumerated constitute what are called the primary forms. The remaining four are called secondary forms, because they arise, as their names generally indicate, out of combinations of the primary 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 the atmosphere.
2. The electrical condition of the clouds.
3. The movements of the atmosphere.
4. The season 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 as twenty square miles of surface, and to be above a mile in thickness, while others are no larger than a house, or a man's hand.
382. How are clouds affected by winds?
If cold winds blow upon the clouds, the cold condenses the vapour, turning the clouds into rain. But if warm dry winds blow upon the clouds, they rarefy the vapour to a greater degree, and temporarily disperse the clouds.
383. How do winds affect the shapes of clouds?
When winds are mild and gentle, the clouds break into small patches, and rise to a considerable height. But when the winds are cold and blustering, the clouds fly low, and roll along in heavy 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 with vapours.
385. Why do west winds generally bring rain?
Because they come across the Atlantic, and are heavily charged with vapour.
386. Why are north winds generally cold and dry?
Because they come from the arctic ocean, over vast areas of ice 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 the hot earth and sands, and as they cross the sea they absorb a large amount of vapour.
388. Why are clouds said to indicate the changes of the weather?
Because, as it is the state of the clouds that, to a great extent, determines the state of the weather, the formation of the clouds must predicate approaching changes.
389. What do cirrus clouds foretell?
Cirrus clouds foretell fine weather, when they fly high, and are thin and light.
They foretell light rain when, after a long continuance of fine weather, they form fleecy lines stretched across the sky.
They foretell a gale of wind when, 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?
Cumulus clouds, when they are well defined, and advance with the wind, foretell fine weather.
When they are thin and dull, and float against the wind, or in opposition to the lower currents, they foretell rain.
When they increase in size, and become dull and grey at sunset, they predict a thunder-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?
Stratus clouds foretell damp and cheerless weather.
392. What do nimbus clouds foretell?
Nimbus clouds foretell rain, storm, and thunder. (Fig. 10.)
393. What do cirro-cumulus clouds foretell?
Cirro-cumulus clouds, in summer, foretell increasing heat attended by mild rain, and a south wind; but in winter they commonly precede the breaking up of a frost, and the setting in of foggy and wet weather. (Fig. 4.)
394. What do cirro-stratus clouds foretell?
Cirro-stratus clouds foretell rain or snow, 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 of bad weather. (Fig. 6.)
395. What do cumulo-stratus clouds foretell?
Cumulo-stratus clouds usually foretell a change of weather—from rain to fine, or from fine to rain. (Fig. 9.)
"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." * * *
CHAPTER XIX.
396. Why are cloudy days colder than sunny days?
Because the clouds intercept the solar rays in their course towards the earth.
397. Why are cloudy nights warmer than cloudless nights?
Because the clouds radiate back to the earth the heat which the earth evolves?
Because, also, the clouds radiate to the earth the heat they have derived from the solar rays during a cloudy day.
398. Why is the earth warmer than the air during sunshine?
Because the earth freely absorbs the heat of the solar rays; but the air derives comparatively little heat from the same source.
399. Why does the earth become colder than the air after sunset?
Because the earth parts 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 the radiation 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 from both their surfaces. A piece of glass, laid horizontally over the earth, would radiate heat both upwards and downwards. But on its lower surface it would receive the radiated heat of the earth, while from its upper surface it would throw off its own heat and become cool. Therefore dew would be deposited upon the upper, but not on the under surface.
402. Why does dew rest upon the upper surfaces of leaves?
Because the under surfaces receive the radiated 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 thus its 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 a bad radiator, but grass is a good radiator.
405. What benefit results from this arrangement?
In cultivated lands, where moisture is required, it is induced by the very necessity which demands it; while in rocky and barren places, where it would be of no good, dew does 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 degree counteract the radiation of heat from the earth; and they also radiate heat from their own substances.
407. Why do heavy morning dews and mists usually come together?
Because they both have their origin in the humidity 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 air has formed a screen over the surface of the earth, which has checked the 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 mist continues 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, and contribute to the formation of clouds, indirectly aid the formation of dew through the formation of clouds, and also by the moisture 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 varying temperature, motion, and direction of the atmosphere.
A warm light atmosphere, of a few day's duration, will elevate the vapours to the region where they are formed into clouds.
A chill air, lying upon the surface of the warmer earth, will occasion mists or fogs.
A cold earth, acting upon the vapours contained in a warmer atmosphere, will condense them and occasion dews.
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 become frozen in the form of hoar-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 great clearness to the nights.
412. Why are hoar-frosts, or, as they are termed, "white frosts," so frequent, and "black frosts" so unusual?
Because white, or hoar frosts, result from the coldness of the earth, which, from its great radiating power, is always varying. But black-frosts result from the coldness of the air, which is liable to less variation of temperature than the earth.
A black-frost results from the coldness of the atmosphere, which is at the time overshadowed by a dull cloud, giving a darkness to everything, and a leaden appearance to the frozen 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 thereof will 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 the radiation of heat, we should be subjected to the most unequal temperatures. The setting of the sun would be like the going out of a mighty fire. The earth would become suddenly cold, and its inhabitants would have to bury themselves in warm covering, to wait the return of day. By the radiation of heat, an equilibrium of temperature is provided for, without which we should require a new order of existence.
The amount of heat which our earth receives from the sun, and the economy of that heat by the laws of radiation, reflection, absorption, and convection, 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 the annihilation 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 would melt 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!
CHAPTER XX.
Rain is the vapour of the clouds which, being condensed by a fall of temperature, forms drops of water that descend to the earth.
It is the return to the earth in the form of water, of the moisture absorbed by the air in the form of vapour.
417. Does rain ever occur without clouds?
It sometimes, but rarely happens, that a sudden transition from warmth to cold will precipitate the moisture of the air, without the formation of visible 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 gather moisture in their descent. The greater the height from which a rain drop has descended, the larger it is, provided that its whole course lay through a rainy atmosphere.
The size of the drops is also influenced by the amount of moisture in the atmosphere, the degree of cold, and the rapidity of the change of temperature, by which the drops are produced.
419. In what seasons of the year are rains most prevalent?
Throughout Central Europe rains are most prevalent in summer, but in Southern Europe the preponderance is on the side of winter rains.
420. In what months of the year does it rain most frequently in this country?
It rains more frequently from September to March, than from March to September; but the heaviest rains occur from March 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 which precipitates 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 greatest quantity of rain falls near the equator, and the amount decreases 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?
The heaviest rains occur in the tropics, 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 a smarting sensation.
424. In what parts of the world do the least rains occur?
There are some parts of the earth which are rainless, 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 of Keswick, 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 the hot air absorbs a large amount of vapour, and rises into the higher regions of the atmosphere, where the vapours are suddenly condensed into heavy rains, by cold currents from the poles.
428. Why does the greatest quantity of rain fall at the equator?
Because the hot air absorbs 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 a dry 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 up a larger quantity than 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 be saturated 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 thousand cubic feet of air contain as much vapour as, were it condensed to water, would yield about two fifths of a pint.
But one thousand cubic feet of air are capable of holding half-a-pint of water; and this may be regarded as the point 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 the temperature of the air.
432. Why are cloudy days and nights not always wet?
Because the air has not reached the state of saturation.
433. Why does rain purify the air?
Because it produces motion in the particles of the air, by which they are intermixed. And it precipitates noxious vapours, and cleanses the face of the earth from unhealthy accumulations.
434. Why are mountainous localities more rainy than flat ones?
Because the mountains attract the clouds; and because the clouds that are flying low are borne against the sides of the mountains and directed upwards, where they meet with cold 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 condenses its 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 is approaching 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 it shrinks, it turns a rod which sends the man out of the house, and when it dries it sends the woman out. Therefore, when the man appears, it is a sign of wet, and when the woman appears it is a sign of dry 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 hair absorbs moisture, which causes its spirals to relax and unfold.
439. Why is it said in mountainous countries that rain is coming, because the mountains are "putting their night-caps on?"
Because the clouds descend when they are heavy 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.
CHAPTER XXI.
Snow is congealed vapour, which would have formed rain; but, through the coldness of the air, has been frozen in its descent into crystalline forms. (Fig. 1.)
Because it reflects all the component rays of light.
442. Why is snow said to be warm, while white garments are worn for coolness?
Snow is warm by virtue of its light and woolly texture. But it is also warm on account of its whiteness; for, had it been black, it would have absorbed the heat of the sun, which would have thawed the snow. Instead of which, it reflects heat; and the reflected heat falls upon bodies above the snow, while the warmth of the earth is preserved beneath it. White clothing is cool, because it reflects from the body of the wearer the heat of the sun. White snow is warm, because it reflects the sun's heat upon bodies.
There are few persons but have felt the effect of the sun's rays reflected by 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 of nine degrees higher than at the surface.
443. Why are lofty mountains always covered with snow?
Because the upper regions of the atmosphere are intensely cold.
444. Why are the upper regions of the atmosphere intensely cold?
Because the atmosphere retains but little of the heat of the sun's rays as they pass to the earth. Because at high altitudes the air is greatly rarefied. And because the radiation of heat from the earth does not materially affect such high 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?
The snow line is the estimated altitude in all countries where snow 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 the snow line lessens in altitude. Had we in England a mountain 6,000 feet high, it would be perpetually crowned 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 the Protoccus 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.
Sleet is snow which, in falling, has met with a warmer current of air than that in which it congealed. It therefore partially melts and forms a kind of wet snow.
Hail is also the frozen moisture of the clouds. It is probably formed by rain drops in their descent to the earth, meeting with an exceedingly cold current of air by which they become suddenly frozen into hard masses.
It is also supposed that the electrical state of the air and of the clouds influences the formation of hail.
"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 the summer than at other seasons, and is frequently attended by storms of thunder 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 by sudden electrical 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.)
CHAPTER XXII.
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 would depart from the sun and travel to the earth.
According to Huyghens, light is caused by an infinitely elastic ether, diffused through all space. This ether, existing everywhere, is excited 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 the effects of light rather than of its cause, we shall avoid, as far as possible, the doubtful points. But let no one 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 a second, through our atmosphere; and 192,500 miles in a second through a vacuum.
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 probably burning with great intensity.
457. What are the minor sources of light?
Light may be produced by chemical action, by electricity, and by phosphoresence, in the latter of which various agencies unite.
A ray of light is the smallest portion of light which we can recognise.
A medium is a body which affords a passage for the rays of light.
A beam of light is a group of parallel rays.
461. What is a pencil of light?
A pencil of light is a body of rays which come 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?
The radiant point is that from which diverging rays of light are emitted.
The focus is the point to which converging 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 of white light, as we receive it from the sun, is composed of a number of elementary rays, which, with the aid of a triangular piece of glass, called a prism, may be separated, and will produce under refraction the following colours:—
1. An extreme red ray—a mixture of red and blue, the red predominating.
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 called fluorescent, which are either of a pure silvery blue, or a delicate green.
465. Why is a ray of light, which contains these elementary rays, white?
Because the colour of light is governed by the rapidity of the vibrations of the ether-waves. When a ray of light is refracted by, or transmitted through a body, its vibrations are frequently disturbed and altered, and thus a different impression is made upon the eye.
Light which gives 37,640 vibrations in an inch, or 458,000,000,000,000 in a second of time, produces that sensation upon the eye which makes the object that directs the vibrations appear red. Yellow light requires 44,000 vibrations in an inch, and 535,000,000,000,000 in a second of time. And the other colours enumerated (see 464) all require different velocities of vibration to 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 of white light contains rays in a 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 appear white. 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 be grey. 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 undergoing refraction, becomes divided into minor rays, which differing in their degrees of refrangibility, vary also in the velocity of their vibrations, and produce the several sensations of colour.
466. Why is a substance white?
Because it reflects the light that falls upon it without altering its vibrations.
467. Why is a substance black?
Because it absorbs the light and puts an end to the vibrations.
Because it imparts to the light that falls upon it that change in its vibratory condition, which produces on our eyes the sensation of redness.
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 a sensation of yellow.
"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 of sound, and the vibratory causes of colour. 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 the air are capable of imparting dissimilar sensations of sounds to the ear, is it not only possible, but probable, that the different vibrations of light may impart the various sensations of colours to the eye?