580. What is the cause of lightning?
Lightning is the result of electrical discharges from the clouds.
581. What develops electricity in the clouds?
Evaporations from the surface of the earth; changes of temperature in the atmospheric vapour; chemical action upon the earth's surface; and the friction of volumes of air of different densities against each other.
"His lightnings enlightened the world: the earth saw and trembled."—Psalm xcvii.
582. Why do these phenomena produce electricity?
Because they disturb the equilibrium of the electric force, and produce positive and negative states of electricity.
583. When does lightning occur?
When clouds, charged with the opposite electricities approach, the forces rush to each other, and combine in a state of equilibrium.
584. Why does lightning attend this movement of the forces of electricity?
Because the atmosphere, being unable to convey the great charges of electricity as they rush towards each other, acts as an insulator, and lightning is caused by the violence of the electricity in forcing its passage.
585. Does lightning ever occur when the conducting power is equal to the force of the electricity?
No; electricity passes invisibly, noiselessly, and harmlessly, whenever it finds a sufficient source of conduction.
CHAPTER XXVIII.
586. Why does lightning sometimes travel through a "zigzag" course?
Because the electricity, being resisted in its progress by the air, flies from side to side, to find the readiest passage.
587. Why does lightning sometimes appear forked?
Because, being resisted in its progress by the air, the electricity divides into two or more points, and seeks a passage in different directions.
588. Why is lightning sometimes like a lurid sheet?
Because the flash is distant, and therefore we see only the reflection.
"He directeth it under the whole heavens, and his lightning unto the ends of the earth."
589. When is the flash of lightning straight?
When the distance between the clouds whose electricities are meeting, is small.
590. What is the cause of the aurora borealis?
The mingling of the electricities of the higher regions of the atmosphere.
591. When does the flash of lightning appear blue?
When the degree of electrical excitement is intense, and general throughout the atmosphere.
592. Why does lightning sometimes appear red, at others yellow, and at others white?
Because of the varying humidity, which affects the refracting power of the atmosphere.
593. Does lightning ever pass upwards from the earth to the clouds?
Yes; when the earth is charged with a different electricity to that which is in the clouds.
594. Does lightning ever pass directly from the clouds to the earth?
Yes; when the electricity of the clouds seeks to combine with the different electricity of the earth.
The mingling of the electricities of the earth and the air must be continually going on. But lightning does not attend the phenomena, because all natural bodies, vapours, trees, animals, mountains, houses, rocks, &c., &c., act more or less as conductors between the earth and the air. It is only when there is a great disturbance of the electrical forces, that terrestrial lightning is developed. When lightning strikes the earth with great force, it sometimes produces what are called fulgurites in sandy soils; these are hollow tubes, produced by the melting of the soil.
595. What is the extent of mechanical force of lightning?
Lightning has been proved, in one instance, to have struck a church with a force equal to more than 12,000 horse-power. A single horse-power, in mechanical calculations, is equivalent to raising a weight of 32,000 lbs. one foot in a minute. The force of lightning, therefore, has been proved to be equal to the raising of 384,000,000 lbs. one foot in a minute. This is equal to the united power of twelve of our largest steamers, having collectively 24 engines of 500 horse-power each. The velocity of electricity is so great that it would travel round the world eight times in a minute.
"After it a voice roareth: he thundereth with the voice of his excellency; and he will not stay them when his voice is heard."—Job xxxvii.
The church alluded to was St. George's church, Leicester, a new edifice, which was completely destroyed on the 1st of August, 1846, by a thunder-storm. The steeple was rent asunder, and massive stones were hurled to a distance of thirty feet. The vane rod and top part of the spire fell down perpendicularly and carried with it all the floors of the tower. A similar disaster occurred to St. Bride's church, Fleet-street, London, about 100 years ago. The lightning first struck upon the metal vane of the steeple, and then ran down the rod and attacked the iron cramps, smashing the large stones that lay between them. The church was nearly destroyed. By the same wonderful force, ships have been disabled, trees split asunder, houses thrown down, and animals struck dead.
596. Why is it dangerous to stand near a tree during an electric storm?
Because the tree is a better conductor than air, and electricity would probably strike the tree, and then pass to the person standing near.
597. If trees are good conductors, why do they not convey the electricity to the ground?
Trees are only indifferent conductors, and the electricity would quit the tree to pass through any better conductor.
598. Why is it dangerous to sit near a fire during an electric storm?
Because the chimney, being a tall object, and smoke a good conductor, would probably attract the electricity, and convey it to the body of a person sitting near the fire.
599. Why is it dangerous to be near water during an electric storm?
Because water is a good conductor, and the vapour arising from it might attract the electricity. Man, being elevated over the water, might form the first point attacked by the electricity.
600. Are iron houses dangerous during an electric storm?
No; they are very safe, because their entire surface is a good conductor, and would convey the electricity harmlessly to the earth.
"To him that rideth upon the heavens of heavens, which were of old; lo, he doth send out his voice, and that a mighty voice."—Psalm lxviii.
601. Why does electricity seize upon bell wires and iron fastenings?
Because copper wires are the very best conductors of electricity; and iron articles are also good conductors.
602. Supposing electricity to attack a bell wire, where would the point of danger exist?
At the extremities of the wire, where the conducting power of the wire would cease, and the electricity would seek to find another conductor.
603. Are umbrellas, with steel frames, dangerous in an electric storm?
They are dangerous in some degree, because they might convey electricity to the hand, and then transfer it to the body. But, generally speaking, when it rains, the rain itself, being a good conductor, relieves the disturbance of electricity by conveying it to the ground.
604. Are iron bedsteads dangerous in electric storms?
No, they are safe, because the iron frame, completely surrounding the body, and having a great capacity for conduction, would keep the electricity away from the body.
605. Why is it safe to be in bed during an electric storm?
Because feathers, hair, wool, cotton, &c., especially when dry, are good insulators or non-conductors.
606. What is the safest situation to be in during an electric storm?
In the centre of a room, isolated as far as possible from surrounding objects; sitting on a chair, and avoiding handling any of the conducting substances. The windows and doors should be closed, to prevent drafts of air.
607. In the open air, what is the safest situation?
To keep aloof, as far as possible, from elevated structures; and regard the rain, though it might saturate our clothes, as a protection against the lightning stroke, for wet clothes would supply so good a conductor, that a large amount of electricity would pass over man's body, through wet garments, and he would be quite unconscious of it.
"God thundereth marvellously with his voice: great things doeth he, which we cannot comprehend."—Job xxxvi.
During a violent electric storm in the Shetland Islands, a fishing boat was attacked by the electric fluid, which tore the mast to shivers. A fisherman was sitting by the side of the mast at the time, but he felt no shock. Upon taking out his watch, however, he found that the electric current had actually fused his watch into a mass. In this case, it is more than probable that the man was saved through the saturation of his clothes with rain.
608. Do lightning conductors "attract" electricity?
Not unless the electric current lies in their vicinity.
609. Why have lightning conductors sometimes been found ineffective?
Because they have been unskilfully constructed; have been too small in their dimensions, and have not been properly laid to convey the electricity harmlessly away.
610. What is the best metal for a lightning conductor?
Copper, the conducting power of which is five times greater than that of iron.
611. Why should a large building have several conductors?
Because the influence of a conductor over the electricity of the surrounding air does not extend to more than a radius of double the height of the conductor above the building: for instance, a conductor rising ten feet high above the building would influence the electricity twenty feet all round the conductor.
612. Why should conductors have at their base several branches penetrating the earth?
To facilitate the discharge of the accumulated electricity into the earth.
613. Why does electricity affect the shapes of clouds?
Because electricity does not penetrate the masses of bodies, but affects generally their surfaces. Hence electricity exists in the surfaces of clouds, and in its efforts to find an equilibrium it causes the clouds to roll in heavy masses, having dark outlines.
"All ye inhabitants of the world, and dwellers on the earth, see ye, when he lifteth up an ensign on the mountains; and when he bloweth a trumpet, hear ye."—Isaiah xviii.
The fact that electricity resides in, and is conducted by, the surfaces of bodies, is well established, and should receive due attention in the protective measures adopted to secure life and property against the effects of lightning. A practical suggestion that arises out of this fact is, that tubes of copper would form far more efficient conductors than bars of the same metal. A copper tube, of half an inch diameter, would conduct nearly double the amount of electricity which could be conveyed away by a bar of copper of the same diameter. The upper extremity of the tube should be open obliquely, that the electric current might be induced to pass over both the inner and outer surfaces.
CHAPTER XXIX.
Thunder is the noise which succeeds the rush of the electrical fluid through the air.
615. Why does noise follow the commotion caused by electricity?
Because, by the violence of the electric force, vast fields of air are divided; great volumes of air are rarefied; and vapours are condensed, and thrown down as rain. Thunder is therefore caused by the vibrations of the air, as it collapses, and seeks to restore its own equilibrium.
616. Why is the thunder-peal sometimes loud and continuous?
Because the electrical discharge takes place near the hearer, and therefore the vibrations of the air are heard in their full power.
617. Why is the thunder-peal sometimes broken and unequal?
Because the electrical discharge takes place at a considerable distance, and the vibrations are affected in their course by mountains and valleys. Because, also, the forked arms of the lightning strike out in different directions, causing the sounds of thunder to reach us from varying distances.
"Lo, these are parts of his ways; but how little a portion is heard of him? but the thunder of his power who can understand?"—Job xxv.
618. Why has the thunder-peal sometimes a low grumbling noise?
Because the electrical discharges, though violent, take place far away, and the vibrations of the air become subdued.
619. Why does the thunder-peal sometimes follow immediately after the flash of lightning?
Because the discharge of electricity takes place near the hearer.
620. Why does the thunder-peal sometimes occur several seconds after the flash?
Because the discharge takes place far away, and light travels with a much greater velocity than sound.
621. Through what distance will the sound of thunder travel?
Some twenty or thirty miles, according to the direction of the wind, and the violence of the peal.
622. Through what distance will the light of lightning travel?
The light of lightning, and its reflections, will penetrate through a distance of from a hundred and fifty to two hundred miles.
623. How may we calculate the distance at which the electric discharge takes place?
Sound travels at the rate of a quarter of a mile in a second. If, therefore, the peal of thunder is heard four seconds after the flash of lightning, the discharge took place about a mile off. The pulse of an adult person beats about once in a second; therefore, guided by the pulse, any person may calculate the probable distance of the storm:—
2 beats, ½ a mile.
3 beats, ¾ of a mile.
4 beats, 1 mile.
5 beats, 1¼ miles.
6 beats, 1½ miles.
7 beats, 1¾ miles.
8 beats, 2 miles, &c.
Attention should be paid to the direction and speed of the wind, and some modifications of the calculation be made accordingly. Persons between 20 and 40 years of age should count five beats of the pulse to a mile; under 20, six beats.
"The clouds poured out water; the skies sent out a sound; thine arrows also went abroad."
624. Why are electric storms more frequent in hot than in cold weather?
Because of the greater evaporation, as the effect of heat; and also of the effect of heat upon the particles of all bodies.
625. Why do electric storms frequently occur after a duration of dry weather?
Because dry air, being a bad conductor, prevents the opposite electricities from finding their equilibrium.
626. Why is a flash of lightning generally succeeded by heavy rain?
Because the electrical discharge destroys the vescicles of the vapours. If a number of small soap-bubbles floating in the air were suddenly broken by a violent commotion of the atmosphere, the thin films of the bubbles would form drops of water, and fall like rain.
627. Why is an electrical discharge usually followed by a gust of wind?
Because the equilibrium of the atmosphere is disturbed by the heat and velocity of lightning, and the condensation of vapour. Air, therefore, rushes towards those parts where a degree of vacuity or rarefaction has been produced.
The name thunderbolt is applied to an electrical discharge, when the lightning appears to be developed with the greatest intensity around a nucleus, or centre, as though it contained a burning body. But there is, in reality, no such thing as a thunderbolt.
"The voice of the Lord is upon the waters: the God of glory thundereth; the Lord is upon many waters."—Psalm xxix.
629. Why do electric storms purify the air?
Because they restore the equilibrium of electricity which is essential to the salubrity of the atmosphere; they intermix the gases of the atmosphere, by agitation; they precipitate the vapours of the atmosphere, and with the precipitation of vapours, noxious exhalations are taken to the earth, where they become absorbed; they also contribute largely to the formation of ozone, which imparts to the air corrective and restorative properties.
Ozone is an atmospheric element recently discovered, and respecting which differences of opinion prevail. It is generally supposed to be oxygen in a state of great strength, constituting a variety of form or condition.
631. Why do we know that electricity contributes to the formation of ozone?
Because careful observations have established the fact that the proportion of ozone in the atmosphere is relative to the amount of electricity.
632. What are the properties of ozone?
It displays an extraordinary power in the neutralisation of putrefactions, rapidly and thoroughly counteracting noxious exhalations; it is the most powerful of all disinfectants.
Schonbien, the discoverer of ozone, inclines to the opinion that it is a new chemical element. Whatever it may be, there can be no doubt that it plays an important part in the economy of nature. Its absence has been marked by pestilential ravages, as in the cholera visitations; and to its excess are attributed epidemics, such as influenza. It was found, during the last visitation of cholera, that the fumigation of houses with sulphur had a remarkable efficacy in preventing the spread of the contagion. The combustion of sulphur ozonised the atmosphere; the same result occurs through the emission of phosphoric vapours; ozone is also developed by the electricity evolved by the electrical machine, and in the greater electrical phenomena of nature. The smell imparted to the air during an electric storm is identical with that which occurs in the vicinity of an electrical apparatus—it is a fresh and sulphurous odour. The opinion is gaining ground that the respiration of animals and the combustion of matter are sources of ozone, and that plants produce it when under the influence of the direct rays of the sun. It is also believed to be produced by water, when the sun's rays fall upon it. The most recent opinion respecting ozone is, that it is electrized oxygen. The subject is of vast importance, and opens another field of discovery to the pioneers of scientific truth.
"The voice of thy thunder was in the heaven: the lightnings lightened the world, the earth trembled and shook."—Psalm lxvii.
Magnetism is the electricity of the earth, and is characterised by the circulation of currents of electricity passing through the earth's surface.
634. What are magnetic bodies?
Magnetic bodies are those that exhibit phenomena which show that they are under the influence of terrestrial electricity, and which indicate the direction of the poles, or extreme points, of magnetic force.
Galvanism is the action of electricity upon animal bodies, and is so called from the name of its first discoverer, Galvani.
636. What is Voltaic electricity?
Voltaic electricity is the electricity that is developed during chemical changes, and is so called after Volta, who enlarged upon the theory of Galvani.
637. What are the differences between mechanical, or frictional electricity, Voltaic electricity, Galvanism, and magnetism?
Frictional electricity is electricity suddenly liberated under the effects of the motion, or the mechanical disturbance of bodies.
Voltaic electricity is a steady flow of an electric current, arising from the gradual changes of chemical phenomena.
Galvanism and Voltaism are almost identical, since the latter is founded upon, and is a development of, the former. But the term Galvanism is frequently used when speaking of the development of electricity in animal bodies.
Magnetism is the electricity of the earth, and is understood to imply the fixed electricity of terrestrial bodies.
"And I heard as it were the voice of a great multitude, and as the voice of many waters, and as the voice of mighty thunderings, saying Alleluia: for the Lord God omnipotent reigneth."—Rev. xix.
Man knows not what electricity is; yet, by an attentive observance of its effects, he avails himself of the power existing in an unknown source, and produces marvellous results. When the Grecian philosopher, Thales, sat rubbing a piece of amber, and watching the attraction of small particles of matter to its surface, he little knew of the mighty power that was then whispering to him its offer to serve mankind. And when Franklin, with the aid of a boy's plaything, drew down an electric current from the clouds, and caught a spark upon the knuckles of his hand, even he little conjectured that the time was so near when that strange element, which sent its messenger to him along the string of a kite, would become one of man's most submissive servants.
So many great results have sprung from the careful observation of the simplest phenomena, that we should never pass over inattentively the most trifling thing that offers itself to our examination. Nature, in her revelations, never seeks to startle mankind. The formation of a rock, and the elaboration of a truth, are alike the work of ages. It was the simple blackening of silver by the sun's rays which led to the discovery of the chemical agency of light. It was the falling of an apple which pointed Newton to the discovery of the laws of gravitation. It was the force of steam, observed as it issued from beneath the lid of a kettle, that led to the invention of the steam-engine. And it is said of Jacquard, that he invented the loom which so materially aided the commerce of nations, while watching the motions of his wife's fingers, as she plied her knitting. As great discoveries spring from such small beginnings, who among us may not be the herald of some great truth—the founder of some world-wide benefaction?
That the area of discovery has not perceptibly narrowed its limits, is evident from the fact that the greatest elements in nature are still mysteries to man. And though it may not be within the power of a finite being to unravel the chain of wonders that enfold the works of an infinite God,—still it is evident, from the progress which discovery has made, and from the good which discovery has done, that God does invite and encourage the human mind to contemplate the workings of Divine power, and to pursue its manifestations in every element, and in every direction.
The wonderful force of electricity astonishes us all the more when we view it in contrast with that equally wonderful element, light. We have seen that light travels with a velocity of 192,000 miles in a second, but that it falls upon a delicate balance so gently, that it produces no perceptible effect. As far as we know the nature of electricity, it is even more ethereal than light; yet, while the ether of light falls harmlessly and imperceptibly—even with the momentum of a flight of ninety-five millions of miles, the ether of electricity, bursting from a cloud only five hundred yards distant, will split massive stones, level tall towers with the dust, strike majestic trees to the ground, and instantly extinguish the life of man! Why does the one ether come divested of all mechanical force, while that which seems to be even more ethereal than it, is capable of exerting the mightiest force over material things? Does it not appear that the Creator of the universe has established these paradoxes of power to testify his Omnipotence—to show to man that with Him all things are possible; and that, in the grand cosmicism of the universe, every attribute of Omnipotence has been fulfilled?
"And the seventh angel poured out his vial into the air; and there came a great voice out of the temple of heaven, from the throne, saying, It is done."—Rev. xvi.
Let us now consider man's relation to this Omnipotence. He sees that electricity smites the tall edifice, and observes that in doing so it displays a choice of a certain substance through which it passes harmlessly, and that its violence is manifested only when its path is interrupted. Man, taking advantage of this preference of electricity for a particular conductor, stretches out an arm of that substance, and points it upwards to the clouds; electricity accepts the invitation, and passes harmlessly to the earth. But this not all: man learns by observation that electricity resides in all matter; that it may be collected or dispersed; that it travels along a good conductor at the rate of half-a-million of miles in a second of time; he constructs a battery, a kind of scientific fortress, in which he encamps the great warrior of nature; and then, laying down a conducting wire, he liberates the mighty force: but its flight must be on the path which man has defined, and its journey must cease at the terminus which man has decreed, where, by a simple contrivance of his ingenuity (the movements of a magnetic needle), the electric current is made to deliver whatever message of importance he desires to convey. Thus, the element which in an instant might deprive man of life, is subdued by him, and made the obedient messenger of his will.
CHAPTER XXX.
The atmosphere is the transparent and elastic body of mixed gases and vapours which envelopes our globe, and which derives its name from Greek words, signifying sphere of vapour.
639. To what height does the atmosphere extend?
It is estimated to extend to from forty to fifty miles above the surface of the earth.
640. Why is it supposed that the atmosphere does not extend beyond that height?
Because it is found, by experiment and observation, that the air becomes less dense in proportion to its altitude from the earth's surface. The gradual decrease of atmospheric density observed in ascending a mountain, or in a balloon, supplies sufficient data to enable us to calculate the height at which the atmosphere would probably altogether cease.
At an altitude of 18,000 feet the air is indicated by the barometer to be only half as dense as at the surface of the earth. And as the densities of the atmosphere decrease in a geometrical progression, the density will be reduced to one-fourth at the height of 36,000 feet; and to one-eighth at 54,000 feet. The effects of the decreasing density of the atmosphere are, that the intensity of light and sound are diminished, and the temperature is lowered. Persons who have reached a very high elevation, state that the sky above them began to assume the appearance of darkness; and there can be no doubt that, if it were possible to reach an altitude of some fifty to sixty miles, there would be perfect blackness although the sun's rays might be pouring through the darkened space, to illuminate the atmosphere. Upon the summit of Mont Blanc, the report of a pistol at a short distance can scarcely be heard. When Gay Lussac reached the height of 23,000 feet, he breathed with great pain and difficulty, and felt distressing sensations in his ears, as though they were about to burst. Upon the high table-lands of Peru, the lips of Dr. Ischudi cracked and burst; and blood flowed from his eyelids.
"For he looketh to the ends of the earth, and seeth under the whole heaven; To make the weight for the winds."—Job xxviii.
641. What is the amount of atmospheric pressure at the earth's surface?
The pressure of the atmosphere at the earth's surface is fifteen pounds to every square inch of surface. That is to say, that the column of air, extending fifty miles over a square inch of the earth, presses upon that square inch with a weight equal to fifteen pounds.
642. Is that the weight of dry or moist air?
That is the weight of air at what is called the point of saturation, when it is fully charged with watery vapour.
643. What is the proportion of watery vapour in the atmosphere?
The proportion constantly varies. Evaporation is not a result of accident; it seems an established law that the air shall constantly absorb vapour until it has reached the maximum that it can hold. Experiments have been tried, in which dry air has been pressed upon the surface of water with great force, but no degree of pressure could prevent the formation of vapour. (See 431.)
644. What is the total amount of atmospheric pressure on the earth's surface?
The total amount of atmospheric pressure on the earth's surface, at 15 lbs. to the square inch, amounts to 12,042,604,800,000,000,000 lbs. This pressure is equal to that of a globe of lead of sixty miles in diameter.
645. What is the pressure of the atmosphere upon the human body?
Estimating the surface of man's body to be equal to fifteen square feet, he sustains an atmospheric pressure of 32,400 lbs., or nearly fourteen tons and a-half. The mere variation of weight, arising out of the changes in the state of the atmosphere, may amount to as much as a ton and a-half.
"I therefore so run, not as uncertainly; so fight I, not as one that beateth the air."—Corinth. ix.
646. Why does not man feel this pressure?
Because the diffusion of air which, surrounding him in every direction, and acting upon the internal as well as the external surfaces of his body, and probably surrounding every atom of his frame, establishes an equilibrium, in which every degree of pressure counteracts and sustains itself.
647. What is the weight of air relative to that of water?
A cubic foot of air weighs only 523 grains, a little more than an ounce; a cubic foot of water weighs one thousand ounces.
648. What is the greatest height in the atmosphere which any human being has ever reached?
M. Gay Lussac, in the year 1804, ascended to the height of 23,000 feet.
A vacuum is a space devoid of matter. The term is generally applied to those instances in which air is drawn from within an air-tight vessel.
650. Is it possible to form a perfect vacuum?
It is probably impossible to do so, even with the most powerful instruments—some portion of air would remain, but in so thin a form that it would be imperceptible.
651. Why does the depression of a pump-handle cause the water to flow?
Because the putting down of the handle lifts up the piston with its valve closed, thereby tending to produce a vacuum; but the pressure of the air upon the water not contained in the pump, forces more water up into the part where a vacuum would otherwise be formed. Then, when the handle is raised, and the piston forced downwards, the valve opens, and the water rushes through.
There is a second valve, below the piston, which closes with the downward movement, to prevent the water from rushing back again.
"The wind bloweth where it listeth, and thou hearest the sound thereof, but canst not tell whence it cometh, and whither it goeth: so is every one that is born of the Spirit."—John ii., iii.
652. How high will atmospheric pressure raise water in the bore of a pump?
It will raise water to an elevation of thirty feet above its level.
653. Why will it raise water to an elevation of thirty-feet?
Because a column of water of thirty feet high, nearly balances the weight of a column of air of equal surface, extending to the whole height of the atmosphere. When, therefore, water is elevated to the height of thirty feet, the power of the pump is enfeebled, as the air and the water balance each other.
654. How is water raised to a greater elevation when it is required?
By mechanical contrivances, by which the water is forced to a greater elevation.
655. Why does water run through the bent tube called a syphon?
Because the atmospheric pressure upon the water on the outside of the syphon forces it into the tube as fast as the syphon empties itself through its longer arm.
656. Why does water run through the longer arm of the syphon?
Because the weight of the water in the longer arm of the syphon is greater than that in the shorter; therefore it runs out by its own gravity. And, as in running out, it creates a tendency towards a vacuum, the pressure of the outer air comes into operation, and forces the water through the tube.
657. Why does water issue from the earth in springs?
Some springs are caused by natural syphons formed in the fissures of rocks, which, communicating with bodies of water, are continually filled by atmospheric pressure, and therefore convey streams of water to the point where they are set free.
"Ascribe ye strength unto God: his excellency is over Israel, and his strength is in the clouds."—Psalm lviii.
658. Why, if a wine glass is filled with water, and a card laid upon it, and the whole inverted, will the water remain in the glass?
Because the pressure of the atmosphere upon the surface of the card counteracts the weight of the water.
659. What has the card to do with the experiment?
It forms a base upon which the water may rest, while the glass is being inverted; and it prevents the air from acting upon the fluidity of the water, and forcing it out of the glass.
660. Why will not beer run out of the tap of a cask until a spile has been driven in at the top?
Because the pressure of the air upon the opening of the tap counteracts the weight of the beer. But when the spile is driven in, the air enters at the top, and counteracts its own pressure at the bottom.
661. Why does a cup in a pie become filled with juice?
Because the heat expands the air, and drives nearly all of it out of the cup. When the pie is taken out of the oven, and begins to cool, air cannot get into the cup again, because its edges are surrounded by juice. A partial vacuum, therefore, exists within the cup, and the pressure of the external air forces the juice into it.
662. Does the cup prevent the juice from boiling over?
No. So long as the heat exists, the cup remains empty; and as it occupies space, the air is driven out of it, into the pie, it rather tends to force the juice over the sides of the dish. It is only when cooling that the juice enters the cup.
663. Why can flies walk on the ceiling?
Because their feet are so formed that they can form a vacuum, under them; their bodies are therefore sustained in opposition to gravitation by atmospheric pressure.
664. How did Mr. Sands perform the feat of walking across the ceiling?
By having large discs of wet leather attached to his feet, so that when they were placed upon a smooth surface, the air was excluded, and when he allowed his weight to act upon one of the discs, it formed a hollow cup and a vacuum. By forming a vacuum of only twelve square inches he gained a pressure of 180 lbs.; this being more than his weight he could accomplish the feat with no other difficulty than that of remaining in an inverted position. The air was admitted underneath the discs by valves, which were closed by springs, which being pressed by the heels of the performer, let in the air, and set the feet free.
"And God made a wind to pass over the earth."—Genesis viii.
665. Why is it difficult to strike limpets from rocks?
Because they have the means of forming a vacuum under their shells, and are pressed on to the rocks by the weight of the atmosphere.
666. Why can snails move over plants in an inverted position?
Because they form a vacuum with the smooth and moist surfaces of their bodies, and are supported by atmospheric pressure.
CHAPTER XXXI.
Wind is air in motion. (See 234.)
667. What are the velocities of winds?
A breeze travels ten feet in a second; a light gale, sixteen feet in a second; a stiff gale, twenty-four feet in a second; a violent squall, thirty-five feet in a second; storm wind, from forty-three to fifty-four in a second; hurricane of the temperate zone, sixty feet in a second; hurricane of the torrid zone, one hundred and twenty to three hundred feet in a second. When wind flies at one mile an hour, it is scarcely perceptible. When its velocity is one hundred miles an hour, it tears up trees, and devastates its track.
Trade winds are vast currents of air, which sweep round the globe over a belt of some 12,000 miles in width.
"They shall be as the morning cloud, and as the early dew that passeth away, as the chaff that is driven with the whirlwind out of the floor, and as the smoke out of the chimney."—Hosea xiii.
669. What is the cause of trade winds?
The air over the tropical regions becomes heated and ascends; it then diverges in two high currents, one towards the north, and the other towards the south pole, where, being cooled, it again descends, and returns towards the equator to replace the air as it ascends therefrom. There is, therefore, a constant revolution of vast currents of air between the tropics and the poles, producing north and south winds.
670. Why do the trade winds blow from east to west, though, in their origin, their direction is from north to south and from south to north?
Because, as the north and south winds blow towards the equator, they are affected by the revolution of the earth from west to east. As the two winds from the poles approach the equator, they are gradually diverted from their northerly and southerly course, to an easterly direction, by the revolution of the earth.
671. Why is there a prevalence of calms at the equator?
Because, as the north and the south winds move towards the equator, they drive before them volumes of atmosphere, which, meeting in opposite directions, resist and counterpoise each other, and abide in a state of stillness between the north and south-easterly winds, one on the north and the other on the south of the equator.
Monsoons are periodical winds which blow at a given period of the year from one quarter of the compass, and in another period of the year from the opposite quarter of the compass.
673. What is the cause of monsoons?
Monsoons are caused by changes in the position of the sun. When the sun is in the southern hemisphere, it produces a north-east wind, and when it is in the northern hemisphere, a north-west wind. The north-east monsoon blows from November to March, and the south-west monsoon from the end of April to the middle of October. The region of monsoons lies a little to the north of the northern border of the trade wind, and they blow with the greatest force, and with most regularity, between the eastern coast of Africa and Hindustan.
"He shall blow upon them and they shall wither, and the whirlwind shall take them away as stubble."—Isaiah xl.
674. What determines the character of winds?
The character of winds is influenced by the condition of the surfaces over which they blow. Winds blowing over dry and arid plains and deserts are dry and hot. Winds blowing across snow-capped mountains and regions of ice are cold. Winds that cross oceans are wet; and those that cross extensive continents are dry.
675. What winds are most prevalent in England?
In England out of a thousand days, north winds prevail in 82; north-east, 111; east, 99; south-east, 81; south, 111; south-west, 225; west, 171; north-west, 120.
676. What is the cause of storms?
Storms result from violent commotions of the atmosphere, and are chiefly the result of extreme changes of temperature.
The magnetic state of the earth, and the electrical state of the atmosphere, also materially influence the phenomena of storms.
By some persons the theory is entertained that storms result from various winds rushing into a centre in which the atmosphere has become extremely condensed. According to this theory, a storm is a mighty whirlwind.
A most violent hurricane occurred in 1780, which destroyed Lord Rodney's fleet, and a vast number of merchant ships. It is said to have killed 9,000 persons in Martinique alone, and 6,000 in St. Lucia. The town of St. Pierre in Martinique was totally destroyed; and only fourteen houses in the town of Kingston, in St. Vincent, were left uninjured.
677. Why do the most violent storms occur in and near the tropics?
Because there the temperature is very high, and the cold currents of air rushing towards the equator from the poles, causes great atmospheric disturbance.
Whirlwinds are produced by violent and contrary currents meeting and striking upon each other, producing a circular motion. They generally occur after long calms, attended by much heat.
Whirlwinds occurring at sea, or over the surface of water, sometimes put the water in motion, and as the wind rises upwards it lifts with it a whirling mass of water, producing a water spout.
"Out of the south cometh the whirlwind; and cold out of the north."—Job xxxvii.