Читать книгу A Guide to the Scientific Knowledge of Things Familiar - Ebenezer Cobham Brewer - Страница 8
CHAPTER II.
ОглавлениеQ. What is the second chief source of heat?
A. Electricity.
Q. What is lightning?
A. Lightning is only an Electric Spark, taken from the clouds.
Q. What causes the discharge of an electric cloud?
A. When a cloud, overcharged with electric fluid, approaches another which is under-charged, the fluid rushes from the former into the latter, till both have the same quantity.
Q. Is there any other cause of lightning, besides the one just mentioned?
A. Yes; sometimes mountains, trees, and steeples, will discharge a lightning cloud floating near; and sometimes electric fluid rushes out of the earth, into the clouds.
Q. What produces electricity in the clouds?
A. 1st—The evaporation from the earth’s surface.
2ndly—The chemical changes perpetually going on: and
3rdly—Currents of air of unequal temperature, excite electricity by friction, as they pass by each other.
Q. How high are the lightning-clouds from the earth?
A. Electrical clouds are the lowest of all clouds; they are rarely more than 700 yards above the ground; and sometimes, they actually touch the earth with one of their edges.
Q. How high are the clouds generally?
A. In a fine day, the clouds are often 4 or 5 miles above our head; but the average height of the clouds is from 1–½ to 2 miles.
Q. Why is lightning sometimes forked?
A. When the lightning-cloud is a long way off, the resistance of the air is so great, that the electrical current is diverted into a zig-zag course.
Q. Why does the resistance of the air make the lightning zig-zag?
A. As the lightning condenses the air, in the immediate advance of its path; it keeps flying from side to side, in order to pass where there is the least resistance.
Q. How does lightning condense the air in the immediate advance of its path?
A. The air is condensed by the rapidity of the lightning-flash.
Q. Why is forked lightning more dangerous than a straight flash?
A. Whatever resists the flash, diverts its course; and when terrestrial objects offer resistance to the current, they are in great danger of being destroyed.
Q. Why are there sometimes two flashes of forked lightning at the same moment?
A. Sometimes (in very severe storms) a flash of lightning will divide into two or more parts; and then each branch assumes the zig-zag form.
Q. Why is the flash sometimes quite straight?
A. When the lightning-cloud hovers near the earth, as the flash meets with very little resistance, it is not diverted; or (in other words) the flash is straight.
Q. What is the cause of sheet lightning?
A. It is only the reflection of distant flashes, not distinctly visible: and sometimes several flashes (from different clouds) intermingle, and form one vast blaze or sheet of lightning.
Q. Which form of lightning is the most dangerous?
A. The ball of fire is by far the most dangerous; and the zig-zag lightning is next in danger. Sheet lightning is not often attended with danger.
Q. Why are balls of fire so very dangerous?
A. Because (whenever they fall) much mischief is occasioned by their bursting, which they always do, with an explosion like that of a cannon.
Q. Do these balls of lightning ever run along the ground?
A. Yes; they often run a considerable way along the ground, then stop for a little time, and burst in numberless pieces: sometimes each of these pieces will explode; and at other times, the whole ball will burst at once, producing most mischievous consequences.
Q. What mischief will these balls of fire produce?
A. They will set houses and barns on fire; and kill all cattle and human beings, which happen to be in their course.
Q. Why does lightning sometimes kill men and beasts?
A. When the electric current passes through a man or beast, it produces so violent an action upon the nerves, that it destroys life.
Q. When is a person struck dead by lightning?
A. Only when his body forms a part of the lightning’s path: i.e. when the electric fluid (in its way to the earth) actually passes through his body.
Q. Why are men sometimes maimed by lightning?
A. Because lightning strikes with amazing force, whatever opposes it: and if a man stand in the way, it strikes him such a blow, as to maim him.
Q. What is thunder?
A. Lightning parts the air through which it passes; and when the parted air closes again, the noise made by the concussion, is called Thunder.
Q. Why does lightning part the air through which it passes? It does not part a rod of iron.
A. Iron is a conductor, and therefore allows the fluid to go freely through it: but air being a non-conductor, resists the lightning; which, therefore, rips it open, in order to pass through it.
Q. Why is thunder sometimes one vast crash?
A. When the lightning-cloud is near the earth, as the flash is straight—the whole volume of air (through which it passes) collapses at once; and produces one unbroken sudden crash.
Q. What is meant by the air collapsing?
A. When the rent air closes again, it is said to collapse.
Q. Why is the peal sometimes an irregular mangling broken roar?
A. When the lightning-cloud is a long way off, as the flash is zigzag, the air does not collapse all at once; and as we hear the concussion of one part after another, the peal is broken, protracted, and irregular.
Q. Which part of the collapsing air do we hear first?
A. That part nearest the earth; then the strata above; and last of all, that in the immediate vicinity of the cloud.
Q. What is meant by “strata of air?”
A. If a board were laid upon the earth, and several other boards were piled upon it, this pile would represent strata of wood.
Q. How does this illustration apply to the air?
A. A layer of air covers the earth; another layer rests upon it; and thus layer is piled upon layer, for 50 miles in height. Each layer is a “stratum” of air; and the plural of stratum is strata.
Q. Why do we hear the collapsing of the air nearest the earth first?
A. Because sound takes a whole second of time to travel 380 yards; but the air is ripped from top to bottom instantaneously: if, therefore, the cloud were 1000 yards off, we should hear the collapsing of the lowest strata nearly three seconds, before we heard that in the immediate vicinity of the cloud.
Q. Why is the thunder sometimes like a deep growl?
A. When the storm is far distant, the thunder sounds like a deep growl.
Q. Does not scenery affect the sound of thunder?
A. Yes; the flatter the country, the more unbroken the peal: Mountain scenery breaks the peal, and makes it harsh and irregular.
Q. What is the cause of rolling thunder?
A. The rolling is produced by the reverberation of the thunder along the massive clouds.
Q. What is meant by the reverberation?
A. The echo.
Q. Why is a flash of lightning generally followed by a pouring rain?
A. The cloud collapses, as soon as the electric fluid has left it; and the water it contained is squeezed out.
Q. Why is a flash of lightning generally followed by a gust of wind?
A. The flash rent the air asunder through which it darted; and when the two parts collapse, a rapid motion is produced, which we call wind: the vibration of the thunder contributes also to agitate the air.
Q. What is meant by the “vibration of the thunder?”
A. The quivering motion it gives to the air, by its loud sound.
Q. Why is there no thunder to what is called summer lightning?
A. Because the lightning-clouds are so far off, that the sound of the thunder is lost, before it reaches the earth.
Q. Do thunder-bolts ever drop from the clouds?
A. No; the notion of thunder-bolts falling from the clouds, arises from the globular form, that is sometimes assumed by a flash of lightning.
Q. Why is the thunder often several moments after the flash?[1]
A. The flash travels nearly a million times faster than the thunder; if, therefore, the thunder has far to come, it will not reach the earth till a considerable time after the flash.
[1] The speed of lightning is so great, that it would go 480 times round the earth in one minute: whereas, thunder would go scarcely 13 miles in the same space of time.
Q. Can we not tell the distance of a thunder-cloud, by observing the interval which elapses between the flash and the peal?
A. Yes; the flash is instantaneous, but the thunder will take a whole second of time to travel 380 yards: hence, if the flash is 5 seconds before the thunder, the cloud is 1900 yards off.
(i.e. 380 × 5 = 1900 yards.)
Q. What places are most dangerous to be in, during a storm?
A. It is very dangerous to be near a tree, or lofty building; it is dangerous also, to be near a river, or any running water.
Q. Why is it dangerous to be near a tree, or lofty building, during a thunder-storm?
A. Because a tall pointed object, (like a tree or spire,) will frequently discharge a lightning-cloud; and then the electric fluid will pass down it, in its way to the earth.
Q. How can a tree or spire discharge a lightning-cloud?
A. A lightning-cloud (floating over a plain) may be too far off to be discharged by it; but as a tree, or spire, would shorten the distance between the cloud and its conductor, it might no longer be too far off a conductor to be discharged.
Q. Is not air a conductor of lightning?
A. No; dry air is not a conductor of lightning; and therefore, the flash rends it in twain, to get to some conductor.
Q. Why would it be dangerous to stand near a tree or spire, while lightning is passing down it?
A. Because the electric fluid (called lightning) always rushes down the outside of the tree or spire; and if any one were standing near, might pass through him, and kill or maim him.
Q. Does lightning go through the inside or outside of a tree?
A. It rolls down the outside of a tree; but passes through the inside of a man.
Q. Why does lightning pass down the outside of a tree?
A. Lightning always makes choice of the best conductors; and the outside of a tree is a better conductor than the inside.
Q. Why does lightning pass through the inside of a man?
A. As the fluids of the human body make a better conductor than the skin, therefore lightning passes through a man, and not down the skin.
Q. Why is it dangerous to be near a deep river, or any other running water, during a thunder-storm?
A. Because running water is a good conductor; and lightning always takes in its course the best conductors.
Q. Why is it dangerous for a man to be near water, in a thunder-storm?
A. Because the height of a man may be sufficient to discharge a cloud: and (if there were no taller object nigh) the lightning might make the man its conductor to the water.
Q. Why is it dangerous to ring church-bells during a thunder-storm?
A. For two reasons: 1st—Because the steeple may discharge the lightning-cloud, in consequence of its mere height.
2ndly—The swinging of the bells causes a current of air, which collects electric fluid.
Q. Why is it unsafe to run or drive fast during a thunder-storm?
A. The rapid motion of running causes a current of air, which collects electric fluid, and is often fatal.
Q. What parts of a dwelling are most dangerous during a thunder-storm?
A. The fire-place, (especially if the fire be lighted); the attics and cellar. It is also dangerous to sit close by the walls; to ring the bell; or to bar the shutters, during a thunder-storm.
Q. Why is it dangerous to sit before a fire, during a thunder-storm?
A. Because the heated air and soot are conductors of lightning; especially when connected with such excellent conductors as the stove, fender, and fire-irons.
Q. Why are the attics and cellar dangerous, during a thunder-storm?
A. Lightning sometimes passes from the clouds to the earth, and sometimes from the earth to the clouds; and therefore, the middle story of a house is always the safest to be in, during a thunder-storm.
Q. When does lightning pass from the earth to the clouds?
A. When the clouds are in a “negative” state of electricity.
Q. When does lightning pass from the clouds to the earth?
A. When the clouds are in a “positive” state of electricity.
Q. What is meant by the clouds being in a “positive state of electricity?”
A. When the clouds contain more electric fluid than they generally do, they are said to be in a positive state of electricity.
Q. What is meant by the clouds being in a “negative state of electricity?”
A. When the clouds contain less electric fluid than they ought to do, they are said to be in a negative state of electricity.
Q. Does the flash proceed from a negative or positive body?
A. Always from a positive body, or one over-burdened with electric fluid.
Q. When lightning flashes from the earth to the clouds, what is the flash called?
A. It is called the “returning stroke;” because the earth (being over-burdened with electric fluid) returns the surplus quantity to the clouds.
Q. Why is it dangerous to lean back against a wall during a thunder-storm?
A. Because the electric fluid sometimes runs down the wall of a house or room; and (as a man is a better conductor than a brick wall), would make him its path, and injure him.
Q. Why is it dangerous to ring a bell during a thunder-storm?
A. Bell-wire is an excellent conductor; and (if a person were to touch the bell-handle), the electric fluid, passing down the wire, might run through his hand and injure it.
Q. Why would the lightning run through a man touching a bell-handle?
A. Because the human body is a better conductor than the wall (between the bell-handle and the floor); and as lightning always chooses the best conductors for its path, it would (in this case) pass through the man, and injure him.
