Читать книгу A Guide to the Scientific Knowledge of Things Familiar - Ebenezer Cobham Brewer - Страница 9

Q. Why is it dangerous to bar a shutter during a thunder-storm?

A. The iron shutter-bar is an excellent conductor; and (if a person were touching the bar), the electric fluid passing down it, might run from the bar through the person touching it, and injure him.

Q. Why is it dangerous to be in a crowd during a thunder-storm?

A. For two reasons. 1st—Because a mass of people form a better conductor than an individual: and

2ndly—The vapour from a crowd increases the danger of such a place.

Q. Why is a mass of bodies a better conductor than a single body?

A. Each living body is a conductor of electricity; and a connected mass of such conductors is more likely to be struck, than a single individual.

Q. Why is the danger increased by the vapour which rises from a crowd?

A. Vapour is a conductor, and therefore, may determine the shock; especially when connected with so many living bodies.

Q. Why is a theatre dangerous, during a thunder-storm?

A. Because the crowd assembled there, and the great vapour arising from so many living bodies, render a theatre an excellent conductor of lightning.

Q. Why is a flock of sheep in greater danger than a smaller number?

A. Because each sheep is a conductor of lightning, and the greater the number, the better its conducting power; besides, the vapour arising from a flock of sheep increases its conducting power, and its danger.

Q. Why is a herd of cattle in danger during a storm?

A. 1st—The number of living bodies increases the conducting power of the animal fluids: and

2ndly—The vapour arising from a herd is also a good conductor.

Q. If a person be abroad in a thunder-storm, what place is the safest?

A. Any spot about 20 or 30 feet from some tall tree or building; unless that spot be near to running water.

Q. Why would it be safe to stand 20 or 30 feet from some tall tree, in a thunder-storm?

A. Because the lightning would always choose the tall tree as a conductor, rather than the shorter man; and he would not be sufficiently near the tree, to be injured by the electric current passing down it.

Q. If a person be in a carriage in a thunder-storm, in what way can he travel most safely?

A. He should not lean against the carriage; but sit upright, without touching any of the four sides.

Q. Why should not a person lean against the carriage in a storm?

A. Because the electric fluid might run down the sides of the carriage; and (if a person were leaning against the sides), would make choice of him for a conductor, and perhaps destroy life.

Q. If a person be in a house during a thunder storm, what place is safest?

A. Any room in the middle story. The middle of the room is best; especially if you place yourself on a mattrass, bed, or hearth-rug.

Q. Why is the middle story of a house safest in a thunder-storm?

A. Because (even if the fluid struck the house), its strength would be exhausted before it reached the middle story.

Q. Why is the middle of the room more safe, than any other part of it, in a thunder-storm?

A. Because, if the lightning came into the room at all, it would come down the chimney or walls of the room; and therefore, the further distant from these, the better.

Q. Why is a mattrass bed, or hearth-rug a good security against injury from lightning?

A. Because they are all non-conductors; and, as lightning always takes in its course the best conductors, it would not select such things as these.

Q. Is it better to be wet or dry during a storm?

A. To be wet: if a person be in the open field, the best thing he can do, is to stand about 20 feet from some tree, and get completely drenched to the skin.

Q. Why is it better to be wet than dry?

A. Because the wet clothes would form a far better conductor than the fluids of our body; and, lightning would roll down the wet clothes, without touching our body at all.

Q. What is the safest thing a person can do to avoid injury from lightning?

A. He should draw his bedstead into the middle of his room, commit himself to the care of God, and go to bed; remembering that our Lord has said, “The very hairs of your head are all numbered.”

Q. What is a lightning-conductor?

A. A metal rod fixed in the earth, running up the whole height of a building, and rising in a point above it.

Q. What metal is the best for this purpose?

A. Stout copper wire.

Q. Why is copper wire better than iron?

A. 1st—Because copper is a better conductor than iron:

2ndly—It is not so easily fused or melted: and

3rdly—It is not so much injured by weather.

Q. What is the good of a lightning-conductor?

A. Metal wire is a most excellent conductor; and as the lightning makes choice of the best conductors, it would run down the metal wire, rather than the bricks of the building.

Q. How far will the beneficial influence of a lightning-conductor extend?

A. It will protect a circumference all round, the diameter of which is (at least) 4 times as long as that part of the rod, which rises above the building.

Q. Give me an example.

A. If the rod rise 2 feet above the house, it will protect the building for (at least) 8 feet all round.

Q. Why are not lightning-conductors more generally used?

A. Because they are often productive of more harm than good.

Q. How can lightning-conductors be productive of harm?

A. If the rod be broken by weather or accident, the electric fluid (being obstructed in its path) will rend the building into fragments.

Q. Is there any other evil to be apprehended from a lightning rod?

A. Yes; if the rod be not big enough to conduct the whole current to the earth, the lightning will fuse the metal, and greatly injure the building.

Q. How stout is it needful for the copper wire to be, that it may conduct the fluid safely to the earth?

A. It should be (at least) one inch in diameter.

Q. Why does lightning sometimes knock down houses and churches?

A. The steeple, or chimney is first struck; the lightning then darts to the iron bars and cramps employed in the building; and (as it darts from bar to bar) shatters to atoms the bricks and stones, which oppose its progress.

Q. Can you tell me how St. Bride’s Church (London) was nearly destroyed by lightning, about 100 years ago?

A. The lightning first struck the metal vane, and ran down the rod; it then darted to the iron cramps, employed to support the building; and (as it flew from bar to bar) smashed the stones of the church, which lay between.

Q. Why did the lightning fly about from place to place, and not pass down in a straight course?

A. Because it always takes in its course the best conductors; and will fly both right and left, in order to reach them.

Q. Why does lightning turn milk sour?

A. Lightning causes the gases of the air (through which it passes) to combine, and thus produces a poison, called nitric acid; some small portion of which, mixing with the milk, turns it sour.[2]

(N. B. Sometimes, the mere heat of the air, during the storm, turns milk sour.)

[2] The air is composed of two gases, called oxygen and hydrogen, mixed together, but not combined. If oxygen is combined with nitrogen, it produces five deadly poisons, viz.—nitrous oxide, nitric oxide, hyponitrous acid, nitrous acid, and nitric acid, according to the proportion of each gas in the combination.

Q. What is the difference between combining and mixing?

A. When different ingredients mingle without undergoing any chemical change, they are said to be mixed; but when the natural properties of each are altered by the union, then those ingredients are said to be combined.

Q. Give me an example.

A. If different coloured sands be shaken together in a bottle, the various grains will mix together, but not combine: but if water be poured on quick lime, the water will combine with the lime, and not mix with it.

Q. Why are the different grains of sand said to be mixed, when they are shaken together?

A. Because they are mingled together, but the property of each grain remains the same as it was before.

Q. Why is water poured on lime, said to combine with it?

A. Because the properties, both of the water and the lime, are altered by the mixture: the lime alters the character of the water, and the water alters the character of the lime.

Q. Do oxygen and nitrogen combine, or only mix together, in common atmospheric air?

A. They only mix together, as grains of sand would do, when shaken in a bottle. When oxygen and nitrogen combine, they do not constitute air, but acid poisons.

Q. Why does lightning turn beer sour, although contained in a close cask?

A. If the beer be new, and the process of fermentation not complete, lightning will so accelerate the process, as to turn the liquor sour.

Q. Why is not old beer and strong porter made sour by lightning?

A. Because the fermentation is complete already; and, therefore, is not affected by electrical influence.

Q. Why is metal sometimes fused by lightning?

A. Because the dimension of the metal is too small, to afford a path for the electric current.

Q. Why does lightning purify the air?

A. For two reasons: 1st—Because the oxygen and nitrogen of the air combine,[3] and produce “nitric acid:”

2ndly—Because the agitation of the storm stirs up the air.

[3] The oxygen and hydrogen are not combined, but simply mixed in the ordinary air; but the lightning causes the mixed elements to combine.

Q. How does the production of nitric acid purify the air?

A. Nitric acid acts very powerfully in destroying exhalations, arising from putrid vegetable and animal matters.

Q. Why is lightning more common in summer and autumn, than in spring and winter?

A. The heat of summer and autumn produces great evaporation; and the conversion of water to vapour, always develops electricity.

Q. Why does a thunder-storm generally follow very dry weather, and rarely succeeds continued wet?

A. The clouds are always charged with electricity; but dry air (being a non-conductor), will not conduct the surplus fluid from the clouds to the earth: so it violently rends the dry air with a flash, in order to relieve the cloud, and reach the earth.

Q. What is the general direction of a thunder-storm?

A. Either from east to west; or else from north to south.

Q. Why is electricity excited by friction?

A. Electricity, like heat, exists in all matter; but is often in a latent state: friction disturbs it, and brings it into active operation. (see p. 31.)

Q. Why is a tree sometimes scorched by lightning, as if it had been set on fire?

A. Lightning scorches it by its own positive heat, just the same as fire would.

Q. Why is the bark of a tree often ripped quite off by a flash of lightning?

A. As the lightning runs down the tree, it develops the latent heat so rapidly, that it carries the bark of the tree along with it, while it seeks to escape.

Q. Why are boughs of trees broken off by lightning?

A. The mechanical force of lightning is very great; and when the flash strikes a tree, it will often break off the boughs by the force with which it strikes against it.

Q. Why is an electric shock felt most at the elbow joint?

A. Because the path of the fluid is obstructed by the joint: and the shock felt at the elbow is caused by the fluid leaping from one bone to another.