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As the excitation which is produced by rubbing with the hand on a tube or plate of glass, is not only very laborious, but inadequate to the production of any material quantity of electric fluid, machines have been constructed of various forms for this purpose. The most common machine consists of a glass cylinder, supported by two glass pillars, and made to turn by a crank or handle. A rubber, or cushion, of leather, spread with an amalgam of mercury and zinc or tin is fastened to a spring, which proceeds from a socket cemented on the top of another glass pillar. A piece of black silk is fastened to the cushion and extended over the cylinder, nearly to the receiving points, to prevent the fluid from flying off. A fourth glass pillar supports what is called the prime conductor, which is made of hollow brass or tin plate, and, at the end towards the cylinder, has a collection of pointed wires, and at the other end, a single wire terminated by à brass ball. A small chain is fastened to the cushion, one end of which extends to the floor or table. It serves to conduct the fluid in passing from the earth to supply the machine. When the cylinder is turned swiftly, the electric fluid passes from the rubber to the glass, and is thence conveyed to the points of the prime conductor, which is thus positively electrified. While the electric fluid is collecting, it produces a crackling noise, and in a darkened room the flame will be seen spread on the surface of the cylinder. If a cylinder be made of resin, the electricity is the reverse of that which is produced by the smooth glass cylinder and rubber of the usual machines; for in this case the rubber partakes of the positive, and the cylinder, and prime conductor, is electrified with the negative. This difference between the resin and glass has given rise to what is called the double current, or vitreous and resinous electricity; but it is generally supposed that the difference arises more from the effect of the surfaces that act on each other, than from any peculiar qualities in the different bodies.

Some of the experiments which may be made with an electrical machine are necessary for illustrating the laws of electricity, and others are merely entertaining. If the inside of a glass tumbler be electrified by presenting it to a pointed wire extending from the prime conductor, and then placed over a few pith-balls laid upon a table, the balls will immediately begin to leap up along the sides of the glass, and then

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back to the table; they are attracted and repelled by the electrified inside surface of the glass, the electricity of which they gradually conduct to the table. If a person having long hair, not tied up, be placed upon an insulated stand, and, by means of a chain be connected with the prime conductor, when the machine is put in motion, the hairs on his head, by repelling each other, will stand out in a most surprising manner. A piece of sponge, filled with water, and hung to a conductor, when electrified in a dark room, exhibits a most beautiful appearance. If a piece of sealing-wax be fastened to a wire, and the wire be fixed into the end of the conductor, and the wax lighted, the moment the machine is worked, the wax will fly off in the finest threads imaginable. Take a two ounce phial, half full of olive-oil, pass a slender wire through the cork, and let the end of it be so bent as to touch the glass just below the surface of the oil; then place your thumb opposite the point of the wire in the phial, and if, in that position, you take a spark from the charged conductor, the spark, in order to reach your thumb, will actually perforate the glass. In this way holes may be made all round the phial.

QUESTIONS.-1. What parts of bodies contain the electric fluid? 2. When is a body said to be electrified? 3. What experiment may, be made with sealing-wax? 4. When is a glass tube said to be excited? 5. What is said respecting an excited tube when in a dark room? 6. What are conductors of electricity? 7. Electrics, or nonconductors? 8. When is a surface positively, and when negatively electrified? 9. When is a conductor said to be insulated? 10. What is said of the human body as a conductor? 11. When do surfaces repel, and when attract each other? 12. What takes place when a conductor receives the electric fluid?-non-conductor? 13. When is a plate of glass said to be charged? 14. What is an electric shock? 15. Describe the electrical machine. 16. What are some of the experiments that may be made with it? (See Electrical Machine, fig. 49.) [NOTE. The earliest account of any known electrical effect is by the ancient naturalists, Thales and Theophrastus, who flourished, the first 600, and the latter 300 years before the present era.]

152

DR. FRANKLIN'S DISCOVERY.

LESSON 68.

Electricity (continued.)

A queous, watery. Collapse', to fall together.

THE Leyden phial is a glass jar coated with tin foil on the inside and outside within about three inches of the top of its cylindrical part, and having a wire with a brass ball at itsextremity. This wire passes through a cork or piece of wood, and at its lower extremity is a small chain, or wire, that touches the inside coating in several places, and serves as a conductor to charge the jar with electric fluid. On bringing the ball of the jar near the prime conductor, after a few turns of the machine, the jar will be charged. The discharging rod consists of two brass balls attached to the ends of a wire, bent in the form of a semicircle, and fixed to a glass handle. When one of the balls of the discharging rod is applied to the ball of the jar, and the other to the outside coating, a communication is made between the outside and inside of the jar, by which the equilibrium is instantly restored by the superabundant electricity passing from one side to the other, appearing in the form of a vivid flash, and accompanied with a loud report. Any number of persons may receive the shock together by laying hold of each other's hands, the person at one end touching the outside of the jar, and the person at the other end bringing his hand near the ball of the jar. If there were a hundred persons so situated, they would every one feel the shock at the same instant. The electric fluid may be thus conveyed many miles in a moment of time. When great force is required from the electric fluid, a number of jars of the above description are connected together by making a communication between all their outsides, and another between all their insides. In this manner any number of jars may be charged with the same facility as a single one, and from the powerful effect of the electric fluid, when it is thus collected, it is called an electrical battery.

The Leyden phial received its name from the birth-place of the discoverer, who was a native of Leyden in Holland. But the greatest discovery that was ever made in electricity was reserved for Dr. Franklin, in America. It had been

THUNDER AND LIGHTNING.

153

imagined before his time that a similarity existed between lightning and the electric fluid; but Franklin brought this supposition to the test, and proved the truth of it by the simple means of a boy's kite covered with a silk handkerchief instead of paper, and some wire fastened in the upper part, which served to collect and conduct the fluid. When he had raised this machine into the atmosphere, he drew electric fluid from the passing clouds, which descended through the flaxen string of the kite as a conductor, and was afterwards drawn from an iron key which he tied to the line at a small distance from his hand. This important experiment immediately led to the formation of conductors to secure buildings from the effects of lightning.

When aqueous vapour is condensed, the clouds formed are usually more or less electrical, and the earth below them being brought into an opposite state, a discharge takes place when the clouds approach within a certain distance, constituting lightning; and the collapsing of the air, which is ra refied in the electrical circuit, is the cause of the thunder, which is more or less intense, and of longer or shorter duration, according to the quantity of the air acted upon, and the distance of the place where the report is heard from the point of the discharge.

In gloomy pomp, whilst awful midnight reigns,

And wide o'er earth her mournful mantle spreads,
Whilst deep-voiced Thunders threaten guilty heads,
And rushing torrents drown the frighted plains,

And quick-glanced Lightnings, to my dazzled sight,
Betray the double horrors of the night:

A solemn stillness creeps upon my soul,
And all its powers in deep attention die;
My heart forgets to beat; my steadfast eye
Catches the flying gleam; the distant roll,
Advancing gradual, swells upon my ear
With louder peals, more dreadful as more near.

Awake, my soul, from thy forgetful trance!
The storm calls loud, and meditation wakes;
How at the sound pale Superstition shakes,
Whilst all her train of frantic fears advance!

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Children of darkness, hence! fly far from me!
And dwell with guilt and infidelity!

But come, with look composed, and sober pace,
Calm Contemplation, come! and hither lead
Devotion, that on earth disdains to tread ;
Her inward flame illumes her glowing face,
Her upcast eye, and spreading wings, prepare
Her flight for heaven to find her treasure there.
She sees, enraptured through the thickest gloom,
· Celestial beauty beam, and 'midst the howl
Of warring winds, sweet music charms her soul;
She sees while rifted oaks in flames consume,

A FATHER GOD, that o'er the storm presides,"
Threatens, to save, and loves, when most he chides.
CHAPONE.

QUESTIONS. 1. What is the description of the Leyden phial? 2. How is it charged?—how discharged 3. What experiment may be made by it? 4. What is an electrical battery? 5. What great discovery did Dr. Franklin make, and by what means? 6. To what did this experiment lead? 7. What is lightning ?-thunder? (See Leyden phial, fig. 50.)

LESSON 69.

Falling Stars, Water Spouts, and Northern Lights.

Lam'bent, playing about, gliding over.

Glory, a circle of rays which surrounds the heads of saints in pictures, praise, celebrity, felicity of heaven.

It is supposed to be owing to the electricity of the atmosphere, that we observe a number of curious and interesting phenomena, such as falling stars, water-spouts, and northern lights. What are called falling stars are seen chiefly in clear and calm weather: it is then that the electric fluid is probably not very strong, and passing through the air it becomes visible in particular parts of its passage, according to the conducting substances with which it may meet. One of the most striking of this kind is recorded by Beccaria, an Italian. As he was sitting with a friend in the open air, an hour after sun-set, they saw a falling, or as it is sometimes

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