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69.-Glimpses of Science.

ABOUT ELECTRICITY.

ae-count'ed, considered.

al-ternate-ly, by turns.

|çîr'euit (sir'kit), continuous electrical communication.

a-năt ́ò-my, an account of the eom'monş, public fields.
structure of the human body, ob- dis-seet'ing, cutting apart.
tained by dissection.
| põleş, extremities.

PREPARATORY NOTES.

(2) Gilbert (1540-1603), an English physician and natural philosopher. (4) Guericke (gā'rik-eh), a German natural philosopher, and the inventor of the air pump (1602-1686). — (4) Hawksbee (— -1730), an English scientist.— (6) Du Fay (1698-1739), a French scientist. — (14) Galvani (gäl-vä’ne), an Italian philosopher (1737-1798). — (16) Volta (1745-1827), an Italian scientist. — (21) Wheatstone and Cooke, two English physicists of the present century. — (21) Steinheil, a Bavarian scientist. (21) Morse (1791-1872), a celebrated American electrician.

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I.

1. So long ago as the time of the Greeks it was already known that amber, when rubbed, will attract or draw towards it bits of straw and other light bodies; and it is from the Greek word electron,-amber,- that our word "electricity" is taken.

2. Until the sixteenth century, however, no one had made any careful experiments upon this curious fact; and it was Dr. Gilbert, an English physician, who first discovered that other bodies besides amber will, when rubbed, attract straws, thin shavings of metals, and other substances; and he also proved that the attrac

tion was stronger when the air is dry and cold than when it is warm and moist.

3. You can easily try this for yourself by rubbing the end of a stick of common sealing wax on a piece of dry flannel, and then holding the rubbed end near to some small pieces of light paper or feathers. You will find that these substances will spring towards the sealing wax, and cling to it a short time.

4. After Gilbert's time very little notice was taken of these facts, till Guericke invented the first rude electrical machine in 1672. He made a globe of sulphur which turned in a wooden frame, and by pressing a cloth against it with his hand as it went round he caused the sulphur to become charged with electricity. His apparatus was very rough, but it led to better ones being made; and, some years later, a man named Hawksbee substituted a glass globe for the sulphur and a piece of silk for the cloth, and in this way electrical machines were made much like those we now use.

5. Guericke also discovered that an electrical body attracts one that is not electrified, but repels it again as soon as it has filled it with electricity like its own. He was also the first to notice the spark of fire and crackling sound which are produced by electricity when it passes between two bodies which do not touch each other.

6. Soon after this, a Frenchman named Du Fay showed that substances filled with different kinds of electricity attract each other. Both these men thought

that electricity was a fluid which was created by the rubbing, and which was not in bodies at other times.

7. But about the middle of the last century Benjamin Franklin began to pay attention to these experiments in electricity, and to make experiments himself. He soon saw clearly that all bodies have more or less electricity in them, which the rubbing only brings out. Franklin began to consider how many of the effects of lightning were the same as those of electricity.

8. Lightning travels in a zigzag line, said he, and so does an electric spark; electricity sets things on fire, so does lightning; electricity melts metals, so does lightning. Animals can be killed by both, and both cause blindness. Electricity always finds its way along the best conductor, or the substance which carries it most easily, so does lightning; pointed bodies attract the electric spark, and in the same way lightning strikes spires and trees and mountain tops. "Is it not probable," thought he, "that lightning is nothing more than electricity passing from one cloud to another, just as an electric spark passes from one substance to another?"

9. Franklin's idea was, that, if he could send an iron rod up into the clouds to meet the lightning, it would become charged with the electricity which he believed was there, and would send it down a thread attached to it, so that he might be able to feel it. He took, therefore, two light strips of cedar fastened crosswise, upon which he stretched a silk handkerchief tied by the corners to the ends of the cross; and to the top of this

kite he fixed a sharp-pointed iron wire more than a foot long. He then put a tail and a string to his kite; and at the end of the string near his hand he tied some silk (which is a bad conductor), to prevent the electricity from escaping into his body. Between the string and the silk he tied an iron key (the metals being the very best conductors), in which the electricity might be collected.

10. When his kite was ready he waited eagerly for a heavy thunderstorm; and, as soon as it came, he went out with his son to the commons near Philadelphia, and let his kite fly. It mounted up among the dark clouds; but at first no electricity came down, for the string was too dry to conduct it. But by and by the heavy rain fell, the kite and string both became thoroughly wet, and the fibers of the string stood out as threads do when electricity passes along them.

11. As soon as Franklin saw this, he knew that his experiment had succeeded: he put his finger to the key, and drew out a strong bright spark,—the most welcome rap upon the knuckles that any man ever received. Franklin soon put his discovery to practical use. Whenever you see a lightning rod guarding a building from destruction, remember that we owe that invention to him and to his kiteflying.

12. Franklin proposed to fix on the highest points of edifices upright rods of iron made as sharp as a needle, and gilded to prevent rusting, and extending down the outside of the building into the ground, or

down round one of the shrouds of a ship, and down the side till it reaches the water.

13. It has since been found that rods of copper form the best conductors. These are now set into the masts of ships, passing down which they pierce the keel, and are fastened underneath by copper bolts in contact with the water.

II.

14. Only a few months before Franklin died, a new fact was discovered about electricity by Galvani, professor of anatomy at Bologna. One day as Madame Galvani was skinning frogs for a soup, one of Galvani's assistants was working an electrical machine near her. Just as the flow of electricity was going on rapidly, this young man happened to touch a nerve of the leg of one of the frogs with a dissecting knife; and to his great surprise the leg began to move and struggle as if it were alive.

15. Madame Galvani was so much struck by this that she told her husband of it; and he repeated the experiment many times, and found that whenever the flow of electricity from the machine was brought near the nerve of the frog's leg it produced convulsions. This discovery of Galvani soon became spoken of far and wide, under the name of galvanism.

16. Among the celebrated men who were attracted by it was Volta, professor of natural philosophy at Pavia. Not satisfied with merely reading about Galvani's experiments, he tried them himself; and he began to

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