534. It is ascertained, that during these shocks an oxydation of the metallic plates takes place. and, after their surfaces become tarnished, th shock diminishes; but, on being wiped, its forc is renewed.

A combination of these troughs forms a galva nic battery, the force of which has produced the most surprising effects; and, by disturbing the close affinity of the constituent parts of many bodies, has led to the analysis of substances hitherto deemed simple and elementary.

Obs. Some fishes, as the torpedo, the gymnotus elec tricus, and the silarus electricus, when touched, communicate shocks to the human body like those of electricity; but as there is no circuit for the fluid, in these cases, no adequate solution has yet been found of this strange phe

nomenon.

535. Since the identity of lightning and electric matter has been ascertained, philosophers have been led to seek the explication of aerial meteors in principles of electricity: and there is no doubt, that the greater part of them, and especially the aurora borealis, are electrical, or gazeous phe

nomena.

It has been observed, that the aurora borealis produces a very sensible fluctuation in the magnetic needle; and that the flashes have been attended with various rumbling and hissing sounds. Now black, and deep, the night begins to fall, Drear is the state of the benighted wretch, Who then, bewilder'd, wanders through the dark, Perhaps, impatient as he stumbles on

Struck from the root of slimy rushes, blue,
The wildfire scatters round, or gather'd trails
A length of flame deceitful o'er the moss;
Whither, decoy'd by the fantastic blaze,

Now lost, and now renew'd, he sinks absorpt,
Rider and horse, amid the miry gulf:

At other times, gleaming on the horse's mane,
The meteor sits; and shows the narrow path,
That winding, leads through pits of death or else,
Instructs him how to take the dangerous ford.

THOMSON.

536. Earthquakes, the most dreadful phenomena of nature, have been ascribed, by some naturalists, to water, fire, steam, and electricity; each of these powerful agents being supposed to operate in the bowels of the earth.

Subterraneous fire, and steam generated from it, are supposed to be the true causes of earthquakes. The elasticity of steam, and its expansive force, are every way capable of producing the stupendous effects attributed to earthquakes; the force of steam being 28 times greater than that of gunpowder.

Can the poor, brittle tenements of man
Withstand the dread convulsion?

Their dear homes, (With shaking, tottering, crashing, bursting, fall ;) The boldest fly; and, on the open plain,

Appall'd, in agony, the moment wait,

When, with disrupture vast, the waving earth
Shall 'whelm them in her sea-disgorging womb.
Nor, less affrighted, are the bestial kind:
The bold steed quivers in each panting vein,
And staggers, bath'd in deluges of sweat:-
The lowing herds forsake their grassy food,
And send forth frighted, woeful, hollow sounds :-
The dog, thy trusty centinel of night,
Deserts his post assign'd and piteous howls.

THOMSON.

537. The most remarkable changes in the form and constitution of the earth, since the deluge, have probably been produced by subterraneous

U

fires in volcanoes and by earthquakes; by which plains are converted into mountains, the ocean into islands, and dry land into pools.

Obs. Half a pound of steel-filings, half a pound o brimstone, and a pint of water, will, when well mixed acquire heat enough to make the mass take fire: and it appears, that volcanic mountains abound in these mixtures; which are ignited, no doubt, by rain falling into their craters, or by the sea communicating with their bases and lower cavities.

The fluid lake that works below,
Bitumen, sulphur, salt, and iron-scum,

Heaves up its boiling tide The lab'ring mount
Is torn with agonizing throes. At once,
Forth from its side, disparted, blazing pours
A mighty river; burning in prone waves,
That glimmer thro' the night, to yonder plain.
Divided there, a hundred torrent streams,
Each ploughing up its bed, roll dreadful on,
Resistless. Villages, and woods, and rocks,
Fall flat, before their sweep.

MALLET.

538. The eruptions of volcanoes exhibit dreadful phenomena, in prodigious inundations of liquid fire, which bear inevitable destruction with

them.

The name of lava is given to these fiery streams, consisting of a mixture of stones, sand, earth, metallic substances, salt, &c., calcined and vitrified.

The last great. eruption of Etna, was in 1669; and the progress of the lava was at the rate of a furlong a day-it destroyed, in forty days, the habitations of 27,000 persons; and of 20,000 inhabitants of the city of Catanea, only 3000 escaped.

The other great volcanoes in Europe, are Vesuvius, and Hecla; but there are two or three hundred in different parts of the world.

XXII. Magnetism.

539. The power of certain ores of iron to attract pieces of iron, was known by the Greeks; but accident led an Italian to discover, that if suspended on a centre, and allowed to turn in any direction, one end would constantly point northward, and, of course, the other southward.

This ore of iron was called loadstone; and it soon became evident, that the property of distin guishing the north and south, was of the highest utility to ships at sea.

540. In the course of time, it was discovered, that loadstone communicated its property of turning northward to all pieces of iron and steel; and that these could communicate it to others, in a degree of strength proportioned to the number of magnets employed in transferring the property.

Natural magnets are not therefore now used; but needles are first made of the most convenient shape, and the magnetic property communicated to them, by contact with other natural, or artificial magnets.

541. The two poles or points of the magnetic needle have different properties-the south poles of magnets attracting the north poles of others; and the same poles acting repulsively towards each other; i. e. a north pole repels a north pole, and a south pole, a south pole.

A white heat destroys the polarity; and so will strokes of the hammer. But it will be acquired, by iron-bars that stand long in an upright position, or that are heated red-hot, and left to cool in a polar direction.

542. If a magnet be laid on white paper, and some steel-dust be suffered to fall gently on the paper, the dust will arrange itself in regular curves, under the influence of the magnetic attraction.

If the magnetic bar be bent round into the form of a horseshoe, its poles in that position will attract and operate in a higher degree than when used separately: and the strength of all magnets is increased by thus keeping their powers in action.

545. Magnets do not point exactly north and south; but in different parts of the world, with a different declination eastward or westward of the north; and different at each place, at different times.

In London, at this time, the needle points 24 degrees to the west of the north; or, rather more westerly than north-north-west; and the declination has increased to the west from due-north, for 160 years past. At Philadelphia the variation is about 2 degrees 24 minutes westerly.

Nor does the needle lie parallel or flat, in any place; but the north point is turned upward in the southern hemisphere, or downward in the northern hemisphere, and in London at this time it dips 72 degrees.

Obs.-The cause of the phenomena of magnetism has hitherto, baffled the investigations of philosophers; and the more we learn of them, the greater becomes our embarrassment; but some accidental discovery will probably hereafter effect more than all reasoning; and till that occur, we must be content to class this, among the many unaccountable secrets of nature.