using plates of a large size?-by multiplying the pairs of plates? 6, What is the invention of Prof. Hare? 7. What experiments may be performed with it? 8. What new view of the subject has Prof. Hare offered? (See the galvanic or voltaic battery described at the begin ning of this lesson, fig. 46.) [NOTE. Prof. Hare has named his new apparatus Calorimator, or heat mover.]

LESSON 72.

Magnetism.

Polarity, that property of a magnet, by which, if left at liberty, it will point towards the poles of the earth, or nearly so the same end always points to the same pole.

ALTHOUGH the phenomena of the magnet have, for many ages, engaged the attention of natural philosophers, not only by their singularity and importance, but also by the obscurity in which they are involved; yet very few additions have been made to the discoveries of the first inquiries into the subject. No hypothesis has hitherto been framed, that will account in an easy and satisfactory manner, for all the various properties of the magnet, nor have the links of the chain, which connect it with the other phenomena of the universe, ever been pointed out. It is certain, indeed, that both natural and artificial electricity will give polarity to needles, and even reverse a given polarity; and hence it may be inferred that there is a considerable affinity between the electric and magnetic powers, but in what manner electricity acts in producing magnetism, is still utterly unknown.

The ancients were acquainted with the attractive and repulsive powers of the magnet; but it does not appear that they knew of its tendency to the pole: this very fortunate discovery was made about the beginning of the fourteenth century, when the spirit of exploring distant regions was gradually forming in Europe. The use which might be made of it in directing navigation was immediately perceived, and that most valuable, but now familiar instrument, the mariner's compass, invented. When navigators found that they could, at all seasons, and in every place, discover the north and south with the greatest ease and accuracy, it became no longer necessary to depend, like the voyagers of former ages, merely on the light of the stars, and the obser

vation of the sea-coast. They gradually abandoned their ancient timid and lingering course along the shore and ventured boldly into the ocean. Relying on this new guide, they could steer in the darkest night, and under the most cloudy sky, with a security and precision till then unknown. The compass may be said to have opened to man the dominion of the sea, and to have put him in full possession of the earth, by enabling him to visit every part of it.

Nearly half a century elapsed, from the time of this discovery, before navigators ventured into any seas which they had not been accustomed to frequent. But in the course of the fifteenth century, discoveries were made far beyond the conception of all former ages. In the first voyage of Columbus, the Spaniards were struck with an appearance, not less astonishing than new. They observed that the magnetic needle, in their compasses, did not point exactly to the polar star, but varied toward the west; and, as they proceeded, this variation increased. This appearance, which filled the companions of Columbus with terror, and which still remains one of the mysteries of nature, is that deviation from the meridian which is called the variation of the needle. It is different in different parts of the world; being west at some places, east at others, and in parts where the variation is of the same name, its quantity is very different. It is the same to all needles in the same place; and for a long time, it was thought to be invariably the same, at the same place, in all ages; but it was discovered, about the year 1625, that it was different at different times, in the same place. From subsequent observations, it appears, that this deviation was not a constant quantity, but that it gradually diminished; and at last, about the year 1660, it was found that the needle at London pointed exactly north. present the declination at London is about twenty-four degrees west. For some years, it has been nearly stationary; but it is understood now to be returning in an easterly direction. Knowing the variation, or declination of the magnetic needle, that is, the angle which the magnetic meridian makes with the meridian of the place, mariners are able to sail by the compass with as much accuracy as if it pointed exactly north.

At

The inclination, or dipping of the magnetic needle, expresses the property which the magnet possesses of inclining

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MAGNETICAL EXPERIMENTS.

one of its poles towards the horizon, and of elevating the other pole above it. This property was discovered in the year 1576. It is found to be always the same at the same place, but different in different places.

QUESTIONS.-1. When was the polarity of the magnet discovered? 2. What use was made of this property of the magnet? 3. When and by whom was the deviation of the needle from the meridian discovered? 4. What is said of this variation with respect to the same place?-to different places? 5. What is said of the declination of the needle at London? 6. What is the inclination or dipping of the magnetic

needle ?

LESSON 73.

Magnetical Experiments.

THE natural magnet, or loadstone, is found in the earth, generally in iron mines, in a hard and brittle state, and for the most part, more vigorous in proportion to the degree of hardness. Artificial magnets, which must be made of hard or highly tempered steel, are now generally used in preference to the natural magnet; not only, as they may be procured with greater ease, but because they are far superior to the natural magnet in strength, communicate the magnetic virtue more powerfully, and may be varied in their form more easily. In making artificial magnets, care should be taken to apply the north pole of the natural magnet or magnets to that extremity of the steel which is required to be made the south pole, and to apply the south pole of the magnet to the opposite extremity of the piece of steel. Very powerful magnets may be formed by first constructing several weak magnets, and then joining them together to form a compound one.

The north or south poles of two magnets repel each other; but the north pole of one attracts the south pole of another. The attraction between the magnet and iron is mutual, for the iron attracts the magnet as much as the magnet attracts the iron; since if they be placed on pieces of wood, so as to float upon the surface of the water, it will be found that the iron advances towards the magnet as well as the magnet towards the iron, or, if the iron be kept steady, then the magnet will move towards it.

MAGNETIC EXPERIMENTS.

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Magnetic attraction will not be destroyed by interposing obstacles between the magnet and iron. If you lay a small needle on a piece of paper, and put a magnet under the paper, the needle may be moved backwards and forwards; and with a piece of glass or board the effect will be the same. This property of the magnet has afforded the means of several amusing deceptions. A small figure of a man has been made to spell a person's name. The hand, in which was a piece of iron, rested on a board, under which a person, concealed from view, with a powerful magnet, contrived to carry it from letter to letter, until the word was made up. If the figure of a fish, with a small magnet concealed in its mouth, be thrown into the water, and a baited hook be suspended near it, the magnet and iron by mutual attraction will bring the fish to the bait.

If you lay a sheet of paper, covered with iron filings upon a table, with a small magnet among them, and then shake the table a little, at the two ends or the poles, the particles of iron will form themselves into lines, a little sideways, which bend, and then form complete arches, reaching from some point in the northern half of the magnet to some other point in the southern half. If you shake some iron-filings through a gauze sieve upon a paper that covers a bar magnet, they will be arranged in beautiful curves.

Soft iron is attracted by the magnet more forcibly than steel, but it is not capable of preserving the magnetic property so long. The gradual addition of weight to a magnet kept in its proper situation, increases the magnetic power, but heat weakens it, and great heat destroys it. Bars of iron that have stood long in a perpendicular situation, are generally found to be magnetical; this circumstance, together with the phenomena of the compass and the dipping needle, leaves no room to doubt but that the cause exists within the earth.

QUESTIONS.-1. Where is the natural magnet found? 2. Why are artificial magnets used in preference to natural? 3. How may very powerful magnets be formed? 4. How do the poles of a magnet attract and repel each other? 5. How does it appear that the attraction between the magnet and iron is mutual? 6. How does it appear that magnetic attraction will not be destroyed by the interposition of bodies? 7. What amusing deceptions has the attractive property of the magnet afforded? 8. How inay the magnetic power be weakened or destroyed? 9. From what is it concluded that the cause of magnetism exists in the earth?

Wick'er, made of small sticks. A'eronaut, one who sails through the air.

Meteorological, relating to the phenomena of the atmosphere, such as the alterations of its weight and temperature, changes produced by evaporation and rain, its excessive agitations, its electricity, &c.

AEROSTATION, in the modern application of the term, signifies the art of navigating through the air, both in its principles and practice. Hence also the machines which are employed for this purpose, are called aërostats, or aërostatic machines; and on account of their round figure, air-balloons. Air-balloons are of two kinds, those filled with rarefied air, and those filled with hydrogen gas. The best forms for balloons are globular or oval. Large balloons, for hydrogen gas, must be made of silk, and varnished over so as to be air-tight. The car, or boat, is made of wicker-work, covered with leather, well varnished or painted, and is suspended by ropes proceeding from the net which goes over the balloon. The hydrogen gas for filling the balloon is procured by putting a quantity of iron-filings, or turnings, with some sulphuric acid diluted with water, into casks lined with lead. From the top of these casks tin tubes proceed, which unite into one that is connected with the silk tube of the balloon. Balloons of oiled silk cannot be made smaller than five or six feet in diameter, as the weight of the material is too great for the air to buoy it up.

In 1729, Bartholomew Gusman, a Jesuit of Lisbon, caused an aërostatic machine, in the form of a bird, to be constructed, and made it ascend, by means of a fire kindled under it, in the presence of the king, queen, and a great concourse of spectators. Unfortunately, in rising, it struck against a cornice, was torn, and fell to the ground. The inventor proposed renewing his experiment; but the people had denounced him to the inquisition as a sorcerer, and he withdrew into Spain, where he died in an hospital. In 1766, the Honourable Henry Cavendish discovered that hydrogen gas (then called inflammable air,) was at least seven times lighter than common air. It occurred soon afterwards to