MAGNETISM.

The earth contains a mineral substance, which attracts iron, steel, and all ferruginous substances; which is called Natural Magnet or Load-stone. The same substance has the power of communicating its properties to all ferrugineus bodies: those bodies, after having acquired the magnetic properties, are called Artificial Magnets.

It is well known, by long experience, that a piece of wire, or a needle, rendered magnetic, and accurately balanced on a pivot, or centre, will settle in a certain direction, either duly, or nearly, north and south. This directive power of the magnet, is its most wonderful and useful property. By it, mariners are enabled to conduct their vessels through vast oceans, in any given direction; miners are guided in their works below the surface of the earth; and travellers conducted through deserts, otherwise impassable.

The usual method is to have an artificial magnet suspended, so as to move freely, which will always place itself in or near the plane of the meridian, north and south; then, by looking on the direction of the magnet, the course is to be directed, so as to make any required angle with it. An artificial steel magnet, fitted for this purpose in a proper box, is called the Mariner's Compass, or simply the compass, and is so well known, as to need no particular description.

There is sometimes a variation in the direction of the

magnetic needle, which differs in degree at different places and times.

GRAVITATION.

The attraction of Gravity, or Gravitation, called also the centripetal force, is that property and power, by which distant bodies tend towards one another. This is the universal principle of nature, from which all motion arises. By this principle, stones fall, and all bodies, on whatever side of the earth, are kept on its surface: by this, the moon preserves her proper distance, and statedly performs her revolution round the earth, the other satellites attend their

respective planets, and the planets, and comets are retained in their orbits. Hence it appears, that downward is a term which has respect entirely to the centre of the system or body to which it is applied; or in other words, to move downwards is to approach toward the centre of a body, or system; and to move upward is to recede from the said centre. Thus, the centre of the sun is the lowest point in the solar system, toward which every object in the system is attracted. With respect to the earth, the lowest point is its centre, and every object belonging to that planet, on whatever side, tends toward it. It is consequently erroneous and improper to suppose that people on the side of the earth opposite to us, walk with their heads downward, or are in danger of falling from its surface.

TIDES.

The regular ebbings and flowings of the sea, twice every day, are called Tides. They are caused by the attrac tion of the sun and moon, but principally that of the lat ter ; the power of the moon, in this case, being to that of

the sun as 5 to 1.

The earth, by its daily rotation about its axis, presents each part of its surface to the direct action of the moon twice each day, and thus produces two floods and wo ebbs. But because the moon is, in the mean time, passing from west to east in its orbit, it arrives, at the meridian of any place later than it did the preceding day; whence the two floods and ebbs require nearly 25 hours to complete them. The tide is highest about three hours after the moon has passed the meridian; for though the force be greater at that time, yet the greatest effect cannot appear on the water till sometime afterward.

When the moon is in conjunction or opposition with the sun, they both tend to raise the water in the same place. The tides are then highest, and are called spring tides. When the sun and moon are 90° from each other, the sun depresses those parts which the moon raises. They are then lower than ordinary, and are called neap tides. Hence the highest tides happen at the time of new and full moon, and the lowest when the moon is at her first and third quarters.

The motion and height of the tides, as they pass over shoals, and run through straits and inlets, become more various. The Mediterranean and Baltic seas have very small tides, because the inlets by which they communicate with the ocean are so narrow, that they cannot in so short time, receive or discharge so much, as sensibly to raise or depress their surfaces. In the bay of Fundy, be tween Nova Scotia and the United States, the tides rise to the height of 60 feet, and flow so rapidly as to overtake animals feeding on the shore. There are no tides in lakes, they being in general so small, that the moon attracts every part of them equally, and therefore no part of the was ter is raised above the other.

THE ATMOSPHERE.

The Atmosphere, or Air, is a fine, invisible, and elastic fluid, surrounding the earth, and extending some miles from its suface. Without this element, no animal or vegetable could exist; there could be neither rain nor dews, to moisten the earth; and though the heavenly bodies might be visible, as bright specks, yet we should have neither day-light nor twilight, but utter darkness. Air is also the vehicle of sound; without it we should not be able to converse with each other, nor even to breath.

By its gravity, air is capable of supporting all lighter bodies ; as smoke, vapors, odors, &c.

WINDS.

When the air is heated, it becomes rarer, and therefore ascends, and the surrounding cold air, rushing in to supply its place, forms a current in some one direction. rent of air, thus put in motion, is called Wind

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Constant, or General Winds, are such as blow always in the same direction. These prevail in the Atlantic and Pacific oceans, between the latitudes of about 28° north and south, and blow constantly from northeast and southeast toward the equator. These are also called Tropical or General Trade Winds.

Periodical Winds are those, which blow in certain directions, during certain ↑ eriods of time. In some parts the

Indian ocean, the wind blows six months in one direction, and then six months in the opposite direction. The changes happen about the time of the equinoxes and are attended with dreadful storms of thunder and lightning. These are called Monsoons, or Shifting Trade Winds. In the

West-Indies, the winds blow from the land in the night, and from the sea during the day, changing their direction every twelve hours. These are called Land and Sea Breezes. Variable Winds are those, which are subject to no regularity of duration or change.

The constant and periodical winds blow only at sea; on land the wind is always variable

Many parts of the world, and the West-Indies in par ticular, are frequently visited by Hurricanes. They are sudden and violent storms of wind, rain, thunder, and lightning, attended with great swelling of the sea, and sometimes with earthquakes. There are signs by which the inhabitants are warned of their approach. Some time before they come on, the sky is troubled; the sun more red than usual; there is a dead calm below; and the tops of the mountains are free from the mists which usually hover about them. In the caverns of the earth, and in wells, a hollow rumbling noise is heard, like the rushing of a great wind. At night, the stars seem much larger than common; the northwest sky has a black and menacing appearance; the sea emits a strong smell, and rises into vast waves, often without any wind. These terrible storms happen principally in the month of August; destroying all the produce of the ground; tearing up trees; overturning buildings; and inundating large tracts of land.

The deserts of Africa and Arabia give a burning heat and blasting quality to the air passing over them. At Goree, on the river Senegal, there is an easterly wind from the inland parts, with which those, who are suddenly met by it in the face, are scorched, as by a blast from a furnace.

At the Falkland Islands an extraordinary blasting wind is felt, but its duration is seldom more than twenty-four hours. It cuts down the herbage, as if fires had been made under them; the leaves are parched up and crumble into dust; fowls are seized with cramps, and never

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recover; men are oppressed with a stopped perspiration, heaviness at the breast, and sore throat, but recover with proper care.

The most dreadful winds, perhaps, are those in the deserts near Bagdad, called Samoul, or Mortifying Winds. The camels perceive their approach, and are said to make. an unusual noise, and cover their noses in the sand. To escape their effects, travellers throw themselves as close on the ground as possible, and wait till the winds have passed over, which is commonly in a few minutes. Thus some escape; but those who die, have their limbs mortified.

In Italy, a wind blows, for several days, called Sirocco, which is fatal to vegetation, and destructive to the inhabitants; depressing their spirits, and suspending the powers of digestion, so that those who venture to eat a heavy supper, while this wind prevails, are frequently found dead the next morning. It is felt with peculiar violence at Palermo.

In the deserts of Africa, there are prodigious pillars of sand, which move with great velocity. Mr. Bruce saw several of these at once, some of which appeared to be ten feet in diameter. They began immediately after the rising of the sun, and his rays shining through them,. gave them the appearance of pillars of fire.

There is a phenomenon, called the Water-Spout, hanging under a deep cloud, in the form of a cone, with the vertex downward; and under it the sea boils up, and rises in a conical form. These cones sometimes meet, and they generally begin to appear together; they sometimes move for a considerable space before they break. When they appear at sea, and approach a ship, it is said the sailors fire at them and break them, as it might be dangerous if they should meet with a ship and break over it. The water-spout is supposed to be an electrical phenomenon.

A Whirlwind is a wind, which rises suddenly; it is extremely rapid and impetuous, taking up all light substances from the earth, which it meets with, and carrying them up in spiral motion. Dr. Franklin supposes that the whirlwind and water spout proceed from the same cause. They have each a progressive and circular motion; they usually rise after calms and great heats, and