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earth, metallic substances, sult, &c., caloined and vitrefied.
The last great eruption of Etna, was in 1009; und 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 Calanca, only 3,000 escaped.
The other great volcanoes in Europe are, Vesuvius, and llcola; but there are two or three hundred in different parts of the world.
XXIII. Magnetism. 640. 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. | Thig ore of iron was called loadstone ; and it soon becaine evident, that the property of distinguishing the north and south, was of the highest utility to ships at sea.
650. lu the course of time, it was discoveted, thut loudstone communicated its property o. 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.
Nntarni magnets are not therefore now used; but tcodles are first made of the most convenient
ibape, and the magnetic property communicated 1o luem, by coulx ct with other natural, or artificial, magn •ts.
55l, The, iwo poles or points of the magnetic needle Lave 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, 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 long stand upright, or that are heated red-hot, and left to cool in a polar direction. 552, If a magnet be laid on white
and some steel-dust be suffered to fall gently upon it, the dust will arrange itself on the paper in regular curves, under the iufluence of the magnetic attraction.
If the magnetic bar be bent round into the form of a horse-shoe, 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.
553. Magnets do not point exactly north and south; but in different parts of the world, with a different inclination eastward or westward of the north; and different in each place, at dif ferent times.com
In London, at this time, the needle points 26 degrees to the west of the north or, rather more westerly than north-north-west; and the incli
rinting has increased to the west from due north, #orld years past,
Nor does the needle lie parallel, or flat, in any place; but the north polut is turned upward or Hownward, and in Lanılou at this time it dàp. 72 degrees.
OM, --- The cause of the phenomenon of magnetism han, bitherto, battled the investigations of philosophers i and the more we think of them, the greater beeoines our em barra ament: but some accidental discovery will effect more than all reasoning i and till that occur, iso must be content to class this, among the many unaccountable wonders of nature,
XXIV. Mathematics.' 501, Mathematies, or the mathematical sci. ences, are divided into -
1. Pia mathematies; containing arithmetic and geometry, which treat only of number and magnitude,
2. Minod mathematies; whiek treat of the properties of quantity applied to matter, as astronomy, geography, Ro.
8. Spreulative mathematies; which contemplate the proportions, relations, &e, of bodies,
4. Pretical mathematics or their applica. tion to the practical uses of life,
585. Geometry is an ancient, perfect, naik benutiful science, which enables its to deter. mine the relations and proportions of supru cies and solids.
Superficies consist of figures of three sides, enlled triangles; of four sides, called! quadam
gles, squares, parallelograms, and trapeziums; of five sides, called pentagons; of six sides, called hexagons; and of many sides, called polygons,
Superficies also are circles, ovals, or ellipses ; sectors of circles, or parts cut out from the centre; and segments of circles, cut off by a strait line, called a chord
The above cut represents a Pentagon or Polygon of five sides, inscribed within a circle.
556. Solids are cubes regular and irregular; spheres; cylinders ; cones ; pyramids; and spheroids, or elliptical spheres.
A Cone cut obliquely to the base, forms an ellipsis; perpendicularly through the side, an hyperbola ; and parallel to the side, a parabola.
Angles are the corners, formed by the meeting of two lines.
A right angle is, when the lines are perpendicular to each other; an acute angle is less than a right angle; and an obtuse angle is greater than a right angle.
SQUARE, AND DIAGONAL AB, AN OOTAGON
867. Parallel lines are those, which are equin distant; diagonals are those whileli cross figures tvom one angle to another,
Tangents are Mnes that only touch a chele in one point.
Every errele is equal to 300 Irgrees; the three angles of every triangle, are equal to 120 degrees; the angles of every quadrangle, are equal to 300 degrees.
658, by means of a scale and a pair of compasses, all kinds of figures may be readily drawn