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 distinguishing 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 artifi- · cial 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, a south pole. A white heat destroys the polarity; and so will stokes 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. U 542. If a magnet be laid on white paper, and some steel-dust be suffered to fall gently upon it, the dust will arrange itself on the paper in regular curves, under the influence 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. 543. 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 in 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, Nor does the needle lie parallel or flat, in any place; but the north point is turned upward or downward, 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. XXIII. Mathematics. 544. Mathematics, or the mathematical sciences, are divided into 1. Pure mathematics; containing arithmetic and geometry, which treat only of number and magnitude. 2. Mixed mathematics; which treat of the properties of quantity applied to matter, as astronomy, geography, &c. 3. Speculative mathematics; which contemplate the proportions, relations, &c., of bodies. 4. Practical mathematics, or their application to the practical uses of life. 545. Geometry is an ancient, perfect, and beautiful science; which enables us to determine the relations and proportions of superfices and solids. Superfices consist of figures of three sides, called triangles; of four sides, called quadrilaterals, squares, parallelograms, and trapeziums; of five sides, called pentagons; of six sides, called hexagons; and of many sides, called po lygons. Superfices also are circles; ovals, or ellipsis; sectors of circles, or parts cut out from the centre; and segments of circles cut off by a strait line, called a chord.. A PENTAGON. The above cut represents a Pentagon or Polygon of five sides, inscribed within a circle. 546. Solids are cubes, regular and irregular; spheres ; cylinders; cones; pyramids; and spheroids, &c. A cone cut obliquely to the base, forms an ellipsis; perpendicular, an hyperbola; and parallel to the side, a parabola. Angles are the inclinations of two straight lines meeting in a point. 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. EQUI-LATERAL RIGHT-ANGLED TRIANGLE. B C SQUARE, AND DIAGONAL A B. AN OCTAGON. A B 547. Parallel lines are those, which are equidistant; diagonals are those, which cross figures from one angle to another. Tangents are lines that touch a circle. The circumference of every circle is equal to 360 degrees; the three angles of every plain triangle, are equal to 180 degrees; the angles of every quadrilateral figure are equal to 360 degrees. 548. By means of a scale and compasses, many kinds of figures may be readily drawn. Triangles contain six parts; viz. three angles, and three sides, and any 1 of one, and 2 of the others being given, the other three may be found either by projection, or by logarithms. This art is called trigonometry; and by means of it are performed most problems in astronomy, geography, navigation, and surveying. It is founded on the great principle that all triangles which have equal angles, have all their |