Conversely, -Multiply the Olympiad by 4, to the product add the current year or years of the Olympiad, and from the whole subtract 5*; then subtract the remainder from 776, and the remainder will be the year B. c. required. Thus, Ol. 111. 3 4 444 Add 3 447 Subtract 5 442 Then, from 776 Remains 334 B.C. or, without subtracting the 5, take the years of the Olympiad found as above from 781, and you get the year B. c. required. Thus, from 781 334 B.C. I have given the longer rules in these cases, for the sake of showing the principle; the shorter are better for practice. For events in the Roman history after the birth of Christ we have only to add the given year of our Lord to 753, to get the year of Rome; or subtract 753 from the given year of Rome, to get the year of our Lord. in no year of no Olympiad, unless on the same principle we add 1 to the Olympiad and 1 to the year. * Because the one current Olympiad is 4 years, and the current year is one year. MODERN GEOGRAPHY. CHAPTER I. INTRODUCTION. Maps of Dr. Butler's Atlas referred to in this Chapter are, Modern Geography (M.G.), plates I. III. IV. XXVI. GEOGRAPHY implies a description of the earth, being derived from the Greek words yãn, the earth, and ypa‡w, to describe. The form of the earth is very nearly spherical; the polar axis being only about 38 miles shorter than the equatorial, which, in a diameter of nearly 8000 miles, can produce no sensible difference. The principal circles on the globe (Pl. I.*) are, the Equator, the Ecliptic, the Tropic of Cancer, the Tropic of Capricorn, the Arctic and Antarctic circles. Every circle, whether greater or less, is divided into 360 degrees; for the ancients supposed that the Ecliptic, or circle which the sun appears annually to describe in the heavens, was completed in 360 days. Each day's advance in this circle they called a gradus, or step, or degree, and applied the same mode of division to circles in general. Each degree is subdivided into 60 minutes, * The references are made to the maps, as numbered in the Index to them. B and each minute into 60 seconds. Degrees, minutes, and seconds are marked thus, ', "; thus 23° 40′ 52′′ means 23 degrees, 40 minutes, 52 seconds. The half of 360 is 180; and the half of 180, or the fourth part of 360, is 90. Hence, if the whole circle contains 360°, a semicircle will contain 180°; and a quadrant, or quarter of a circle, will contain 90°, or an angle called a right angle. Hence it will be seen that the Equator dividing the earth equally, must divide each circle passing through the poles into two semicircles, containing 180° above, and 180° below; or, reckoning by quadrants, into two quadrants of 90° each above, and two of 90° each below the Equator.* The Ecliptic, or circle which the sun appears to describe in the heavens, sets out from the Equator, and rises through the first quadrant to the Tropic of Cancer; it then turns † towards the Equator, which it again meets 180° from the place where it set out; it then descends for the third quadrant, * A straight line passing through the centre of any circle, till it meets the circumference in two points, is called the diameter of the circle, because it diapeтpeĩ —measures through it. Half this diameter (or a line drawn from the centre to the circumference in one point) is called the radius of the circle. And it is a property of the circle to have all its radii, or diameters, of equal length. If a circle be supposed to turn round on its diameter, it will generate a solid figure called a sphere. Such is the figure of the earth very nearly. The diameter on which the circle revolves is called its axis. The extreme points of this diameter are called its poles, from woλɛïv—to turn round. A great circle is any circle described on a sphere, whose diameter is equal to the diameter of the sphere. The Equator and Ecliptic are called primary great circles. A secondary, is a great circle, whose axis is at right angles to the axis of the primary; the poles, therefore, of the secondary will be 90° from the poles of the primary. An arc is any part of the circumference of a circle contained between two radii, and is denominated from the number of degrees it contains. Thus 30° of the circumference, contained between two radii, is called an arc of 30°; a quadrant is an arc of 90°; a semicircle is an arc of 180°. Parallels are lesser circles which every where keep at the same distance from the primary circle, and so run, as it were - παρ' ἀλλήλους — by the side of each other. † Hence the name of Tropic, from rρéπε, to turn. |