250 GEOGRAPHY AND ASTRONOMY. SECT. XI.-Of Maps and Sea Charts. THOUGH nothing can represent the heavens or the earth in their natural appearances so exactly as a globe, yet the two hemispheres either of the heavens or of the earth may be repre- sentedupon a flat or plain surface, which are generallycalled pro- jections of the sphere. If you suppose a globe to be cut in halves just at the equator, and each hemisphere represented on a plane, it is called a " pro- jection of the globe upon the plane of the equator. Then the equinoctial line will be the circumference, and the two poles of the world will be the centres of those two projections, and all the meridian lines will be so many strait lines or semidiameters meet- ing in the centre. This is the most common method ofrepresent- ing the celestial globe and the stars. ' If theglobe be cut asunder at the horizon of any particular place and thus represented on a plane, it is called the " projec- tion on the plane of the horizon." Then the zenith and nadir will be the centres of those projections, and the horizon is the circumference. The two poles will be placed at such a distance from the circumferenceas the pole of the world is elevated above the horizon of that place ; and the meridian will be represented as curve lines meeting in thepole point, excepting Only that meri- dian that passes through the zenith which is always a right line. This is a more uncommon projection of the sphere, though it is much used in dialling. The most usual way of describing the earthly globeon a plane, or a map, is to suppose the globe cut in halves about the first meridian at the island of Ferro or Teneriff. This is a " projection on the plane of the meridian : then the first meri- dian will determine the circumferences : The pole points will stand in the upper and lowerparts of that circle and the other meridians will be curve lines meeting in the pole points, except that which passes through the centre to the projection, which is a right line. Here the equator will be a strait line or diameter crossing all the meridians at right angles, and at equal distances from the two poles. Here the two tropics of cancer and Capricorn are drawn at their proper distances of 231 degrees from the equa- tor ; and the two polar circles at the same distance from the poles. In this projection the ecliptic is sometimes a strait line cut- ting the middle of the equater obliquely in each hemisphere, and ending where the two tropics meet the meridian : But sometimes the ecliptic is drawn as a curve line or an arch beginning where the equator meets the meridian, and carried Upward just to touch-the tropic of cancer in one hemisphere, and downward to touch the tropic of capricorn in the other. It is in this form
RkJQdWJsaXNoZXIy OTcyMjk=