The construction of the .globillar or equidistant projection is as follows (fig. 1): De scribe a circle NESW, to represent a meridian, and draw two diameters, NCS and WCE, • leerpendicular to each other, .he one for a central meridian, the other for the equator. 'Then N and S will represent the north and south poles. Divide each of the auadrants into 9 equal parts, and each of the radii ON, CE, and C also into 9 equal paits. Pro Aluce NS both ways, and find on it the centers of circles which will pass through the three points 80 x 80, 70 y 70, etc., and these arcs described on both sides of the equator -will be the parallels of latitude. In like manner, find on WE produced, the centers of .eircles which must pass through a, b, c, and the poles. Having selected the first meridian, number the others successively to the east and west of it. A_ map in this way may be constructed on the rational horizon of any place.
The impossibility of getting a perfect representation of special parts of the sphere by any of the previous methods, led to the desire for others less defective. Of all solid bodies whose surfaces can be accurately developed or rolled out upon a plane without alteration, the cone and cylinder approach nearest to the character of the sphere. A portion of the sphere between two parallels not far distant from each other, corresponds very exactly with a like conical zone; whence it is that conical developments make the best projections for special geographical maps, and even with some modifications for large portions of the globe.
A conical projection of Europe (fig. 2) is constructed thus: Draw a base line AB of .indefinite length; bisect it in E, and at that point erect a perpendicular ED, to form the .central meridian of the map. Take a space for 5° of latitude, and since Europe lies between the 35th and 75th parallels of latitude, mark off eight of these spaces along ED for the points through which the parallels must pass. The center from which to de -scribe the parallels will be the point in ED where the top of a cone, cutting the globe at the 45th and 65th parallels, would meet the axis of the sphere. This point will be found to be beyond. the north pole at C. Since on the parallels of 45° and 65°, where the cone ,cuts the sphere, the degrees of longitude are exactly equal to those on the globe, if on -these parallels distances be marked off equivalent to 5 degrees of longitude, in propor lion to the degrees of latitude in those parallels, and through these points straight lines be drawn from C, they will represent the meridians for every 5 degrees.
Since all meridians on the globe are great circles passing through the poles, the north .and south points at any place correspond with the poles of the earth. The east and. west points, however, are indicated by a line at right angles to the meridian, and do not, except at the equator, correspond with those of the earth. In all the projections hitherto described, the direction either of the north and south, or of the east and west -points, is represented by a curved line, so that on such a map the course of a vessel would almost always be laid down in a curve, which could only be described by contin ually laying off from the meridian a line at au angle equal to that made with the merid ian by the point of the compass at which the ship WAS sailing. If the vessel were to steer in a direct n.e. course by one of the previous projections, she would, if land did
not intervene, describe a spiral round, and ultimately arrive at the north pole; there fore, the mariner requires a chart which will enable him to steer his course by compass in straight lines only. This valuable instrument is supplied by Mercator's chart, in which all the meridians are straight lines perpendicular to the equator, and all the par allels straight lines parallel to the equator.
It is constructed as follows (fig. 3): A line AB is drawn of the required length for the equator. This line is divided into 36, 24, or 18 equal parts, for meridians at 10°, 15°, or 20° apart, and the meridians arc then drawn through these perpendicular,to AB. From a table of meridional parts (a table of the number of minutes of a degree of longitude, at the equator comprised between that and every parallel of latitude up to 89°), take the distances of the parallels and of the tropics and arctic circles from the equator, and mark them off to the north and south of it. Join these points, and the projection is made.
This projection, of course, does not and is not intended to give a natural representa tion of the earth, its effect being to exaggerate the polar regions iinmensely. The dis tortion in the form of countries and relative direction of places, is rectified by the de grees of latitude being made to increase proportionably to those of longitude. This is the only map which gives an unbroken view of the whole surface of the earth.
The term map is specially applied to representations of land, or land and water to gether; while that of chart is limited to the coast and water surface only, showing cur rents, rocks, anchorage, light-houses, harbors, soundings, and other objects of impor tance to seamen.
A geographical map proper is a general map of the world, or of a large extent of country. A topographical map differs from it in being limited in area, and much more detailed. The ordnance survey of Britain is a good example of a topographical map. Besides purely geographical and topographical maps, others are constructed for special purposes, which may be physical, political, or civil, military, statistical, historical, etc.
In order to construct a map, and to determine accurately the positions of places on it, a knowledge of two elements is essential—viz., latitude or distance from the equator, and longitude or distance east or west of the meridian adopted.
Every inap, whatever its dimensions, is in some definite relation to the actual size of the globe. This relation is indicated by a scale—a graduated line showing, by its divi sions, the number of miles corresponding to any space measured on the map. The scales of geographical maps range from about 800 m. to an inch (for maps of quarters of the globe) to 10 m. to an inch; those of topographical maps range from 1 in. to 25 in. to a mile, the largest topographical maps we have, admitting of the most minute details.
The ordnance survey of Great Britain is on the scale of „i „ of nature, or 1 in. of paper to 1 m. of surface.
A recent improvement introduced into our best maps is that of printing the water courses in blue ink, making the orography and skeleton of every country stand out inr clear rel;ef, thus avoiding the confusion resulting from all the lines being black, as in older maps.