STEREOGRAPHIC PROJECTION 459 



latitude to vary from about ^-J^ of the earth's circumference 

 at the equator to infinity at the poles ; and a degree of 

 longitude, which near the poles has almost no length, is 

 made to have a length everywhere equal to that of a 

 degree on the equator. Thus passing from the equator 

 toward the poles, the areas of surfaces on the earth are 

 increased when represented on this projection, but the 

 increase east and west and the increase north and south 

 are not equal. This causes the shape of the portions of 

 the earth farthest from the equator to be much distorted. 



The Mercator projection is the most commonly used of 

 all projections. It is a simple modification of the cylin- 

 drical, in which the exaggeration north and south is made 

 equal to that east and west. In this projection the polar 

 regions are greatly enlarged. This explains why Green- 

 land, which on the globe is of comparatively small size, 

 when seen on the ordinary map of the world is half the 

 size of North America. The great advantage of this pro- 

 jection is that the meridians and parallels are both repre- 

 sented by straight lines. A navigator can thus at any 

 time find his course 'by drawing a straight line joining the 

 places between which he is sailing. This is why most 

 nautical charts are constructed on this projection. But 

 to geographers this projection is not of as great value as 

 some others since the shapes of the land masses are so 

 much distorted. 



222. Stereographic Projection. Of the hemispherical pro- 

 jections probably the best for study is the stereographic. 

 This, or a slight modification of it, is the projection upon 

 which are constructed the hemispherical maps usually seen. 

 In it a plane is considered as held tangent at a certain 

 point on the globe and from a point on the globe directly 

 opposite the point of tangency, lines are drawn to the 

 plane through the intersections of the parallels of latitude 



