PROJECTIONS 457 



be determined with great accuracy, for its height will be 

 indicated by the line passing through it or near it. These 

 maps are called contour maps. 



219. Contour Maps. Although it is easy to find the ele- 

 vations of places on a contour map, it is hard to get a 

 clear idea of what a contour map really expresses. The 

 best way to gain an appreciation of a contour map is to 

 get a map of the region in which you live, take it into the 

 field, and study map and region together. Another ex- 

 cellent way is to make a contour map of a model. When 

 once you have made a map of this kind, you will readily 

 understand all other similar maps. 



We must remember that a contour is the projection on 

 a flat surface of a line which passes through places of 

 equal elevation. It shows where the margin of water 

 would come if the place in question were submerged to a 

 given depth. No two contours can possibly cross each 

 other, as no place can have two elevations. No contours 

 can ever end except at the edge of the map, for a sheet of 

 water must have a continuous boundary and only where 

 the map terminates can the line representing the edge of 

 the water appear to end. 



Experiment 133. Provide each pupil with a contour map represent- 

 ing the home locality if possible ; if not, use the contour map facing 

 page 345. Let the teacher or different pupils pick out places and ask 

 some one to give their elevations. In this way you will get an idea 

 of how elevations can be determined by use of a contour map. 

 Notice the different topographical symbols used on the map. 



220. Maps of Curved Surfaces. Projections. The accu- 

 rate mapping of small areas offers no great difficulty 

 because these are practically flat, but when an attempt is 

 made to represent a curved surface upon a flat surface, 

 difficulties present themselves which are insurmountable. 

 If the rind of an orange is taken off, it cannot be made to 



