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THE BOOK OF NATURE STUDY 



This method has the advantage of showing precisely how 

 much land is above and how much below a given level, and 

 what is the exact shape of the land when followed along any 

 one level. A map constructed on this plan, with contour lines 

 drawn at any given intervals (usually, in Britain, 100 feet), is called 



A 



FIG. 44. Contour map of part of area shown in Fig. 40. Contour lines are 

 drawn at 100 feet intervals. A, B and C, D are the lines of section given 

 on Figs. 46 and 47. 



a contour map, and is at once seen to have many uses (Fig. 45). 

 To take an absolutely level walk, going neither uphill nor downhill, 

 it is only necessary to follow a contour ; and to construct a canal 

 without locks, cuttings, or tunnels, it would be necessary that it 

 should follow a contour line throughout. Crossing from one con- 

 tour to another will involve going up or down, and the quicker 



