220 FIRST YEAR COURSE IN GENERAL SCIENCE 



is farther west than the point 0. It cuts the line just as 

 far to the west of as the point 1 on the circle is west of 

 the point on the circle. 



Land forms are not perfectly symmetrical, as a cone or 

 a globe is. Let the dotted outline in Fig. 115 represent the 

 real profile of an eroded volcano. The contour lines of this 

 volcano are not circles. The horizontal distance between 

 them is not uniform, but the vertical distance between them 

 is uniform. (LABORATORY MANUAL, Exercise XX.) 



FIG. 116. CONTOUR OF AN ISLAND 



240. Reading a Contour Map. Fig. 1 16 is a contour map 

 of an island. The difference of elevation represented by 

 successive lines is 50 feet. The island is 5 miles long. 

 Determine the elevation of .the highest part. Name by 

 direction the line of steepest slope; that of the most gentle 

 decline. Compute the average descent on the longest slope, 

 giving the number of feet of descent to the mile. 



A line at the base of a mountain range may be 500 feet 

 above sea level. Calculating from that, we can estimate 

 the altitude of the summit above the surrounding country 

 and above sea level by counting the number of lines above 

 the base and multiplying by the number of feet in the con- 

 tour interval. (See Fig. 113.) 



241. Valley Contours. In following streams on a topo- 

 graphic map, it is seen that the contour line nearest one 

 bank of a stream is of the same elevation as the line nearest 



