H 



PHYSIOGRAPHY. 



[chap. 



from ordinary hill-shading. Where the ground is very steep 

 the contour-lines run close together; where very flat they 

 stand far apart. The relation between the contour-lines and 

 the form of the ground is clearly shown in Fig. 4. In the 

 upper part of this figure, a hill is represented by contour- 

 lines ; and supposing this hill were cut through, along the 

 line A B, it would give such an outline as that drawn in 

 the lower part of the figure, the corresponding points in 

 the plan and the section being connected by broken lines. 



Fig. 4. — Cjntour-lines round a hill. 



Inspection of the map in Plate I. shows, as might have 

 been expected, that the river there represented flows 

 from high ground to low ground. In fact, if the reader 

 were to travel up the Thames, by walking along its banks, 

 he would find himself continually going up-hill. Between 

 Thames Head and London Bridge, a distance of about 

 170 miles, as measured along the winding course of the 

 river, there is a difference of level of 370 feet. We 

 have seen that the head of the navigation is at Lechlade, 

 about 146 miles from London ; between Lechdale and 



