I.] THE THAMES. 13 



quently lighter. Such a system of shading, however effec- 

 tive by its combination of .light and shade, shows in most 

 cases merely that one part of the country is higher or 

 lower than another, without enabling us to judge how 

 much higher or how much lower. But in very accurate 

 maps, such as those employed for military purposes, a 

 definite scale of shade is often used. The same object 

 may, however, be attained by an entirely different system, 

 such as that used in the map of the Thames forming 

 Plate I. 



It will be observed that, instead of hill-shading, a number 

 of curved lines have been traced over the map, giving it a 

 pecuhar character. These curves are called contour lines, 

 and their meaning is extremely simple. Suppose the valley 

 of the Thames were flooded with water, and that this water 

 could be dammed in or prevented from escaping, by a 

 wall built across the mouth of the valley. If sufficient 

 water covered the ground to stand 100 feet above the 

 level of the sea, the surface of the water would form a 

 plane, and its margin would trace a Hne, winding round 

 every hill and up every valley, at a height of exactly too feet. 

 Such a line has been traced on our map, and being the first 

 of the series of curves, it is numbered i. This is conse- 

 quently the 100-feet contour line. The second line, No. 2, 

 is drawn at a height of 200 feet above the sea-level, and 

 therefore represents the margin of a body of water standing 

 in the Thames Valley 200 feet above the sea. In like 

 manner a succession of these contour-lines has been drawn, 

 each at a distance of 100 feet from the next one below, just 

 as though the flood had risen in the valley and stopped at 

 every 100 feet to leave its mark around its margin. It is 

 evident that a system of such lines conveys a far better 

 notion of the character of the ground than can be obtained 



