1-1 THE THAMES. 



II 



north. This declination is shown in Fig. 2. Knowing 

 the amount of declination, it is easy to make the proper 

 allowance and thus find the true points of the compass. 

 By means of the compass the direction of the river in all 

 its windings may be traced, and the course of the mean- 

 dering stream laid down upon a map, as has been done, 

 for example, in Plate I. 



This map, however, does something more than show 

 simply the direction of the Thames and its tributaries ; it 

 gives us, in addition, some notion of their length. A map, 

 as we have seen, is a kind of picture, and the size of this 

 picture must bear a certain relation to the size of the object 

 represented. This relation or proportion is called the 

 scale of the map. If a map is said to have a scale of one 

 inch to the mile, it is simply meant that a mile measured 

 along the ground is represented by an inch measured on 

 the map ; or a square mile of country is represented by a 

 square inch on the map, and so on. Most of the wonderfullj'- 

 accurate maps of the Ordnance Survey are constructed on 

 this scale of one inch to the mile. In other words, the 

 map of a given district is es^ao th of the size of the natural 

 area, since there are 63,360 inches in one statute mile. 

 The fraction which denotes the ratio of the map to the 

 true area is sometimes termed the representative fraction. 

 A map of the Thames on the one-inch scale would 

 extend to a length of about 120 inches, since the greatest 

 width of the basin of the river from east to west is about 

 120 miles. Maps on a scale even much greater than this 

 are occasionally constructed. The Ordnance Surve3^ for 

 example, issues county maps on a scale of six inches to a 

 mile, the representative fraction being here -^%^ts- ^'^^^ 

 it is evident, from the size of a page of this book, that 

 our map must be on a very much smaller scale ; in fact, 



