I.] THE THAMES. 17 



limits of tliis page. Hence it is a common practice in 

 constructing such diagrams to draw the heights to a scale 

 many times greater than that used for the lengths. This has, 

 in fact, been done in Fig. 5, where the vertical is as much as 

 twelve times greater than the horizontal scale. Without such 

 exaggeration, the surface of the country would appear in 

 a sm.all diagram to be almost flat; and even with it, the 

 extreme shallowness of the Thames basin is strikingly ap- 

 parent. There is clearly no harm in the practice of drawing 

 diagrams on this principle, provided that the exaggeration 

 of one dimension is always acknowledged by the draughts- 

 man, and borne in mind by the reader. Great misconception, 

 however, constantly arises from mistaking these intentionally 

 distorted diagrams for true figures. 



To the north-west of London, the margin of the Thames 

 basin is formed, in part, by a hne of low hills called the 

 Chiltern Hills ; and, on the south of London, there is an- 

 other series known as the North Downs ; while, if we go far 

 enough to the west in the Thames basin, we come to a still 

 higher country forming the Cotteswold Hills in Gloucester- 

 shire. Suppose the reader were to ascend one of these 

 ranges of hills, say the Cotteswolds, on the west. As he 

 went up he would meet with many little streams which are 

 flowing down to feed the affluents of the Thames. But 

 having reached the summit, and walked on in the same 

 direction, he would soon begin to go down-hill, and 

 then meet fresh streams running in an opposite direction to 

 those he had left. These new streams cannot possibly flow 

 into the Thames, for to do that the water would have to run 

 up-hill. By following these streams, however, he would find 

 that they ultimately flow into a river entirely distinct from 

 the Thames ; thus, on the other side of the Cotteswolds, the 

 streams find their way, sooner or later, into the Severn. On 



c 



