I.] THE THAMES. 15 



London the river has a total fall of about 250 feet; and as 

 the descent is tolerably uniform, it may be taken at an 

 average of 21 inches per mile. At Teddington, where the 

 river ceases to be tidal, the ordinary summer-level of low 

 water is i6| feet above low-water level at London Bridge; 

 and the fall of the bed of the river below Teddington is 

 nearly a foot in every mile. The rapidity with which a river 

 flows will of course depend upon the amount of slope in 

 its bed ; where the fall is great the stream is rapid ; where 

 small the stream is slow. The bed of the Thames, for- 

 tunately, is tolerably uniform in its descent, so that the 

 stream is free from rapids. 



What has been said of the Thames is equally true of any 

 one of its tributary streams : the source is always higher 

 than the mouth. It is seen from the contour-lines on the 

 map that, if we travel along any of the little rivers which 

 open into the Thames upon its left bank, we go up-hill in 

 passing from south to north ; if we travel along any of the 

 streams on the opposite side of the nver, we go up-hill in 

 passing southwards. As a consequence of all this, it follows 

 that the tract of country drained by the Thames and its 

 tributaries must be bounded on at least three sides — the 

 west, the north, and the south — by comparatively high 

 ground. It thus forms a shallow depression, with an outlet 

 to the east through which the river flows out to sea. Such 

 a depression is known as a river-basin, and the country 

 through which the Thames and its tributaries flow is conse- 

 quently called the Thames basin; while the deepest part of 

 the basin, that in which the main stream flows, is termed 

 the Thames Valley. The basin of the Thames, depicted in 

 Plate I., includes a very large tract, extending over 6,160 

 square miles ; the Thames, in fact, drains more than one- 

 seventh of all England. 



