2 PHYSIOGRAPHY. [chap. 



raised by the passing breeze, the whole body of water is 

 in a constant state of motion. At one time in the day the 

 water sweeps down below bridge in the direction of Green- 

 wich and far onwards to the Nore ; after this movement 

 has been continued for some hours, it gradually slackens, 

 and the water comes almost to a stanJ-still; then the motion 

 begins afresh, but its direction is reversed, the water flowing 

 this time towards Westminster and far away up the river ; 

 but, after a while, this motion slowly subsides, and is followed 

 once more by renewed movement in a contrary direction. 



Every one knows that this regular backward-and-forward 

 movement of the great mass of water is due to tidal 

 action. For about seven hours during ebb tide, the water of 

 the Thames runs down towards the sea ; and for about five 

 hours during flood tide the movement takes place in the 

 opposite direction, the water being then driven up the 

 river. At the end of the ebb-tide the river is shallowest ; 

 at the end of the flood-tide it is deepest. The water at 

 London Bridge is consequently twice in every four-and- 

 twenty hours at its highest, and twice at its lowest level. 



As we go up the river we find the effect of the tide 

 gradually diminishing, until at length it ceases to be felt. 

 In point of fact, the tidal wave has no perceptible influence 

 beyond Teddington Weir, some nineteen miles above 

 London Bridge. The very name Teddington is indeed said 

 to be a corruption of " Tide-end-ton," the town where the 

 tide ends. Up to Teddington, then, the Thames is a 

 tidal river, moving alternately backv;ards and forwards at 

 definite intervals. The tidal wave occupies about two 

 hours in coming up from the Nore to London, a distance 

 of about 40 miles ; that is to say, it is high water at London 

 Bridge two hours later than at the Nore. Then it takes 

 two hours more for the wave to travel onwpnk from London 



