28 Observations on taking doivn and 



overflowed, partly by land floods, but chiefly by high tides, 

 which rise above a foot higher below bridge than they do at 

 present above bridge: the evil will therefore be proportionably 

 increased both in degree and frequency. But besides the 

 common operations ol" land floods and tides in calm weather, 

 all the river above bridge will, when the dam is removed, be 

 further exposed to the influx of heavier v,aves driven from the 

 Nore, with storms from the northward, which have hitherto 

 been checked by the almost solid mass of the upper part ot 

 London Bridge. These observations apply to all the banks 

 and low grounds on each bide of the river from Westminster 

 to Teddington, and which are very extensive. 



Instances of such influx and rising of the tide have been 

 already mentioned, and another has come to my knowledge 

 while engaged in the jiresent survey. At the Caslicn river in 

 Kerry, which falls into the sea near the mouth of the Shan- 

 non, a bar has been lately cut across to make a more direct 

 navigation : the upper river has thereby been lowered two or 

 three feet at low water, and at high water raised so as to over- 

 flow the marshes more than before; and the direct stream is 

 now cutting a channel through the sandy shoals above the 

 bar. This information I received from the able engineer 

 (Mr. Nimmo) who advised the measure. 



The Effects Eastvcard of the Bridge. — No longitudinal or 

 cross sections having been taken to the eastward of the bridge, 

 I have no accurate knowledge of the state of the river bed, 

 and can therefore only observe generally, that my investiga- 

 tions have led me to the conclusion that more water will pass 

 with a greater velocity in every jiart of the river ; but as the 

 difference will diminish as the section increases, the effects will 

 of course disappear in the lower parts of the river. When 

 operations do take place, they will scour and deepen the I'iver, 

 where the matter is alluvial and loose. 



24, Abingdorirstreet, Westminster, ThOMAS TeLFORD. 



June 11, 1823. 



IX. Observations on the Project of taVing doxcn and re- 

 build ing London Bridge *. 

 J T is a matter certainly' of great interest to men of science, to 

 know what effect the removal of a dam producing a fall of 

 water westward at high water sometimes of two feet, and east- 

 ward at low water sometimes of nine feet, from a great river 

 like the Thames, would ha\e westward and eastward of that 

 dam in respect to the bed and shores of such a river ; and 



^ From the Quarterly Journal of Science, &c. No, xxx. 



whether 



