24- Mr. Telford on the Infects on the Thames 



and if London Bridge be removed, so that there be no ma- 

 terial dam at low water, we have also to fill the pond now 

 caused by that dam. This pond is from 4 to 6 feet deep at 

 the bridge, at low water ; and we find that the level of low 

 water above bridge meets the bottom of the Thames between 

 Putnev and Kew, viz. lOi miles above bridge: taking this as 

 the head of the pond, the average breadth at 600 feet at low 

 water, the mean depth to be filled at 2 feet, we have an addi- 

 tion of 15 millions of cubic feet, or l-57th of the quantity of 

 tide-water between London and Gravesend, or only l-2S4th 

 of the whole quantity of tide-water within the Nore; therefore 

 the whole water which must pass the New Bridge, to raise the 

 upper river to the level of high water below bridge, is 657 

 millions, or l-32d of the entire quantity of tide-water within 

 the Nore below bridge. 



It is a well-known fact that the tide in narrow channels with 

 funnel-shaped mouths, or against coasts which oppose its re- 

 gular course, rises considerably higher than at the places 

 which are situated in retired bays, or under the wake of pro- 

 jecting points : thus the Atlantic tide running up the Channel 

 rises 6 or 7 fathoms against the French coast near St. Malo 

 and Havre; while on the opposite English coast, at Port- 

 land and Poole, we have only one fathom rise. In St. George's 

 Channel, the tides at Milford and along the Welsh coast rise 

 four fathoms; on the opposite Irish coast, from Carnsore 

 Point to Wicklow, hardly one fathom. Many similar instances 

 might be given. Again, as to funnel-shaped mouths : the 

 spring-tide at the entrance of Bristol Channel rises 22 to 24 

 feet ; but as that channel contracts in breadth, the velocity and 

 A'ertical rise increase in pi'oportion so much, that in King Road 

 it rises between 7 and 8 fathoms. Many other similar in- 

 stances may be shown. As may be pei'ceived by the position 

 of the banks of the Thames' mouth, the flood-tide comes from 

 the N.E. or German sea: at half-past eleven it is high water at 

 Harwich, Kentish Knock, and Margate ; the oscillation or 

 rise at springs is from 15 to 16 feet; at twelve it is high water 

 at the Nore; and although the rise thei'e is only 14 feet, yet 

 in the Swale, which is in the direct course of the tide, the rise 

 is 17 to 21 feet at half-past twelve. 



The general set of the current running up the Thames forms 

 a branch which at the Nore at noon rises, as we have said, 

 14 feet; but from thence the funnel-shape produces a gradual 

 increase in the oscillation until we arrive near London : that 

 at Gravesend, at one, the rise is 1 6 feet ; at Woolwich, at 

 three-quarters past one, it is 18 feet; at Deptford, at two 

 o'clock, we have 18-^- feet; but at Billingbgate, at a quarter 



past 



