B70 Observations on taking doum and 



of the water-way of London Bridge having been able to decide 

 upon these matters without the data Mr. Telford now thinks 

 necessary. Such a river as the Thames, which, at a mean 

 width between London and Blackfriars Bridges, even now the 

 dam exists, having a velocity in the mid stream of sixty-three* 

 inches per second, or Z^^ miles per hour, at half flood, requires 

 some respect to be paid to its speed, its windings, and its 

 fences, and will be found indignant to an alteration of its ancient 

 habits. The paradoxes which experiments on the flowing of 

 waters present, the recent history of the Eau Brink as to its antici- 

 pated and its actual effect on the harbour of Lynn, the erroneous 

 calculations of the Royal Academy of Paris in respect to the 

 apparently simple question of the Paris aqueduct, and those of 

 Desaguliers and M'Laurin as to that of Edinburgh, might cause 

 some doubt of any opinion with sufficient data, and much more 

 of the determinations of mere theory, from one of very advanced 

 age, without any. The question relating to the effects of the 

 removal of the dam westward, put in the following manner, 

 would cause more inquiry than the present seems to have 

 done. 



What effect would the introduction of another river on the 

 west side of London Bridge, of the same dimensions as the river 

 Thames at London Bridge, with a fall into it of two feet, have 

 upon the bed and banks westward at high water? What effect 

 would the subtraction ofaquantity of water, at low water, equal to 

 the surface of the river, six feet in depth at that subtraction, have 

 upon the river westward at that time of the tide? It has been main- 

 tained, with reference to a compensation clause in the bill for the 

 new bridge, that, in cases of land-floods, the removal of the dam 

 of London Bridge would not cause an increased height of the 

 waters in the up country, but have a contrary effect. This position 

 is true at all times of the ebbing, but not of the flowing ; a high 

 sea-flood meeting a high land-flood must dam back the latter, 

 and at times two feet higher than at present, when the dam of 

 the bridge is removed. For example, on the 28th of December, 



• See 3d Report, Appendix. G. London Port, and Plate 20, Appendix. 

 At Westminster, Mr. Labelye ascertained the velocity to be tliirty-six 

 inches per second. 



