28 Observations on the taking down and 



templated the grandeur of the work about to be erected. But 

 having received from a friend the Act " for the rebuilding of Lon- 

 don Bridge," and a drawing of the new bridge designed by the 

 late Mr. Rennie, his attention has been more particularly drawn 

 to it ; and the examination makes him regret that he cannot be 

 convinced of the expediency of a new bridge ; for, to see across 

 the Thames an elliptical arch in stone, whose transverse axis is 

 150 feet, and its semiconjugate 29 feet 6 inches, with two other 

 arches on each side of it, of a like character, standing on bearing 

 piers 45 feet high, would afford him a gratification which he is 

 unable to express. In such a work it seems proper that the most 

 able person should be selected to furnish the design, and erect the 

 bridge ; and doubtless the late Mr. Rennie was that person. It 

 also seems proper, now that gentleman is no more, that the faith 

 he would have inspired, should be obtained by investigation and 

 proof. In the absence of any shewing of the expediency of the 

 demolition of London Bridge, and of the reality of any beneficial 

 effects to arise, or of the fallacy of the evil effects anticipated from 

 it, and in the want of every scientific explanation of the principles 

 by which the late Mr. Rennie regulated the proportions of his 

 bridges ; the observations in respect of the taking down the present 

 bridge, and the following inquiry in respect of the new one, are 

 submitted to you, not without a hope that in your next Journal 

 all the desiderata will be supplied by some one of those who have 

 pledged themselves to the success of these undertakings. 



When the heights of the piers, and form of an arch of a bridge, 

 and the kind of stone of which it is to be constructed, are deter- 

 mined, the first object of inquiry is the thickness of the arch 

 at the vertex. Mr. Rennie, in his stone bridges, adopted the 

 rule* which M. Perronett says is customary in large arches sur- 

 baissees, namely, to make the thickness at the vertex of an arch 



* Examples — Waterloo, Darleston, and Weston. The key-stone in the 



faces of Waterloo is said to be only 4 feet 6 inches, but the thickness of the 



i. • c i 120 

 arch is 5 teet = — . 

 24 



t CEtwres de Ptrronet, pages 635 and 664. 



