30 Observations on the taking down and 



in a work* published in the commencement of 1822 ; the latter 

 had been geometrically solved by the same author about fourteen 

 years ago. His formulae may be applied to the case of the pro- 

 posed new London Bridge, as follows : 



Let the material be Cornish granite ; then it may be deduced 

 from the experiments of Mr. George Rennie, in the Phil. Trans. 

 1818, that a prism of this material will crush at its base, if it be 

 5500 feet high. Let a prism one foot square in horizon- Feet 



tal section, one-fourth of that height t, be denoted by /= 1375 J 



Let the height of another prism of the same section, 

 and of the same material, equal in weight to the 

 weight to be borne, allowing for contingencies, be de- 

 noted by . . . . . . . . w = 24 § 



One cubic foot of the same material, by . g =: I 



The thickness of the given arch at the vertex, by n = 6,25 || 

 The breadth of the given arch, from face to face, by b = 50H 

 The semi-transverse, by . . . • . t = 75 

 The semi-conjugate, by . . . . . c = 29,5 



The radius of curvature of the arch at the vertex, by r = 190 

 (2Q' = 22500 _ 1Qf) 

 4c 118 



* Tracts on Faults and Bridges, pages 36 and 67, Tract 2. 



t Mr. Emmerson considered that one-fourth of the breaking strength should 

 be considered the practical strength, which rule seems to have been adopted 

 by the late Mr. Renuie, according to his evidence before Committees of the 

 House of Commons. 



J The weight necessary lo crush a thin piece of Cornish granite 1| inch 

 square, is 14302 lbs. avoirdupois. A foot cube of Cornish granite weighs 



2662 ounces avoir. = 165,4 lbs. and !i?2? x 64 = 6500 and S50 ° = 1375. 



165,4 4 



§ M. Perronet considered that 14 times the probable weight should be added 



for insurance weight, taking 4000 lbs. = the possible pressure and concussion 



- , ,, 4000x16 

 per foot sup. : then = 24. 



2662 



|| — = 6,25, being ith of the span. 

 24 



*T Mr. Myloe recommended the bridge should be, when rebuilt, 50 feet. 



The engraving daes not show the intended width of Mr. Hennie's design. 



