49
breaking weight, which, in such a curvature as theangle 11 8 15' at the point of suspension, gives, shouldnot have been assumed but with very great allowance.(See also Tract 3 , page 29.)
Abstract, shewing the limit of weight on a foot superficial at the vertex, thefollowing bridges are capable of sustaining, derived from the foregoing cal-culations, supposing them equally strong in respect to compression in everyother part; also the probable and insurance weights.
Architect.
Material.
Limit ofweight in lbs.avoirdup.
Probable
avoird.
Insurance 01excess forcontingen-cies, lbs.avoirdupois.
The bridge of the Holy")Trinity, at Florence j
— Tete, Picardy
— Southwark, London —Menai pend ent bridge, 1
Bangor, North Wales J
— Proposed pendent (bridge at Runcorn J
Ammanati
Perronet
Rennie
Telford
Coffinette
Weibeking
Rennie
Telford
Telford
Statuary marble,not veinedSaillancourt stoneCornish graniteAberdeen graniteFir
Fir
Cast-iron *Wrought-iron
Ditto
13313
3966
39846
102172
1106
1359
19926
221
60
250
250
250
250
150
150
250
13063
3716
39596
101922
956
1209
19676
Gauthey says (vol. 1, page 67), “The great thick-ness which the ancients have every where given tothe supports in their edifices, lead to the conclusionthat they had little idea of the resistance of material;
* The excess in respect to the bridges of Bonar and Craigel-lachie could not be calculated for want of the distance betweenthe ribs and the thickness of them; but by referring to the dateof their erection, and the note page 33, it may be presumed thatthey are 40 times weaker than intended in respect to compression.
When it is understood to what extent the expence ofpublic works is increased in order to insure the risk arisingfrom a want of knowledge; probably there may arise a Colbert tolimit the waste, by causing an inquiry to be made into the qua-lities of matter applicable to building purposes in respect to ten-sion and compression, and by determining measures of fracture,and measures of strength.