16

they have in some instances been mismanaged bybungling constructors, they have, in others, beensuccessfully extended to 130, 140, and 240 feetarches, with scarcely any other approach to a maxi-mum than what is determined by locality and ex-pense.” In comparing the tables of wooden andiron bridges together, it would seem that wood isa stronger material than iron ; but as that is not thefact, it may be concluded, that the most successfulconstructors have yet a great deal to learn. Assum-ing iron and wood to be of the same strength incompression, the most successful constructors aredeficient as 90 to 60. But taking the ratio of strengthof iron to wood as the experiments give, then evenWiebeking may be more hardy in his opinions inrespect to the proper height of the frame of an archat the vertex of an iron bridge.