SCHUYLKILL PERMANENT BRIDGE^ X5 



ed. The weight of transportation here is uncommon 

 and constant, and the friction of course incessant. 

 Strength, symmetry and firmness, are required here ; of 

 which one very extended arch is incapable. Although 

 wood or iron may be so framed, as to have the least 

 possible djiftjOY lateral thrust, on the abutments or piers, 

 yet there is a point, beyond which it is dangerous to 

 pass. Of stone or brick it would be adventurous, be- 

 yond all common discretion, to risque an arch of such 

 . a span. Nor is the undulatory motion of an extensive 

 arch, (however composed) an unimportant objection. 



A bridge of so extended a span must have been (to 

 be safe) so much more elevated, that the filling would 

 have pressed the walls too dangerously. Some relief 

 might have been given b}^ culverts, or reversed ai'ches, 

 to $ave filling ; but these ai'e not without their disadvan- 

 tages. The pressure on the walls of the present west- 

 ern abutment and wings, is quite as much as masoniy 

 on piles will bear ; and no other foundation could have 

 been had, but at an unwarrantable expence, the rock at 

 the site of the abutment, being covered with mud and 

 gravel 38 to 40 feet deep. It was deemed, and found 

 prudent, to sink the whole frame of the present struc- 

 ture, three feet into the piers, and imposts of the abut- 

 ments, as well to avoid over weight of filling, as to de- 

 press the platform, or travelling floor, to a point easv of 

 access. An approach of the abutments, for an arch of 400 

 feet span» would have created a necessity (not known 

 when sucli a plan was proposed) for coffer dams, and all 

 their dangers and expence. The present bridge enlarges 

 the passage for the water, at least, a fifth. One for an arch 

 of 300 to 350 feet. Mould have diiriinished it in a greater 



