300 Mr D. Stevenson on Long-s American Frame-Bridge. 



bridge the several parts of the two truss-frames are, in the 

 first place, properly fitted and put together in the work-yard ; 

 and after this has been done, and the beams and flooring tim- 

 bers prepared, and the abutments built for the reception of 

 the bridge, preparations are made for erecting it by forming a 

 temporary bridge or support of pile-work in the bottom of the 

 river or ravine which it is to span. This support is construct- 

 ed in a substantial manner, and raised to the level of the lower 

 part of the bridge. When this has been done, the truss-frames 

 are taken asunder and erected piecemeal, and the lateral braces, 

 flooring timbers, &c. applied in the manner shewn in the 

 drawings. 



The only peculiarity that remains to be noticed is the driv- 

 ing home of the keys or wedges, an operation which is done 

 in the following manner before the scaffolding is removed : — 

 Four workmen, two stationed on each side of the bridge, com- 

 mence by driving the keys of the posts on each side of the 

 centre post (taking care to drive the keys at the bottoms of 

 the posts first, and afterwards those at the tops), and so ad- 

 vancing gradually with each post towards the abutments at 

 each end of the bridge. The counter-brace keys are next 

 driven home, and in doing this the workmen commence at the 

 abutments and drive regularly along to the centre of the 

 bridge, driving them as hard as they may be driven without 

 splitting or injuring the wood, with a smooth-headed hammer 

 weighing four or five pounds. This operation having been 

 completed, the flooring is put on, and the scaffolding removed, 

 when the bridge is ready for the passage of goods and pas- 

 sengers. 



In conclusion, I may remark that bridges constructed on 

 this principle, varying from 100 to 150 feet in span, are very 

 commonly met with in the United States ; and on the Ameri- 

 can railways I have crossed bridges of this construction, va- 

 rying from 50 to 100 feet in span, in a heavily loaded train 

 drawn by a locomotive engine, weighing from 12 to 15 tons, 

 without observing the smallest appearance of fracture, or even 

 of yielding in the woodwork of the bridge. 



Edinburgh, April 1839. 



