Mr D. Stevenson on Long's American Frame-Bridge. 299 



ter braces to the upper string-piece. In this diagram b is the 

 string, c the post, d the main-brace, e the counter-brace, and 

 g is a wedge of hardwood, by which the whole woodwork is 

 tightened up. Fig. 7 shews the fixture employed at the lower 

 string. In this diagram b is the string, c the post, d the main- 

 brace, e the counter-brace,^ a wedge of hardwood, and/a block 

 on which the counter-brace rests. The frames are connected 

 at the top by cross beams, x, and at the bottom by the beams, 

 marked letter y, which support the planking of the roadway. 



The advantages attending this construction of bridge, which 

 is called in America " Long's Frame-Bridge," are, 



1*/, A comparatively small quantity of materials, arranged 

 as shewn in the plate, possesses a very great degree of strength 

 and rigidity. 



2d, The bridge exerts no lateral thrust tending to over- 

 threw the piers or abutments, which may consequently be 

 made in a much lighter and less expensive manner than the 

 abutments or piers of arched bridges of the ordinary construc- 

 tion. ' 



od, The joinings of the different parts of the bridge are 

 effected by the use of comparatively few bolts or spikes, a 

 method which admits of its being very easily repaired when 

 decay of the materials or other causes render it necessary. 



Vv hite Pine (Finns Strobus), which grows in great abundance 

 and perfection in the United States, is generally considered 

 best suited for the construction of frame-bridges. This tim- 

 ber is preferred on account of its lightness and rigidity, and 

 also because it is found to be less liable to warp or cast on 

 exposure to the atmosphere, than most other timbers of that 

 country. 



The peculiarities in the construction of this description of 

 bridge, and the manner in which the mortice and tenon joints 

 and the seats for the different wedges are formed, can best 

 be shewn by a reference to a model or working drawings on 

 a large scale. The wedges are introduced for the purpose of 

 tightening the truss-frames, and thereby rendering the whole 

 structure stiff and rigid, and on the nice fitting of these wedges, 

 and mortice, and tenon joints, the stability and rigidity of the 

 structure in a great measure depends. In erecting a frame- 



