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



surface of the water. The water-way of the bridge measures 

 5800 feet, and its whole length, including piers and abutments, 

 is one mile and a quarter. The supei-structure (consisting of 

 the arches and roadway) is wholly of timber. The bridge is 

 supported by three wooden arcs, forming a double roadway 

 which is adapted to the passage both of road and railway car- 

 riages. There are also tvvc footpaths, which make the whole 

 breadth 30 feet. 



In a country where so many bridges exist, it is not difficult 

 to conceive that their construction must be very various, and 

 it would be tedious to enumerate the different principles of 

 construction which have been introduced and successfully 

 practised in America. Some of these are very excellent and 

 well adapted to many situations. So much is this the case, 

 that having been consulted lately by H. F. Brown, Esq. as to 

 the erection of abridge of 150 feet span for his estate in India, 

 I recommended the adoption of the bridge invented by Colonel 

 Long of the United States Engineers' 1 Corps as that which, 

 under all circumstances, appeared to be most suitable, and a 

 model was made by Messrs J. 13. Maxton & Co. of the Leith 

 Engine Works, from working drawings furnished by me, which 

 was exhibited by Mr Brown's permission at a meeting of the 

 Society of Arts. 



The model was constructed on the same principle as that 

 shewn in Plate VI. Fig. 1 is an elevation, Fig. 2 a plan, 

 and Fig. 3 a cross section. The level of the road is indicated 

 by letter a, b represents the " string pieces,' 1 as they are called 

 in America ; c the " posts'" ; d the " mainbraccs' 1 ; and e the 

 •' counter-braces."" 



The string pieces are formed of three beams, in the manner 

 shewn in the plan and cross section. The posts and main- 

 braces are in two pieces, and the counter-braces are formed 

 of a single beam. Figs. 4, 5, 6, and 7 illustrate the manner 

 in which the joining is formed, at the points where the posts 

 and braces are attached to the string-pieces. Figs. 4 and 5 

 are enlarged diagrams, shewing the manner in which the posts 

 are fixed to the strings. In Fig. 4 the strings are shewn in 

 section at letter b, and the posts passing between them at c. 

 In Fig. 5 the posts are shewn in section at c, and the strings 

 at b. Fijy. G shews the manner of fixing the main and coun- 



