Mr Stevenson’s Descript ion erf Bridges of Suspension. 24? 
other with rails, or pieces of timber mortised into them. The 
side-rails answering the purposes of hand-rails, are formed with 
diagonal braces and cross ties. The roadway is finished with a 
cleading of boards laid across the direction of the roadway, lea- 
ving openings of about Jths of an inch between each of the boards, 
to let off* the moisture in wet weather. Under the platform, two 
chains made of circular rods, 1 inch in diameter, are stretched 
beneath the beams, and connected with the abutments of mason- 
ry on each side, as an additional security. 
The back braces or landward chains employed for keeping 
the upright posts erect, and for counteracting the weight of the 
bridge, are made of rod-iron, 1 inch in diameter, which are sunk 
a considerable way into the ground, and pass through large 
flat stones, which are loaded with a mass of masonry, built in an 
arched form, and acting as ballast, as shewn in Fig. 3. 
An occurrence took place, during the erection of Dryburgh 
Bridge, which deserves to be particularly noticed. It was ob- 
served, that the catenarian curve was not the same when the 
main chains were simply suspended with their own weight, as 
when they came to be loaded with the roadway. At the ex- 
tremity of the chains on each side, and in the centre of the 
bridge, the points of attachment remained stationary after the 
catenarian chains were loaded, but between the centre and 
either abutment, the roadway made two distinct curves, the 
versed sine of* which measured about 7 inches. This defect 
was easily rectified, by shortening the suspending chains ; but 
it serves to shew the liability of the catenarian curve to alter, 
when loaded in the direction of the horizontal plane of the con- 
necting roadway. 
For the erection of a bridge at Dryburgh, on a ferry of com- 
paratively small importance, the public are under no small obli- 
gations to the Earl of Buchan : and the enterprise which marks 
the design and execution of it, confers honour on the architects. 
Union Chain-Bhidge.— -The work to which we next refer 
the reader, is the Union Bridge across the river Tweed at Nor- 
ham Ford, about five miles from Berwick, of which we have 
given a sketch in Fig. 4. The work here was begun in the 
month of August 1819, and the bridge was opened on the 26th 
July 1820, having required only a period of about twelve 
