Report on the Progress and Stale of Applied Mechanics. 221 



have also been referred to ; they comprise examples of carpentry, 

 unparalleled for magnitude. But the most characteristic of the time 

 are those of iron, the designing and construction of which is almost a 

 new art, called into existence by the requirements of Railways. To 

 this remark some exceptions have to be made ; first, as regards cast iron 

 arches, of which several remarkable examples, such as that at Sunder- 

 land over the Wear, and those of Southwark Bridge over the Thames, 

 were constructed before the development of the Railway system ; and, 

 secondly, as regards Suspension Bridges, of which the Menai Bridge of 

 Telford, on the Holyhead Road, long remained an unrivalled example, 

 and whose use on lines of road has extended over all civilized countries. 

 But the various forms of the wrought iron bridge, known as the Tubu- 

 lar Bridge, the Bow-string Bridge, and the Lattice Bridge, have been 

 invented in succession, for the purpose of carrying Railways across wide 

 spans and at great heights. The Tubular Bridge of Stephenson has 

 already been referred to, and is too well known to require lengthened 

 explanation ; but it may be mentioned, that although the Conway and 

 Britannia Bridges were the earliest examples of tubular girders, large 

 enough to allow trains to pass tlu-ough them, and stiffened by cells, still 

 wrought iron rectangular hollow beams, or box-beams as they are called, 

 after having long been used on the platforms of blast furnaces, were 

 first employed in Railway construction, about 1834, by Andrew Thom- 

 son, of Glasgow, in the Bridge by which the Butterbiggins Road is 

 carried over the PoUok and Govan Railway. One of the finest examples 

 of the boio-string girder, is Mr. Stephenson's celebrated " High-Level 

 Bridge" at Newcastle. A well known example of the use of Trellis- 

 work or Lattice girders, is the Viaduct over the Boyne at Drogheda ; 

 it was designed and executed by Mr. Barton, for a line of which Sir 

 John Macneill was chief engineei', and is a striking instance of the com- 

 bination of strength with lightness, and of the exact verification of 

 theoretical calculations in practice. The most simple of all Lattice 

 girders, is that invented by Captain Warren, and known by his name ; 

 it has been used in what is in some respects the most remarkable bridge 

 in the world, the Crumlin Viaduct in South Wales, of which the honour 

 is shared between Captain Warren, the inventor of the girders, Messrs. 

 Liddell & Gordon, the engineers, and the Messrs. Kennard, the con- 

 tractors. The piers are skeleton frames of cast iron, and rise to the 

 enormous height of 220 feet above the valley of the Taff; yet this 

 gigantic structure has cost about one-half of the sum, per cubic yard 

 covered, that had been considered cheap for previous Viaducts. Mr. 

 Brunei has designed some very peculiar iron Viaducts, of immense size, 

 such as that over the Wye at Chepstow, and that over the Tamar at 



