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Pont Cysyllte: 1795-1805

In 1795 Thomas Telford applies cast-iron technology in a bold new context. In 1793 he has been appointed engineer and architect to the Shropshire Union canal, which is to link the Mersey with the Severn. Near Llangollen the proposed route crosses the Dee valley, which is more than 300 yards wide and drops down about 120 feet to the river level below.

The number of locks needed to get a barge down and up again would represent a costly delay for the bargees. Yet an aqueduct of this height and length is a daunting project. The valley is much wider and deeper than the one spanned by Brindley in his heavily buttressed aqueduct at Barton. But Telford accepts the challenge.
 









Telford constructs at Pont Cysyllte what is in effect an enormous cast-iron gutter. Cast to the correct curves and then welded together, Telford's plates combine to form a channel which is nearly 12 feet wide, with a path alongside for the carthorse. The metal is much lighter than the thick layer of pounded clay and sand used by Brindley to contain the water of the Bridgewater canal. So Telford's aqueduct can be a slender structure of nineteen tall stone arches.

Pont Cysyyllte is ready for the first barge to make the journey across the valley in 1805. Walter Scott describes it as 'the most impressive work of art' which he has ever seen.
 






The roads of Telford and McAdam: 1803-1815

Improvement in the speed of coaches, seen in Britain with the introduction of the mail coach in 1784, is accompanied by similar advances in road technology. Travel in horse-drawn vehicles becomes increasingly sophisticated during a period of about fifty years, until the success of the railways results once again in roads being neglected. The early decades of the 19th century are the great days of coaching, commemorated in many paintings and prints.

Clear evidence of this new priority is the government's appointment of Thomas Telford in 1803 to undertake extensive public works in his native Scotland.
 










Telford constructs more than 900 miles of road in Scotland, together with 120 bridges, before transferring his attention to the important route along the north coast of Wales (leading to Anglesey and the shipping lanes to Ireland). With justification Robert Southey describes Telford as the Colossus of Roads.

Meanwhile another Scot, John McAdam, has been making great improvements in the surface quality of the new roads. He devises a system, first put into practice in the Bristol region in 1815, for improving the durability of a carriage way.
 








A McAdam road is well drained and is raised slightly above ground level. McAdam achieves this by laying three successive layers of graded stones, with the largest ones at the bottom. Each layer is compacted by a very simple method. The road is opened to traffic for several weeks, until the metal-rimmed wheels of carriages and carts have compressed and levelled the stones sufficiently for the next layer, of a finer grade, to be added.

Roads made by this method come to be known all over the world as macadamized. When tar is added to bind the top layer, later in the 19th century, the result is the tar macadam road - and eventually the trade name 'tarmac'.
 







Sections are as yet missing at this point.
 






Glass, iron and prefabrication: 1837-1851

The public first becomes aware of the glorious potential of cast-iron architecture in the 1840s, when extraordinary conservatories are erected at Chatsworth and in Kew Gardens. But the technology derives from factory construction in the 1790s.

With Boulton and Watt's steam machinery in operation, conventional factories using timber for joists and floors are prone to disastrous fires. The occasional use of cast iron for structural purposes goes back many centuries in China, for temple pagodas, but it is an innovation in Britain when William Strutt builds the first fireproof mill at Derby, in 1792-3, with floors on shallow brick arches supported on cast-iron pillars.
 









Strutt's mill still contains some massive wooden beams, but an entirely wood-free factory is constructed at Ditherington, near Shrewsbury, in 1796-7. Arched brick floors, on cast-iron beams and pillars, become the standard factory and warehouse interior of the 19th century.

The next and most glamorous stage in cast-iron architecture is linked above all with the name of Joseph Paxton. As superintendent of the duke of Devonshire's gardens at Chatsworth, he builds there in 1837-40 a great conservatory, shaped like a tent (277 feet long and 67 feet high) but consisting entirely of cast iron and glass.
 







In a ducal garden this building is not much visited, but it astonishes all who see it. Queen Victoria notes in her diary in 1842 that it is 'the most stupendous and extraordinary creation imaginable'. Two years later a similar building is commissioned from Richard Turner and Decimus Burton for the royal gardens at Kew. Since 1841 these gardens have been open to the public, so the beauty of the Palm House, completed in 1848, becomes more widely known than the Chatsworth conservatory.

But it is Paxton's building for the Great Exhibition of 1851, the astonishing Crystal Palace, which reveals to the millions the potential of the new architecture.
 







The Crystal Palace is gigantic compared to its predecessors in cast iron and glass. It is five times as long as the Palm House in Kew and nearly twice as high; or, put another way, it is longer than the palace of Versailles and higher than Westminster Abbey. But even more significant is the famous speed of its design (one week of detailed drawing, after a preliminary jotting by Paxton on a piece of blotting paper) and of its construction (six months).

The reason, and the reason for its lasting architectural significance, is that Paxton's building is the first thoroughgoing example of prefabricated architecture (a concept perfectly suited to cast iron, and pioneered seventy years earlier for the bridge at Coalbrookdale).
 







The statistics of the Crystal Palace are bewildering (3300 iron columns, 2150 iron girders, 250 miles of sash bar, 293,635 panes of glass), but the crucial detail is that these all conform to a basic 24-foot module. The manufacture of the pieces can be subcontracted to several foundries and glass factories; assembly on site is like putting together a giant's dolls' house. Hence the fact that this palace of glass is created, from scratch, in less than 200 days. As if to emphasize the point, it is dismantled in 1852 and moved to another site at Sydenham - where it stands until its contents catch fire in 1936.

The modular steel-frame tradition of late 20th-century architecture has in this building its most distinguished ancestor.
 







This History is as yet incomplete.
 






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