CHAP. VII. IRON LOCKS AND GATES. 361 



application of iron." This iron aqueduct had now been 

 cast and fixed ; and it was found to effect so great a 

 saving in masonry and earthwork, that he w T as afterwards 

 induced to apply the same principle, as we have already 

 seen, in different forms, in the magnificent aqueducts of 

 Chirk and Pont-Cysylltau. 



The uses of cast iron in canal construction became 

 more obvious with every year's successive experience ; 

 and Telford was accustomed to introduce it in many cases 

 where formerly only timber or stone had been employed. 

 On the Ellesmere, and afterwards on the Caledonian Canal, 

 he introduced cast iron lock-gates, which were found to 

 answer admirably, being more durable than timber, 

 and not liable like it to shrink and expand with alter- 

 nate dryness and wet. The turnb ridges which he in- 

 troduced upon his canals, instead of the old drawbridges, 

 were also of cast iron ; and in some cases even the locks 

 themselves were of the same material. Thus, on a 

 part of the Ellesmere Canal opposite Beeston Castle, in 

 Cheshire, where a couple of locks, together rising 1 7 feet, 

 having been built on a stratum of quicksand, were re- 

 peatedly undermined, the idea of constructing the entire 

 locks of cast iron was suggested ; and this extraordinary 

 application of the new material was successfully accom- 

 plished, with entirely satisfactory results. 



But Telford' s principal application of cast iron was 

 in the construction of road bridges, in which he 

 proved himself a master. His experience in these struc- 

 tures had now become very extensive. During the 

 time that he held the office of surveyor to the county 

 of Salop, he erected no fewer than forty-two, five of 

 which were of iron. Indeed, his success in iron bridge- 

 building so much emboldened him, that in 1801, when 

 Old London Bridge had become so rickety and incon- 

 venient that it was found necessary to take steps to 



1 Letter to Mr. Andrew Little, Langholm, dated Shrewsbury, 18th March, 

 1795. 



