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 used. On the Ellesmere, and afterwards on the Caledonial Canal, he adopted cast iron lock-gates, which were found to answer well, being more durable than timber, and not liable like it to shrink and expand with alternate dryness and wet. The turnbridges which he applied to his canals, in place of the old drawbridges, were also of cast iron; and in some cases even the locks 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 17 feet, having been built on a stratum of quicksand, were repeatedly undermined, the idea of constructing the entire locks of cast iron was suggested; and this unusual application of the new material was accomplished with entirely satisfactory results. But Telford's principal employment of cast iron was in the construction of road bridges, in which he proved himself a master. His experience in these structures had 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 inconvenient that it was found necessary to take steps to rebuild or remove it, he proposed the daring plan of a cast iron bridge of a single arch of not less than 600 feet span, the segment of a circle l450 feet in diameter. In preparing this design we find that he was associated with a Mr. Douglas, to whom many allusions are made in his private letters.*[6] The design of this bridge seems to have arisen out of a larger project for the improvement of the port of London. In a private letter of Telford's, dated the 13th May, 1800, he says: "I have twice attended the Select Committee on the Fort of London, Lord Hawkesbury, Chairman. The subject has now been agitated for four years, and might have been so for many more, if Mr. Pitt had not taken the business out of the hands of the General Committee, and got it referred to a Select Committee. Last year they recommended that a system of docks should be formed in a large bend of the river opposite Greenwich, called the Isle of Dogs, with a canal across the neck of the bend. This part of the contemplated improvements is already commenced, and is proceeding as rapidly as the nature of the work will admit. It will contain ship docks for large vessels, such as East and West Indiamen, whose draught of water is considerable.