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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。
〃There are now two other propositions under consideration。 One is
to form another system of docks at Wapping； and the other to take
down London Bridge； rebuild it of such dimensions as to admit of
ships of 200 tons passing under it； and form a new pool for ships
of such burden between London and Blackfriars Bridges； with a set
of regular wharves on each side of the river。 This is with the view
of saving lighterage and plunderage； and bringing the great mass of
commerce so much nearer to the heart of the City。 This last part of
the plan has been taken up in a great measure from some statements
I made while in London last year； and I have been called before the
Committee to explain。 I had previously prepared a set of plans and
estimates for the purpose of showing how the idea might be carried
out； and thus a considerable degree of interest has been excited on
the subject。 It is as yet； however； very uncertain how far the
plans will be carried out。 It is certainly a matter of great
national importance to render the Port of London as perfect as
possible。〃*＇7＇
Later in the same year he writes that his plans and propositions
have been approved and recommended to be carried out； and he
expects to have the execution of them。 〃If they will provide the
ways and means；〃 says he； 〃and give me elbow…room； I see my way as
plainly as mending the brig at the auld burn。〃 In November； 1801；
he states that his view of London Bridge； as proposed by him； has
been published； and much admired。 On the l4th of April； 1802； he
writes； 〃I have got into mighty favour with the Royal folks。 I have
received notes written by order of the King； the Prince of Wales；
Duke of York； and Duke of Kent； about the bridge print； and in
future it is to be dedicated to the King。〃
The bridge in question was one of the boldest of Telford's designs。
He proposed by his one arch to provide a clear headway of 65 feet
above high water。 The arch was to consist of seven cast iron ribs；
in segments as large as possible； and they were to be connected by
diagonal cross…bracing； disposed in such a manner that any part of
the ribs and braces could be taken out and replaced without injury
to the stability of the bridge or interruption to the traffic over it。
The roadway was to be 90 feet wide at the abutments and 45 feet
in the centre； the width of the arch being gradually contracted
towards the crown in order to lighten the weight of the structure。
The bridge was to contain 6500 tons of iron； and the cost of the
whole was to be 262；289L。
＇Image＇ Telford's proposed One…arched Bridge over the Thames。
The originality of the design was greatly admired； though there
were many who received with incredulity the proposal to bridge the
Thames by a single arch； and it was sarcastically said of Telford
that he might as well think of 〃setting the Thames on fire。〃
Before any outlay was incurred in building the bridge； the design
was submitted to the consideration of the most eminent scientific
and practical men of the day； after which evidence was taken at
great length before a Select Committee which sat on the subject。
Among those examined on the occasion were the venerable James Watt
of Birmingham； Mr。 John Rennie； Professor Button of Woolwich；
Professors Playfair and Robison of Edinburgh； Mr。 Jessop；
Mr。Southern； and Dr。 Maskelyne。 Their evidence will still be found
interesting as indicating the state at which constructive science
had at that time arrived in England。*＇8＇ There was a considerable
diversity of opinion among the witnesses； as might have been
expected； for experience was as yet very limited as to the
resistance of cast iron to extension and compression。 Some of them
anticipated immense difficulty in casting pieces of metal of the
necessary size and exactness； so as to secure that the radiated
joints should be all straight and bearing。 Others laid down certain
ingenious theories of the arch； which did not quite square with the
plan proposed by the engineer。 But； as was candidly observed by
Professor Playfair in concluding his report〃It is not from
theoretical men that the most valuable information in such a case
as the present is to be expected。 When a mechanical arrangement
becomes in a certain degree complicated； it baffles the efforts of
the geometer； and refuses to submit to even the most approved
methods of investigation。 Thi