按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！
But the dock accommodation was still found insufficient； and； as
the recognised head of his profession； Mr。 Telford； though now
grown old and fast becoming infirm； was called upon to supply the
requisite plans。 He had been engaged upon great works for upwards
of thirty years； previous to which he had led the life of a working
mason。 But he had been a steady； temperate man all his life； and
though nearly seventy； when consulted as to the proposed new docks；
his mind was as able to deal with the subject in all its bearings
as it had ever been； and he undertook the work。
In 1824 a new Company was formed to provide a dock nearer to the
heart of the City than any of the existing ones。 The site selected
was the space between the Tower and the London Docks； which
included the property of St。 Katherine's Hospital。 The whole extent
of land available was only twenty…seven acres of a very irregular
figure； so that when the quays and warehouses were laid out； it was
found that only about ten acres remained for the docks； but these；
from the nature of the ground； presented an unusual amount of quay
room。 The necessary Act was obtained in 1825； the works were begun
in the following year； and on the 25th of October； 1828； the new
docks were completed and opened for business。
The St。 Katherine Docks communicate with the river by means of an
entrance tide…lock； 180 feet long and 45 feet wide； with three
pairs of gates； admitting either one very large or two small
vessels at a time。 The lock…entrance and the sills under the two
middle lock…gates were fixed at the depth of ten feet under the
level of low water of ordinary spring tides。 The formation of these
dock…entrances was a work of much difficulty； demanding great skill
on the part of the engineer。 It was necessary to excavate the
ground to a great depth below low water for the purpose of getting
in the foundations； and the cofferdams were therefore of great
strength； to enable them； when pumped out by the steam…engine； to
resist the lateral pressure of forty feet of water at high tide。
The difficulty was； however； effectually overcome； and the wharf
walls； locks； sills and bridges of the St。 Katherine Docks are
generally regarded as a master…piece of harbour construction。
Alluding to the rapidity with which the works were completed；
Mr。 Telford says： 〃Seldom； indeed never within my knowledge； has there
been an instance of an undertaking； of this magnitude； in a very
confined situation； having been perfected in so short a time；。。。。
but； as a practical engineer； responsible for the success of
difficult operations； I must be allowed to protest against such
haste； pregnant as it was； and ever will be； with risks； which； in
more instances than one； severely taxed all my experience and
skill； and dangerously involved the reputation of the directors as
well as of their engineer。〃
Among the remaining bridges executed by Mr。 Telford； towards the
close of his professional career； may be mentioned those of
Tewkesbury and Gloucester。 The former town is situated on the
Severn at its confluence with the river Avon； about eleven miles
above Gloucester。 The surrounding district was rich and populous；
but being intersected by a large river； without a bridge； the
inhabitants applied to Parliament for powers to provide so
necessary a convenience。 The design first proposed by a local
architect was a bridge of three arches； but Mr。 Telford； when
called upon to advise the trustees； recommended that； in order to
interrupt the navigation as little as possible； the river should be
spanned by a single arch； and he submitted a design of such a
character； which was approved and subsequently erected。 It was
finished and opened in April； 1826。
This is one of the largest as well as most graceful of Mr。 Telford's
numerous cast iron bridges。 It has a single span of 170 feet； with
a rise of only 17 feet； consisting of six ribs of about three feet
three inches deep； the spandrels being filled in with light
diagonal work。 The narrow Gothic arches in the masonry of the
abutments give the bridge a very light and graceful appearance；
at the same time that they afford an enlarged passage for the high
river floods。
The bridge at Gloucester consists of one large stone arch of 150
feet span。 It replaced a structure of great antiquity； of eight
arches； which had stood for about 600 years。 The roadway over it
was very narrow； and the number of piers in the river and the small
dimensions of the arches offered considerable obstruction to the
navigation。 To give the largest amount of waterway； and at the same
time reduce the gradient of the road over the bridge to the
greatest extent； Mr。 Telford adopted the following expedient。
He made the general body of the arch an ellipse； 150 feet on the
chord…line and 35 feet rise； while the voussoirs； or external
archstones； being in the form of a segment； have the same chord；
with only 13 feet rise。 〃This complex form；〃 says Mr。 Telford；
〃converts each side of the vault of the arch into the shape of the
entrance of a pipe； to suit the contracted passage of a fluid； thus
lessening the flat surface opposed to the current of the river
whenever the tide or upland flood rises above the springing of the
middle of the ellipse； that being at four feet above low water；
whereas the flood of 1770 rose twenty feet above low water of an
ordinary spring…tide； which； when there is no upland flood； rises
only eight or nine feet。〃*＇1＇ The bridge was finished and opened in
1828。
＇Image＇ Dean Bridge； Edinburgh。
The last structures erected after our engineer's designs were at
Edinburgh and Glasgow： his Dean Bridge at the former place； and his
Jamaica Street Bridge at the latter； being regarded as among his
most successful works。 Since his employment as a journeyman mason
at the building of the houses in Princes Street； Edinburgh； the New
Town had spread in all directions。 At each visit to it on his way
to or from the Caledonian Canal or the northern harbours； he had
been no less surprised than delighted at the architectural
improvements which he found going forward。 A new quarter had risen
up during his lifetime； and had extended northward and westward in
long lines of magnificent buildings of freestone； until in 1829 its
further progress was checked by the deep ravine running along the
back of the New Town； in the bottom of which runs the little Water
of Leith。 It was determined to throw a stone bridge across this
stream； and Telford was called upon to supply the design。 The point
of crossing the valley was immediately behind Moray Place； which
stands almost upon its verge； the sides being bold； rocky； and
finely wooded。 The situation was well adapted for a picturesque
structure； such as Telford was well able to supply。 The depth of
the ravine to be spanned involved great height in the piers； the
roadway being 106 feet above the level of the