Chap. VIII.
437
h
87 feet. The sides of the bridge are defended by an open balustrade; the carriage road is28 feet wide, and the footpath on each side 7 feet.
The roadway is nearly horizontal throughout, andlevel with the terrace at the back of Somerset House.
This was nearly the first bridge so constructed, and greatcredit is due to the engineer in thus triumphing overthe vulgar prejudice, that it was necessary to inclinethe thoroughfares to obtain a greater elevation of themiddle arch : in the arrangements at Blaekfriars thecolumns are made to diminish in height towards theabutments; in the present case they are of the samealtitude throughout; consequently the entablature isnot distorted, and from the excellent effect producedby this level line of cornice being preserved, the bridgeis justly considered the most beautiful structure of thekind.
The cofferdams were formed of three concentric rowsof piles, placed about 3 feet 6 inches apart; the stratuminto which they were driven was gravel; each elm orbeech pile was 12 inches in diameter, and about 20 feetin length.
'JHie form given to the cofferdams was that of anellipsis; and the heads of the piles, after they were driveninto the gravel, were cut off, about 5 feet above the levelof high water mark spring tides; to secure them moreeffectually, wrought-iron screw-bolts were passed throughthem, as well as three rows of waling pieces, the endsof which w'ere covered by stout cleets. Cross and dia-gonal braces of whole timber connected and strutted theseveral rows of piles; the dam was braced longitudinallyin several places, and struts were introduced in everyposition where they were not found to interfere with themasons in their work. The water which accumulatedor passed into the cofferdam could be let out at low tideby means of a short tunnel formed in the end; and asluice, moved by iron racks and wheels, enabled theworkmen to open or close the mouth of the tunnel atpleasure. The spaces between the piles were filled withwell puddled clay, that had been beaten and preparedfor the purpose before it was put on board the lighterwhich conveyed it to the cofferdam. In the executionof this work great attention was paid by the engineer
to obtain a perfect command over the water at all times, ■ i -y i ■
and thus prevent any interruption to the men employed I Or i*/y ili
upon the masonry of the piers, which was completelyeffected by means of a steam-engine erected for thepurpose: neither hand-buckets, pumps and machinesworked by men or horses, nor the numerous contrivancesdescribed by the French engineers, could have emptiedthe space comprised within the area of a cofferdam inso short a time as it was accomplished by this; hencethe labour was continued almost without interruption,on many occasions being proceeded with for twenty-four successive hours, a most important advantage inconstructions of this kind. The application of the
steam-engine to bridge-building has wonderfully eco- if §|^f pjf
nomised both labour and time; piles are driven, theirheads cut off by its power: manual labour has thusbeen greatly abridged, and the use of horses almostrendered unnecessary; in the present instance, the stoneand other materials were so laid upon the temporaryplatforms or timber constructions, as to be within thecommand of the power of the engine at all times, andheavy weights were moved by very simple tackle andmachinery.
The arches being all equal, and of an elliptical form, produce an admirable effect; thedepth of the key-stone is between a twenty-sixth and twenty-seventh of the span, which is
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Fig. 430. WATERLOO BRIDGB.