Chap. VIII.

BRITAIN.

417

the centre; the shallow depth given at the key proves that there strength is not required,so long as the extremities are well maintained in their position. Hie arrangement of thestones, as in this curious girder, is a strong proof that at its introduction the theory of thearch was understood, and, its use not being apparent, we are tempted to imagine that it wasso placed as a model for study, and an evidence of its practical strength.

After the reign of Ilenry VIII. bridge-building underwent a considerable change;timber constructions again became very common, and some of the principal rivers werecrossed by them. In the year 1636, Inigo Jones erected a bridge at Llanwast in Den-bighshire, after the method practised in Italy , which was the model for some of the suc-ceeding structures.

It was formed of three segmental arches, the middle spanning 58 feet, with a versed sineof 17, and the breadth of the soffite of the arch 14 feet. 'Hie depth of the voussoirs,measured on the face, was 18 inches, the piers were 10 feet in thickness. The pointed archwas no longer used, and the defences of towers and gateways were unnecessary : the passagewas made more convenient, and the roadway approached a horizontal line, in consequenceof the substitution of vehicles for the pack-horse for the transit of merchandize.

At the commencement of the eighteenth century we find evidences of an attempt toimprove the bridges throughout England, but there is no account of any principles bywhich the engineer could be directed, nor are there any names upon record to whom suchconstructions were particularly entrusted ; what had been done in Italy does not seem to havefound many imitators here, and though Newton had discovered the principles upon whichmechanical science was based, it was long before the equilibrium of the arch occupied theconsideration of practical men. Dr. Hooke had, however, drawn attention to the figurewhich a heavy chain or rope assumes when suspended at the two ends, and shown the pro-perties of the catenaria ; but it was not then applied to the construction of bridges.

One of the first essays on the subject was written by Isaac Gadsdon, and published in1739 ; he sets forth very clearly the nature and properties of arches, mechanically considered,with respect to their shape and duration ; and, as this essay is interspersed with someremarks on the intended bridge at Westminster, some portion of its contents may be in-teresting as evidences of the state of knowledge on this subject at the period he wrote.

“ By the word arch is to be understood some regular curve or crooked line, which, whenapplied to practice in building, is always put with the convex side uppermost, in order tosupport some weight. But without the abutments it would not support itself, for theaction of gravity pressing upon it would soon tumble it down; and if a flexible or mal-leable body, would soon reduce it to a straight line. From hence it is plain that thestrength of all arehc-s depends upon the abutments, and that dependency is in proportion tothe height they rise from the chord line of their respective arches.

“ The truth of this will plainly appear by setting up two pieces of wood in the shape oflevers, one against the other, in the nature of a pediment. The pediment requires a greaterweight or resisting power, to beplaced at the foot e,e, to keep itfrom falling in the arch A, than inthe other; therefore the arch Bwill support a greater weightthan the arch A, if their abut-ments are equal. The reason of e e e e

this is, that the lines a,a, are 4 ^~*

nearer a perpendicular than the lines o, o, for if those before-mentioned pieces of wood wereplaced perpendicular one against the other, with a hinge at the top, and wheels put in thebottom, for the sake of experiment, to take off the friction, they would in that position wantno abutment to keep them together at bottom ; but if they are a small space asunder atthe bottom, they will want an abutment to keep them in that position, otherwise they willrun one from the other, and the further they recede from each other, the greater must bethe resisting power or abutment be to keepthem from falling. From what has beensaid, it appears that gravity acts on thesetwo levers, according as they are elevatedor depressed, and if so, it must be in someproportion, which proportion may verynearly be determined by the followingfigure G, where the theory depends uponthat of the steelyard, which, for the betterunderstanding of what is to follow, I shallendeavour to explain by the figure E.

“ Let the line c and o represent the beam,and O the centre of a pair of steelyards, Flg ‘ 418,

and suppose two weights, as n and r, were to be hung at equal distances from the centre

E e