1390
THEORY AND PRACTICE OF ENGINEERING.
Book II.
Fig. 2304.
BALANCING BRIDGE, BRUSSELS.
Fig. 2305.
The platforms of all draw-bridges should have a pre-ponderance. to render themsteady, when in their le-vel position; counterpoisingweights are, however, ex-ceedingly useful, in affordingfacility in moving them whenrequired, and for loweringthem into their place. Draw-_ bridges of considerable extent.should be formed out of'curved timbers, or slightlycambered. Provision mustbe made for the load to whichsuch bridges may be subject, from the possi-bility of their being crowded with humanbeings, such a load amounting to onehundredweight at least for every super-ficial foot, independent of the weight of thematerials with which the bridge is con-structed: if thercfote it be calculated forthree times that weight, there will not bemore than the requisite strength ; and thelevers, chains, and pivots must be propor-tioned to the duties they have to perform.
Where drawbridges were used to crossa ditch or moat around a fortified houseor castle, they were hung so that whendrawn up they covered the face of the
portcullis, and constituted a doubleprotection: the under side of theplatform should be cased with iron,or rendered secure against the at-tempts of the enemy either suddenlyto burn or cut it away. The weightof the bridge thus increased requiresgreater pow r er to hoist it than wasordinarily obtained by the levers andchains manoeuvred by the soldiersstationed within the gate-house;
Fig. 2303.