T. P OPE’S BRIDGE.
205
PROP. 3.
2nd, The manner in which the arms of a Bridgeon this plan are constructed in the internal part alsqconstitutes each rib distinctly a compound lever ofthe first order, with continuity.
The above proposition is obviously true, whilstthe arms of the Bridge, which are finally to composethe arc, remain disunited in the centre; as everylast pair of angular levers lowered down on each rib,in the building of this Bridge, by their gravity, notonly contributes to the locking back, and perfectlysecuring those logs or levers previously added, butthey furnish also new fulerums or supports for thesucceeding levers to rest on: (see plate I, fig. 1, at G.also plate 5,fig. 1, at R) but as soon as the arms of thisBridge are united in the centre, then the said leversare wholly relieved from that gravity which beforeconstituted them in action. But although this bethe case, there cannot ensue a lateral pressure fromthe abutment to the centre, as exists in all otherarcs, for two obvious reasons: first, the two armsand the two abutments of the Bridge are now becomeone perfect whole. Second, they are tied together,longitudinally, by more strength than their ownweight could tear asunder perpendicularly.
COR. 1.
Hence, were it not the fact that a space or vacuumdid exist in the joint between each log at this period,instead of a medullary substance, the whole might