Chat. XXIII.

CENTERING.

1401

Oj.

Fig. 2333,

BIUDGB AT MANTES.

\yy i

Fig. 2334.

BRIDGE AT MANTES.

the same throughout to its abutments ; as there were six principals or ribs to each centre, itrequired considerable power to raise them into their places, which was well accomplishedby strong tackle attached to lofty poles.

'We have evidence that this systemof Perronet’s was previously adoptedby Ilardouin Mansard, when thebridge at Moulins was constructedunder his direction, and it wouldappear at first sight that the timberwas placed in a manner to be sus-ceptible of the greatest resistance :but it has not always succeededwell where it has been applied; themultiplicity of timbers occasionstoo much play about their joints,and centres so formed are veryliable to change their figure. Thecentres of the bridge at Mantes, theribs of which were 6 feet 6 inchesapart, were formed of four coursesof rafters or inclined pieces 18 inchessquare, and their settling was foundto be very considerable; when the voussoirs were placed at the haunches, the centres bentunder the weight, and rose at the summit, which it was necessary to load considerably,to keep the centres in their true form.

CENTRES OF ARCHES.