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that 250 lbs. may be taken for the probable in-cumbent weight, 3966 — 250 =3716 and ?|1 = notquite 15, that is, 15 times more weight than it is pro-bable will ever come upon it, will be necessary tocrush it. Perronet says, the piers sustain 12 timesless than would crush the stone, page 626 of hiswork. Perronet was a man of experience and science,and knew by experiment the strength of the stonewhich he used ; it may not be prudent at present tobe more hardy than he was.

The discretion and experience of the bridge-builderwill determine the extra weight, beyond the weightlikely to come upon a bridge, which it should be cal-culated to sustain. From the examples given (Seeabstract, page 49), it appears that the difference of opi-nion upon this subject is very great; wooden bridgesare thought to be safe, if they are limited to carry aweight six and eight times more than it is probablewill be placed upon them. Stone bridges, from 15to 400 times more. Iron bridges, when insistent, 78times more ; but when pendent, an excess of strengthfor insurance is disregarded, though the hazard isaugmented by the motion and jerks # contingent tosuch bridges.

* Le belier hydraulique de Montgolfier may illustrate analo-gically this motion. See Mollet’s Hydraulique Physique, 1809.