The Civil engineer and architect's journal, scientific and railway gazette . the extreme edge of the voussoir, andits direction must also be that of the tangent to the intrados, orextrados, at A,, Aj, &c., because if not, the line of resistancepasses without the boundary of the voussoirs, either on one orotlier side of the point A, and tlie structure has already failed, bythe turning over of some other voussoir. Therefore, wlien thearch is in the condition of unstable equilibrium, then, at all thepoints of rupture, the directions of the resultant pressures are tan-gents to the intrados, or ext
The Civil engineer and architect's journal, scientific and railway gazette . the extreme edge of the voussoir, andits direction must also be that of the tangent to the intrados, orextrados, at A,, Aj, &c., because if not, the line of resistancepasses without the boundary of the voussoirs, either on one orotlier side of the point A, and tlie structure has already failed, bythe turning over of some other voussoir. Therefore, wlien thearch is in the condition of unstable equilibrium, then, at all thepoints of rupture, the directions of the resultant pressures are tan-gents to the intrados, or extrados. Art. 9. Pruhlem 2.—To find the second point of rupture, in anarch whose voussoirs are incompressible, the form of whicli and thepressure sustained by it, as regards position, direction, and amount,being similar on either side of the crown of the arch. Also to find the amount of pressure at the crown and at thesecond point of rupture. Take for example an arch with a backing, or superstructure,diagram 9. It is required to find the second point of rupture, that. Diagram 7. Diagram 6. diagram 3. But when the arch is in the condition of unstableequilibrium, that is, when it is on the balance between standingand falling, and when the voussoirs are on the point of turning ontheir edges at A„ Aa, &c., then the point of application of the re- Diagram 9. is, that point in the haunches, at which the voussoirs will he aboutto turn on their edges, when the arch is in the condition of un-stable equilibrium. As the form of the structure leads to the supposition, that, iffailure take place, it will be by the sinking of the crown and thespreading of the haunches, let it be first assumed that the arch isabout to fail in that manner. Then the point C, in the extrados,at the crown, will be the first point at wliich the voussoirs areabout to turn ; and the horizontal line C E, will represent thedirection and position of the pressure upon the side of the archdrawn in the figure, caused by
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