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TO DETERMINE THE WEIGHT AN ARCH IS CAPABLE OFSUSTAINING ON A FOOT SUPERFICIAL AT THE VER-TEX, WHEN THE RADIUS OF CURVATURE THERE,THE HEIGHT OF THE KEY, AND THE WEIGHT ANDSTRENGTH OF THE MATERIAL, ARE GIVEN.

STONE BRIDGES.

EXAMPLE.

Bridge of the Holy Trinity , Florence.

(White Statuary Marble, not veined.)

This arch is pointed at the vertex, and the angleof the joint with a vertical line is 5° 56', the secantof which varies so little from 1, that the arch may beconsidered to be round headed.

By the table of stone bridges, c = 172.63 feet.

n = 2.75 feet.

By the table of the strength of materials, f'~ 5058 feet

~ _ 2760

By the formula to' — n (— — 1 ) = » (

16 '5058

= 172 lbs.

172.63

= 2.75 x 28.3 = a prismatick column of the same ma-terial 77-8 feet in height of one square foot base, and77-8 x 172lbs. = 13382 lbs. on afoot superficial, whichis the limit of weight the bridge is capable of sus-taining at the vertex.

Bridge of Neuilly .

Perronet, in his Descr. des Proj. et de la Cons, desPonts, pages 616 and 624, states, that a cubic foot

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