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OF THE LIMIT OF WEIGHT WHICH A BRIDGE OUGHTTO BE ABLE TO SUSTAIN, OF THE PROBABLE WEIGHT,AND INSURANCE WEIGHT.

The moving force on an insistent bridge may betaken at 40lbs. per foot superficial, and to move at therate of about four miles per hour, or fi feet per second jits motion will cause percussion when it meets withobstacles, and therefore at that rate may be taken at55lbs. ; the puddling or cementitious covering uponthe extrados, and the gravel and matter to form theroadway at 195lbs. more, together 250lbs. avoirdupoise,on stone bridges. On wooden bridges, 150lbs. maybe taken.

Rondelet, page 101. vol. iii. says, from all the ex-periments given by him and by an infinite number ofothers, that stone begins to split under a pressure ofa little more than half the weight which would crushit. Gauthey says, page 272 , “ consequently therecannot be any danger in making stone bear one-thirdof the weight which would crush it.” Gauthey, page274, vol. I. says, that the weight supported by thestone of Bagneux, in the pillars of the dome of St.Genevieve, is between 8 and 9 times less than wouldcrush it. He says, page 275; that the key stone ofthe bridge of Neuilly supports a weight 14 times lessthan would crush the stone. By the calculationgiven, it would be crushed by an incumbent weightper square foot of 3966 lbs. and it has been shown

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