48
c = 312 feet, n = 6 feet, g =. = 439*56 lbs. /'•=.
51229 ft* mo' -n\f- -l) = n [ 51 ~ -l) = 6 x 163.19
= a prismatick column of the same material onesquare foot base 979-14 feet high, and 979.14 x4 39-56 lbs. z= 430390 lbs. and because the ribs are onlyinches thick, and there are in 4.5 feet, 21.6 times
2-J inches, r= 19926 lbs. avoirdupois on a foot
superficial, which is the limit of weight the bridge iscapable of sustaining at the vertex.
Menai Bridge * Bangor, North Wales .
Mr. Telford states that the weight per foot super-ficial to break down the bridge is =
221 lbs. per foot superficial, derived from an expe-riment made on a bar of good iron, one inch square,by a weight suspended from it in a curve of a catenaryprecisely that of the Menai bridge; in which thefibres of the material must have lost in strength fromthe deflection, those on the concave side sufferingcompression, and on the convex side extension.
At the Runcorn bridge, 60 lbs. per foot superficial800 x 2240 „„ ,, ., n . 0 A/r
T ooo x" ~ 3 0 = 60 was lmit : See P a S e ^ r *
Barlow’s essay; though the strength of iron in thecomputation was derived from an experiment upon it1 vertically suspended, and 27 tons assumed as the
* See tract 3, page 37. Dr. Young, lec. 13, Nat. Phil., says,“ Although iron is much stronger than wood, yet it is more liableto accidental imperfections ; and when it fails, it gives no warningof its approaching fracture.”
t See page 25, 3d Report, Menai bridge, House of Commons ,29th April, 1819.