242 ~ REPORT—1857. 
experiment was repeated several times with the same result, was 0°64 inch 
with 56 lbs., the deflection without the chains being 2°375 with 8 Ibs., or 25 
times the amount. 
In determining how far this result was effected by the resistance produced 
by the change of figure in the curve of the chain, I removed all the weights 
from the plank, and found the result as follows, with 56 lbs. at 4 from the 
high tower :-— 
; from H. T. Centre. i from L. T. 
—'85 +°20 +°75 
With 56 Ibs. placed + from low tower— 
+°81 +12 —'86 
From this experiment it appears that the deflection is decreased by loading 
the bridge to th of that of the girder unattached, and if the chain were 
without weight “it would be still further reduced ; in praehien however, the 
weight on the bridge will much exceed that on a model, and 54th will be the 
least amount that will arise, a result so at variance with the. preconceived 
notions of many engineers, that it is to be expected in some instances it 
will be received with incredulity ; but an investigation will show that the 
ak : B . 
result is in accordance with the law a8 constant quantity. 
If the girder were supported only in the middle, the deflection of the half 
girder would be ith, but as one half of the girder cannot — without the 
other half rising, from the action of the chain, it is reduced to —,th; but the 
girder is not supported at one point only, but at various points, whieh will 
still further reduce the deflection. 
However, whether this view is precisely the correct one or not, the fact is 
established, that the deflection of the wave of a gieder attached to the chain 
and loaded as in the actual bridge, will not exceed jth of the same girder 
without the chain, from which we may estimate the weight of girder sufficient 
to produce in a suspension bridge or arch the requisite rigidity. 
In order to show the importance of this result in the cost of bridges, I will 
compare the deflection.and weight of metal in a bridge similar to the Lon- 
donderry Bridge, with a girder of equal span, in each case assuming that 
3 tons per foot on the bridge will bring no strain exceeding 5 tons per inch ~ 
on the metal. 
The weight of chain, such that 3 tons per foot on the girder will 
not exceed 5 tons per inch, is (see Appendix B) ........ 150 tons. — 
The weight of girder sufficient to give no wave or deflection 
greater than 1°32in. with 100 tons (see Appendix A) . 150 ,, 
The weight of metal in cast-iron columns, so that the greatest 
compression with 3 tons per foot is 4 tons per inch (see Ap- 
pends Gy, SPSL SPIN 2s OURS Ts 60 5, 
Weight of suspension bars, so that the tensile strain does not ex- 
ceed 5 tons men inch with 3 tons ial foot load ea: 15 
pendix D).. PMS he ae eee olga via le «Ath 15) 9% 
375 
To this must be added the value of the cost of the anchorage 
of the chains, which in the Londonderry Bridge will be 15 -* 
per cent. of the iron-work of the main girder portion of the 
bridge, so that I have added 57 tons to represent the value ; g 
GE THGIGORE TS EERIE Saki wn stele ee 2 iets . ib eae 57 -» 
432 tons. 
