GIRDERS AND SUSPENSION CHAINS COMBINED. 241 
The deflection of the Londonderry girder, deduced from the mean results 
_ of the deflections of the Boyne Viaduct and Newark Bridge and the Britannia 
Tube (see Appendix A), was 33 inches with 100 tons in the centre, }2=2°20 
inches, the deflection here indicated in the Londonderry Bridge with 100 
tons placed at a quarter the length of the girder. 
It was still obvious from the deflection at the centre and little rise exhi- 
bited in the wave, that the stretching of the chain to bring the metal surfaces 
to bear, still sensibly influenced the result, and I had another wooden girder 
‘made, consisting of a plank 74 inches in width and 3th of an inch thick, in 
order still more to magnify the wave, and to diminish the error from the 
stretching of the chain. 
The deflection without the chain attached was 1:48 inch with 10 lbs. 
_ Experiments with the Chain attached.—With 56 lbs. placed at + from the 
high tower on the girder which was previously quite unloaded, the deflec- 
$ from + from 2 from 3 from 3 from 2 from 2 from 
H. T. i. T. H.T. Hi. T. HH. T. A. T. HT. 
—31 — 48 82 — 02 +'22 +°29 +:15 
_ Experiment 2.—70 lbs. being equally distributed over the girder, and 
56 Ibs. at + from high tower— 
—'28 —"42 —"25 +04 +°23 as weitere 
Experiment 3.—150 lbs. all over weight in same place— 
—‘20 —'35 —*20 +°02 +°20 +'23 +'14 
Experiment 4.—193 lbs. equally distributed, 56 lbs. as before— 
1s —31 re +°05 St +'20 +'14 
The deflection here indicated with the model loaded with a weight repre- 
senting 96 tons on the bridge (which experiment was several times repeated), 
was *31 with 56 lbs.=°055 with 10 lbs., or 3};th of the deflection of the girder 
without the chain; 33=1-27 is therefore the deflection of the wave indicated 
by the experiment of the Londonderry Bridge, with a load of 100 tons at 1 
from the tower. 
o obtain the comparative rigidity of the experimental girder, we have 
as— 
6lbs.: 10lbs.:: 1 in. : °0485, the deflection of a true modei with 10 lbs.; 
or sas represents the rigidity of the experimental girder ; aoe X 33= 
35, the deflection by a weight on the bridge of 56 x 33°=97 tons. 
_ 27: 100:: ‘335 : 1:97, the deflection as previously calculated. 
_ This result being so much at variance with the general view of the subject, 
hough very nearly in accordance with my calculations, I determined to 
y it by a smaller girder, 6 inches by ths of an inch thick, which would 
er the wave still more visible, the observations being made with great 
ty. 
E deflection at the centre when not attached to the chain was 2:375 
es with 8 lbs. 
Girder attached to the chains, 193 lbs. being equally distributed over it. 
lbs. from H. T. Centre. 3 from L. T. 
56 —'64 +13 +°53 
_ Experiment 2.—With 56'lbs at the centre of the bridge the deflection was 
30. 
he deflection of the wave here exhibited at 1th of the length with the 
ge loaded to a weight equivalent to 100 tons on the actual bridge, which 
. R 
