weighs 31.5 long tons or 70,500 pounds. Another single track lattice girder of 
ninety- four feet span between Kioto and Osaka weighs 47 long tons or 105,280 
pounds. A double track truss bridge of one lmndi'ed feet span weighs 77.6 long tons 
or 178,824 pounds. 
By interpolating in Table I we find tho weight of iron per lineal foot for a 67^ 
span to bo about 800 poumls, and that for a 94 r span 852 pounds, making the total 
weights for these cases respectively 58, G00 and 80,088 pounds. 
A special calculation gave tlie total weight of iron for a 100， span double track 
briclgo as 176,750 pounds. 
Now as the bridges of this treatise are provided with iron stringers and guard 
rails and oak ties, while the Japanese bridges have only wooüen stringers it is evi- 
dent that the former are at any rate the more economical ； and, I think, that if you 
will take tlio trouble to carefully read tlic following chapters, you will conclude that 
they are also muoli better designed. 
Tlie reason why the double track bridge that I designed is proportionately so 
much heavier than the single track bridges is that tlie overhead bracing for reasons, 
wliicli will appear further on, is necessarily very heavy. 
But to return to the subject of American railroad bridges ; I do not wish you to 
imagine that I consider them all perfect and in every way superior to tlie European. 
Unfortunately such is not the case, for many existing bridges in the United States 
are tho work of inferior bridge companies and engineers, who have failed to pay 
proper attention to detail. Then again tlie bridges of twenty years ago are not 
heavy enough for tlie rolling loads of to-day, and moreover tlie science of bridge de- 
signing has made great progress in tho last twenty years. But the lately erected 
bridges of the better class of Amoricau bridge companies are uncloubtedly good, and 
it is with these in view that I have prepared this treatise, endeavouring in every res- 
pect not only to equal tlietn in excellence of design but to improve upon them where- 
ever I saw the opportunity. The styles of truss adopted are those of tlie Pratt and 
Whipple systems, or the single and double quadrangular trusses. That these forms 
are both the best and most economical is proved by thoir being almost universally 
adopted by the leading bridge builders of tho United States ; besides, I have shown 
in a paper entitled “Economy in Struts and Ties,” by a method entirely practica], 
that vertical posts and inclined ties are more economical than any other arrange- 
ment ; and these are tlie essential features of the Pratt and Whipple trusses. 
You will notice that double track bridges and deck bridges have not been as 
fully treated as through and pony truss bridges*: deck bridges are applicable to only 
high grade crossings, few of which will be found necessary in tliis country ; while tlie 
double track bridges will not bo needed, in all probability, for tlie next twenty years, 
by which time steel will have replaced iron in bridge construction. Nevertheless 
you -will find that both these styles of structure have received sufficient attention to 
enable an engineer to design them with ease, the only difference being that lie will 
have no diagrams similar to those on Plates XIV 一 XLII to guide him. In referenco 
to tliese diagrams I would stato that tlie dead loads and wind pressures liad to be 
first assumed then checked, so that they do not agree exactly with those given in tho 
