CHAPTER XVI. 
ROONOMY. 
TIig first point to be considered, when deciding upon the stylo of bridge for a 
certaiu stream crossing, is the number of spans. It is, in reality, a consideration 
of economy which determines this; for the best bridge to build, provided that the 
water-way be not too much contracted, is the one for which the sum of the cost of 
superstructure and the cost of foundations is a minimum. If the water-way be 
to ° mu cli interrupted, the design would not be an economical one, even if its first ， 
cost were tlie least, because of the risk of washout to which the bridge would always 
l)e subject. 
In most cases there is not much choice concerning the number of spans, local 
considerations often determining it ; but there is occasionally a choice between two 
0v even three numbers. The only way, then, to decide is to make a rough es- 
of the cost of the superstructure and the foundations for each number ; tlien ， 
tlle choice fall about equally between two numbers, it is better nearly always to 
a ^°pt the longer spans, because the actual expense for the foundations usually ex- 
ceeds the amount of the preliminary estimate. 
Tlie spans iu this country at river crossings are in the author s opinion al- 
ogetlier too short considering the sudden rises and the immense volumes of water 
ln 山 6 mountain torrents. 
Tlie recent washouts in the neiglibourhood of Kioto will give force to this 
statement. 
The next economic consideration is that of depth of truss. Upon this subject 
^ucli has been written, and many investigations have been made ; the general con- 
C Us ' otl being, that the depth should be from one- seventh to one- tenth of the span : 
oniG English writers say from one- tenth to one- fourteenth of the span ; "while only 
了 11 ^’ as fai. as the author knows, — Benjamin Baker, Esq. C. 1^., in his treatise on 
e aras f Columns, and Arches, n — makes it from one-fifth to one- seventh of the 
span. 
