. The railroad and engineering journal . ns where the traffic is continuous andheavy and foundations are easy, bridges of masonry aremost suitable. But such bridges offer greater obstructionboth to the stream and to navigation, as compared withother types of bridges, because in them the ratio of thethickness of the pier to the span is very great. For spansbetween 30 and 150 ft. an iron girder is most economical,and for shorter spans wooden bridges may be preferred toother types when the first outlay must be small. .Suspen-sion bridges, or those of the cantilever type, are to be rec-ommended wh
. The railroad and engineering journal . ns where the traffic is continuous andheavy and foundations are easy, bridges of masonry aremost suitable. But such bridges offer greater obstructionboth to the stream and to navigation, as compared withother types of bridges, because in them the ratio of thethickness of the pier to the span is very great. For spansbetween 30 and 150 ft. an iron girder is most economical,and for shorter spans wooden bridges may be preferred toother types when the first outlay must be small. .Suspen-sion bridges, or those of the cantilever type, are to be rec-ommended where the span is too great to be crossed byarches or girders at a reasonable cost. Our present object of inquiry is how best to bridge overa river with the least possible disturbance to the regimen ofthe river. With a span of too ft. or thereabouts, and byreducing the thickness of the pier as much as possible, wewill be able to cross most economically and at the sametime in harmony with the regimen of a river which is sub-ject to gated by rivers, so that the level of such rivers must notbe lower than that of the fields. Such being the case, we have many examples of riverswhose beds are considerably elevated above the neighbor-ing ground. The most noticeable examples of this kindare seen between Kobe and Osaka. Travelers by rail fromKobe to Osaka will remember that they passed throughthree short tunnels. These tunnels are under three riversknown as Ishiyagawa, Ashiyagawa, and these rivers materials in mechanical suspension weregradually deposited, and at the same time the inhabitantsraised the banks. The result was that, when the railroadwas to be constructed, the rivers had to be tunneled under. In such cases, when the bed of the river is sufficientlyhigh to be tunneled under, an engineer can carry out hiswork with comparative ease as long as a flood doesnot burst the banks. But in case there is not sufficientheadroom to carry the line below, or when dr
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