46 TECHNOLOGY. 
side of the principal opening, each of 50 feet span. The under side of the 
roadway is 100 feet above the high-water line. 
In France wire suspension bridges have been extensively introduced. In 
the United States also suspension bridges have been erected to a limited 
extent. At Philadelphia there is a wire suspension bridge over the Schuyl- 
kill upon the site of Wernway’s wooden bridge, burned in 1838. At 
Wheeling, over the Ohio, one of the finest structures of this kind in the 
world has been erected by Mr. Charles Ellet, Jr., with a span of 1,010 feet, 
which is 152 feet longer than the celebrated bridge at Freyburg, which has 
the greatest span heretofore constructed. 
The flooring of the Wheeling bridge is 24 feet wide, and is suspended 
from twelve cables of iron wire 4 inches in diameter, and 1,380 feet long. 
The same architect has also constructed a suspension bridge over the 
Niagara river, between the falls and the whirlpool, and in sight of both; the 
span is 800 feet, and the roadway 230 feet above the surface of the river. 
Thus it will be seen that distances have been spanned by suspension 
bridges far exceeding anything even attained by any other species of con- 
struction, while the experiments made by Vicat during an examination 
of the state of the suspension bridges over the Rhone lead to the conclusion, 
that with proper care they will prove as durable as the most solid stone 
structures. It is to be regretted, however, that they do not afford that 
stability which is necessary to enable them to be used as railroad bridges. 
A few years ago it became necessary to construct a bridge over the 
Menai straits for the passage of the Chester and Holyhead Railway. It 
was impossible to make use of the chain suspension bridge, as was at first 
intended, its flexibility rendering it unsuitable to the passage of trains of 
ears. It was also necessary that no centring or scaffolding should be used, 
as this would interrupt navigation ; a stone bridge was therefore out of the 
question. . 
Mr. Stephenson offered a design of an iron tube, a proposition which was 
received at the time with general incredulity; the company, however, 
having confidence in their engineer, after some preliminary experiments, 
decided to adopt the plan, and it has since been built. The tube is not 
eylindrical, but rectangular; it is constructed of thick plates of boiler-iron, 
and is made of several sections resting upon piers. The distance spanned 
by the longest section is 460 feet clear, the greatest distance ever yet 
attempted except in suspension bridges. One of the longest sections is esti- 
mated to weigh 1600 tons. These tubes were floated upon pontoons to 
near the position they were to occupy, and raised to their place by huge 
hydraulic presses. The trains pass through the interior of the tubes. 
8. Intanp NAVIGATION. 
Hitherto we have treated of communication by land only; but streams 
dlso form an important means of communication wherever they are capable 
of ‘bearing shipping. All streams, however, are not adapted to this pur- 
626 
