380. The following Table, showing the velocities of some of the principal rivers, may assist in giving more accurate ideas on this interesting subject; and we have only to regret that it is not so complete as might have been expected, owing to the few observations that have been made.

Name of River.

Place of Observation.

Stale of River.

Velocity in feet per second.

Observer.

Chames

Above London Bridge

Mean state

3.1667

Labelye.

" Westminster Bridge

Mean state

2,25

"

Seine ..

Between Tuileries and Pontneuf..................

....

1.54

Mitrriotte.

(Between Sureneand Neuilly

..

2.51

Chezy.

Fiber..

Rome..............

Low wafer

3.28

Danube

Ebersdoff..............

Low water

3.45

High water

from

7.2

to

12.2

Loire ..

Declivity of the bed .000382

...

4.25

Rhone

At Arless...............

Low water

4.8

At Beaucaire..........

Low water

8.2

Durance

From Sisternon to its mouth

Mean state

8.2

In 1818 an immense quantity of water accumulated in the Val de Bagnes, in Switzerland, by a glacier sliding into the valley; when the ice gave way, the torrent burst forth with the tremendous velocity of 33 feet per second, and swept two bridges away in its course, and still retained a velocity of

* Respecting the velocity of rivers, and the fall of water under bridges, the reader may consult Du Buat, ' Principes d'Hydraulique,' edit. 1816; Dr. Robison's art. River, ' Encyclopaedia Britannica; Playfair's ' Outlines of Natural Philosophy;' ' Rees's Cyclopaedia,' art. River, by John Farey, jun.; ' Hutton's Tracts,' vol. i.; Sir H. Douglas ' On Military Bridges; and Dr. Brewster's ' Encyclopaedia,' art. Hydrodynamics.

6 feet per second, until it flowed into the Lake of Geneva, a distance of nearly five miles.*

The Span or Width of Opening.

381. The extent of the span is the next point to he considered; it will he obvious that this is in some degree determined by what has been said respecting the area of the water-way. The span of the opening, however, must also be regulated by the form of the banks, the height of the highest floods, the depth and rapidity of the river, and the kind and dimensions of the timber that can be procured. In rivers that are tranquil, of little depth, and not subject to high and rapid floods, the number of piers may be increased without inconvenience, provided they do not interrupt the navigation of the river, nor contract too much the water-way.

But if the bridge has to cross a torrent, the fewer the number of supports placed in the stream the better. When the banks are not too low, and the width of the river does not exceed about 300 feet, the engineer whose choice is confined to a wooden bridge should give the preference to a single span or opening. For greater widths it is desirable to erect piers in the bed of the river. There is very little advantage as regards cost in adopting large spans, because in such cases the piers require to be carefully constructed, which will cause additional labour and expense.