THE

London

JOURNAL OF ARTS AND SCIENCES.

No. XLVIII.

[SECOND SERIES.]

Original Communications.

ON FLYING BRIDGES.

To the Editor of the London Journal of Arts, &c. SIR, Flying Bridges are rarely if ever used in this country, because whenever there is any intercourse between the opposite banks of a river, it is generally sufficient to justify the erection of a permanent bridge, and the breadth of our rivers is not in general such as to render the construction of permanent bridges impracticable; moreover, where they are broad, they are seldom rapid enough to prevent a ferry boat from crossing with tolerable ease and expedition.

From their simplicity and cheapness, and the expedition with which they can be constructed, flying bridges are of great use in military operations; they are also very common on the broad and rapid rivers of the continent, and although little used in England, are not undeserving of attention.

VOL. VIII. SECOND SERIES.

2 P

A flying bridge is formed by fastening a floating body to the end of a cable or chain, moored in a river, and keeping the body by a rudder, oblique to the direction of the stream; the action of which against the oblique side of the floating body, drives it away towards one of the banks, moving it in an are of a circle, about the moorings of the cable.

Plate XIV. fig. 1, represents a floating bridge; a, a boat fastened by a cable b, to a buoy or an anchor at c. The current running in the direction of the arrow No. 1, moves the boat a, in an arc of a circle about c, away from the bank h, towards the bank i.

The force which urges the boat a, in the direction h, i, depends upon the obliquity of the boat to the current, and is greatest when the side x, makes with it an angle, of 54° 44'.*

A bridge of this kind is in use on the Rhine, for crossing opposite Nymuegen. From the bank opposite to Nymuegen, a bridge of boats is built, extending rather more than half across the river. The flying bridge (is a platform or piece of road, laid on a strong barge, to which one end of a chain is made fast; the other end of the chain carried over and fastened to the masts of seven boats, to support its weight, and is moored in the river at some distance up the stream. The barge is steered oblique to the stream, and according to the direction of its obliquity, swings round in an arc of a circle, from the end of the bridge of boats to a jetty on the Nymuegen bank, or vice The stream runs about three to three and a half

versa.

miles an hour.

In very rapid rivers, flying bridges should not be made both to cross and re-cross by the action of the current, for the resistance to the motion of the boat in the ascending part of the arc (viz. in the direction of the arrow,

* Vide Douglas on Military Bridges, p. 94.

No. 2,) is very great, and the descending force of the stream, to drive the boat down the river, negatives the effect of its oblique action on the boat, to drive it upwards about the moorings of the cable.

In such cases, it is advisable to make the flying bridge move only one way by the current, viz. in a descending arc, and to haul it back by a second cable; thus the boat a, fig. 2, crosses to the bank e, through a descending arc from the bank d, to e, taking with her a rope b, c, by which she is hauled back from e, to the bank d.

On the other hand, a flying bridge will not act well in a river with a very slow current at the sides, unles jetties or bridges of boats are built out from the banks, some distance into the river, for the flying bridge to come up to, for when the current at the sides is slight, it will not carry the boat close up to the shore.

A flying bridge was established a few years ago across the Kistna, in India: the breadth at the part where the bridge was made is between 700 and 800 yards in the rainy season, when the river is full, and the stream then runs in the middle of the river, at about four to four and a half miles per hour. The bed of the river is deep, but the sides shelve up. In the rainy season, when the river was quite full, the flying bridge acted very tolerably; but when the waters were out, the strength of the current at the sides, owing to the extreme shallowness of the water, was insufficient, and the flying boat could not be made to come up to the banks within about 40 yards.

When a river is too wide for a simple flying bridge, two boats may be used, one moving in an arc c, d, fig. 3, about the centre a, and the other in an arc e, f, about b, and a boat or a raft moored in the middle of the river, for shifting the passengers from one boat to the other. Or the raft may be dispensed with, and the cables shifted when the boats come close up to each other; the boat d,

being then made fast to the cable b, e, and the boat e, to the cable a, d, so that each boat will go across in two stages from one bank to the other, through c, d, and e, f.

Sir H. Douglas recommends this plan, in preference to having a raft in the middle (see Douglas on Military Bridges p. 96); we apprehend it might be attended with more danger of the boats fouling and doing injury if the current were very rapid, for both boats would be moving at their greatest speed, just where they would meet, viz. in the middle of the river; and hence the shock, if by accident they were to strike each other, would be twice as great as the shock of one boat against a stationary raft. Also the difficulty of managing them in a rapid current to change the cables, would be considerable, and the operation tedious.

We are not aware whether this plan has been extensively adopted in practice. The other is adopted in effect, and answers very well in the Nymuegen flying bridge; for though it has not two boats, the flying boat comes up to the end of the stationary bridge of boats, nearly in the strongest part of the stream.

In flying bridges the cable should be of a good length, for when it is long, the flying boat moving through the arc of a large circle, has to ascend the stream less than when the cable is short; that is, its direction is nearer to a strait line across the stream; and, consequently, less of the effective force of the current to impel the boat across, is abstracted by the resistance of the current to the ascent of the boat. For instance, in figure 1, with the cable c, a, the boat moves upwards, equal to a distance a, l; but if the cable were only as long as c, l, the boat in crossing, would move through a distance 7, n, against the stream, which is much greater than a, l.

The whole motion of the flying boat from side to side, should not exceed a right angle, and then the angle a, c, s,

fig. 1, will not exceed 45°; for when the angle a, c, s, exceeds 45°, the force c, l, that impels the boat sideways, in opposition to the current, becomes less than the force l, s, which holds it to the centre. This is shewn by the triangle c, n, m, fig. 1, where the angle n, c, m, is more than 45°. The force n, m, is greater than the force c, n, and the boat would not, in fact rise by the oblique action of the stream so far as m.

In narrow rivers, not exceeding two hundred yards in width, and with a tolerably rapid current, a flying bridge may be applied with effect, in the following manner :—

Let a cable be stretched across thé river from bank to bank, and attached on each side to a frame secured in the bank, and drawn tight by a windlass; then attach the flying bridge to this cable, by means of a short rope,. with a running block on the cable; and by keeping the boat in an oblique direction to the course of the stream, it will be carried across by the force of the current with considerable effect and expedition. This mode of establishing a flying bridge is more easy of adoption than the former one, and is attended with far less expense and trouble, as the buoys for the support of the cable may be dispensed with, and also the anchor for mooring it in the river.

This plan was used, to establish a communication across the Thames at Gravesend, during the threat of invasion from France. The cable was suffered to sink to the bottom, not to interrupt the navigation; and as the boat crossed, the rope rose to the point of suspension on the bank.

The cable should not be sunk when it can be avoided, because the boat has then to move the weight of the rope that connects it to the cable; and, moreover, the running block will run with a great deal more friction upon the wet cable than when it is out of the water.

A triangular raft may be floated over a river, as well