crossed diagonally on a bridge of one span, giving about 12 feet headway to the streets. The approach then continues on the diagonal to the intersection of Third and Iowa streets, which is crossed by a single span of about 120 feet; then it is on a curve of about 1,700 feet radius to the junction of Second and Front streets, which is crossed by a span of 196 feet to the main bridge. There is 12 feet headway above the wagonroad floor. The main bridge has one span of 260 feet, then three of 220 feet, then one of 260.5 feet, and then the draw, 370 feet from end to end, with two clear openings of 160 feet each. A span of 80 feet then carries the railroad-track to the embankment on the island. This embankment extends across the island on a grade of 30 feet to the mile. Then comes a span of 80 feet over the island wagon-way; then the embankment continues 700 feet further to the abutment of the slough bridge. This latter has four spans of 150 feet each, and then there is an embankment on a 40 curve to the main track.

Masonry.-The masonry of the bridge has already been described. The piers in the slough are of masonry, resting on piles. This channel has but little current, it being only the tail-race of the water power at the head of the island. No special care was observed as to direction in crossing it, or in the obstruction of the water-way.

Superstructure.-The bridge is of wrought iron throughout. The upper chords are of rolled channel-bars, and the lower, open links in the fixed spans. In the draw-span both upper and lower chords are of channelbars. The draw is opened and closed by an engine placed on the outside of the truss, on a level with the railroad-floor. Great care is necessary in handling this heavy draw in time of high wind. The posts are Phoenix beams. The railroad-floor beams are attached to the posts so as to form a system of lateral bracing. The bridge over the main river was built by the Baltimore Bridge Company, Mr. C. Shaler Smith, chief engineer. On account of the limitation of cost provided in the law, the specifications called for wooden floors for both rail and high way. If the means admitted, this should have been changed to iron. When it becomes necessary to renew them, iron should be substituted for the wood.

INFLUENCES ON NAVIGATION.

This bridge has met the wants of navigation as fully as a low drawbridge is capable of doing. The locality is particularly favorable to this kind of bridge, as there is sufficient depth of water for boats and rafts from shore to shore, and the rock bed renders a change in this impossible.

ALTERATIONS TO LESSEN OBSTRUCTIONS TO NAVIGATION.

Sheer-booms.-The board of engineers on sheer-booms recommended that a sheer-boom 1,200 feet long be placed with its lower end, when in position, resting against the Davenport rest-pier.

Finishing the work in left draw-opening.-This board also recommended the timbering and planking of the rocky shore of the island for 500 feet above the bridge. This was part of the original plan, and should have been carried out if there had been funds to do it.

HIGH BRIDGE.

It is probable that this is one of the last places on the Mississippi River that will require the low draw-bridge to be replaced by a high one. When done, it should be placed at least 77 feet above low water, to enable boats to pass under it at all times when they can pass the bridge at Saint Louis. (See Discussion, Chapter iii.)

BURLINGTON RAILWAY DRAW-BRIDGE.

(Opened for railway travel in 1868.)

Authority. This bridge was built by the Burlington Bridge Company in 1867 and 1868, under act of Congress approved July 25, 1866.

DESCRIPTION OF THE RIVER AND VALLEY AT THE LOCALITY.

(See Diagram 21.)

This bridge crosses the Mississippi River about 4,000 feet below the steamboat-landing at Burlington, Iowa. The river at this point is about 2,100 feet wide between the low-water banks, with a large extent of bottom-land overflowed 5 to 10 feet at highest water, extending to the Illinois bluffs, making the width of the high-water river about 5 miles.

The channel of the river for 5,000 feet above the bridge and 3,000 feet below runs along the west bank in a direction for the most part parallel with the shore, which is rocky. The bluffs rise abruptly from the shore except at the site of the town, which is in an opening in the bluffs through which Hawkeye Creek flows. The river for two miles above and one below the bridge is confined to one channel, in which there is always a good navigable depth. The slope of the river at low water is 0.27 foot per mile. The bed is generally sand. The depth to bed-rock is not known, except for that part from the west or right bank to the pivot-pier, where it is about 30 feet below low water. The rise from low to high water is 20 feet.

SURVEYS.

A survey was made by us here in 1866 extending about 2 miles above and 2 miles below the bridge-site. The location of the bridge had not at that time been made, or, if made, was not generally known, so that no special examinations of the bridge site were made. The survey consisted of a meander of both shores of the river; location of sand-bars; levels to ascertain the slope of the water's surface; soundings over the section surveyed; and measurements for volume of discharge. A survey including current-observations was made in 1868 after the bridge was built. The water then was about 4 feet above extreme low water. The maximum velocity at the bridge was about 23 miles per hour; that in the channel 2,000 feet above was about 24 miles per hour. Currentobservations were again made in October, 1869, with the water 11.6 feet above low water. The maximum velocity was about 4 miles per hour.

DESCRIPTION OF THE BRIDGE.

(See Diagram 22.)

The bridge is approached from the west on an embankment with a curve of about 720 feet radius. The first span is 175 feet long from center of abutment to center of first pier-part of the span being over the land. The second span is 200 feet long. The trusses of these two spans are wider than those of the rest of the bridge, to allow the curve of the approach to be extended on the bridge. The next span is a pivotdraw 360 feet long over all, with two clear openings of 160 feet each; then there are 6 spans of 250 feet each from centers, and then trestling for a few hundred feet, which last is considered temporary and is

to be replaced by an embankment. The whole length of the bridge is 2,235 feet.

Masonry and foundation.-The masonry is built of stone from Athens, Ill., and Le Claire, Iowa. All of the piers rest upon piles cut off about 5 feet below low water. The piers, in this report, are numbered from the right bank. The piles for foundation of piers Nos. 1 and 2 were driven to bed-rock, which was struck about 30 feet below low water. Part of the piles in the foundation of pier 3 (pivot) were driven to bed-rock. The piles in the foundation of the other piers were driven from 20 to 25 feet into the bed of the river. The distance to the rock is not known. Under pier No. 1 there were 140 piles; under pier No. 2, 170 piles; under pier No. 3 (pivot), 333 piles; under pier No. 4, 170, and under each of the others about 140. The masonry was built upon a grillage of timber, which rests upon the heads of the piles.

The dimensions of pier 5 (see Diagram C') are, length 24 feet, width 8 feet. The batter is one-balf inch to the foot. At a point 9 feet below the level of the bridge-seat, 2 feet above highest water, the starling commences. The horizontal section is in the form of an equilateral triangle, and the cut-water has a slope of 8 inches to a foot vertical. The lower ends of the piers are frustums of a right semi-cone with a circular base. Where the piers rest on the grillage the length is about 57 feet, and width 16 feet; the lowest 5 feet being of three courses, each offsetting 6 inches all around. All the others are similar to pier No. 5 except the rest-piers Nos. 2 and 4, which are one foot wider. All of the piers except the draw-pier have detached ice-breakers; these were not built when our examinations were made in 1868, and when the bridge was surveyed in 1869 the high water prevented our examining them. The draw-rests are pile-cribs built to above high water, with their sides planked; at the upper and lower ends there are timber-cribs filled with riprap.

The superstructure of this bridge is of iron. The fixed spans are quadrangular girders with cast-iron upper chords. Phoenix rolled posts, and bottom chords of open links. The draw-span has both chords of rolled channel-beams riveted together. The whole superstructure was built by the Detroit Bridge and Iron Works, at the same time and under the same requirements as the bridge at Quincy. No account of the building of this bridge has been published that we know of.

INFLUENCE UPON NAVIGATION.

At the time of the examinations in October, 1868, the current measurements show that the general direction of the current made an angle of about 50 with the axis of the piers. The maximum velocity was through the west draw-opening.

A hard gravelly point makes out from the Iowa shore, about 500 feet above the bridge, which, at the time of this examination, deflected the current from the shore. This gravel-bar is probably composed of the coarser material brought into the river by Hawkeye Creek, and has become packed together so hard that it resists the action of the water even at flood velocity. This point acts differently at different stages of water; with the water high enough to run over it, it deflects the current but little; but when the water falls below, the current is deflected toward the right draw-opening, so as to cause a cross-current through this opening. The filling out of the shore-line above this point would probably remedy this somewhat; this filling had been commenced by the railroad company to make ground for depot-buildings.