Report of Mr. E. F. Hoffman, Assistant Engineer.

UNITED STATES ENGINEER OFFICE,
Rock Island, Ills, May 6, 1874.

COLONEL: In obedience to your order, contained in the letter of the 23d ultimo, I proceeded to Prairie du Chien, Wis., for the purpose of examining the crossings of both channels of the Mississippi River at that place. I report as to the extent to which this bridge at Prairie du Chien will obstruct the navigation of the river, as follows: The river between Prairie du Chien, Wis., and North McGregor, Iowa, is divided into two branches by an island. The eastern channel, Prairie du Chien side, is solely used by the large steamers of the different packet companies; the western channel, North McGregor side, is exclusively the water-way which is chosen by rafts of any size. The towns of Prairie du Chien and North McGregor are connected by a bridge. The bridge is constructed of piles driven into the bed of the river, over which rests a single railroad-track. Length of the bridge, measured upon the track, 7,200 feet from one shore to the other. There are two ponton-draws in this bridge. The draw inserted into the pile-bridge of the eastern channel consists of three pontons connected lengthwise firmly, and representing a distance of 396 feet. Two pontons have each one 28 feet beam-the middle one 17 feet. The draw, inserted into the pile-bridge of the western channel, consists of but one ponton, having a length of 408 feet by 28 feet beam. The pontons have a height in their sides of 5 feet and 44 feet, and a draught of 10 inches and 12 inches. When trains are passing over the draught increases to 16 and 18 inches. The varying height of the planes between the pile-bridge and the pontons is overcome by aprons or inclined planes. The draws are opened by the current of the river in less than one minute, as soon as the connection of the ponton-draw with the pile-bridge is broken. The closing of each draw is effected by a small engine which pulls the end of the draw, by means of a chain, into its connection with the bridge again. This operation requires from 3 to 4 minutes. In reference to navigating the draws, I state that both are located in good, deep water, but the direction of the current strikes the eastern draw at an angle of 75° 30', and the western channel at an angle of 55°. It will be therefore seen that the benefit of the length of the 396 feet in the eastern draw and the length of 408 feet in the western draw practically is reduced to 383 feet and 334 feet, viz, 396 feet, sin 75° 30′=383 feet and 408 sin 55°=334 feet. The bill pending in Congress in relation to this bridge demands not less than 250 feet for

the eastern draw, and not less than 320 for the western draw. It will be therefore seen that the builders of the bridge and draw have not only complied with the requirement but even increased the lengths of each of the draws, and that the bridge as constructed seems to be in accordance with the requirements of the act approved 4th of June, 1872, "further regulating the construction of bridges across the Mississippi River," as far as concerns the ready passing of boats and rafts through said bridge. At the same time it should be remarked that the draw-openings alone afford the way of passing through, and that such a bridge as the above-described combination of pile-work and pontons can only be authorized across the Mississippi River by the enactment of a special law as proposed, and the act should be entitled an act to legalize and establish a pile-bridge with ponton-draws upon which to locate a railway across the Mississippi River; and in view of the fact that the draw-openings at each channel are single,

the width which said draws respectively have, as described in this report, shall not be diminished. In relation to the navigation, I have to state that the captains and pilots unanimously opine this bridge to be the easiest to pass on the whole river, and consider it not to be obstructive to navigation. The accompanying sketch shows approximately the location of the bridge.

I have the honor to be, colonel, your most obedient servant,

Col. J. N. MACOMB,

E. F. HOFFMANN,
Assistant Engineër.

Corps of Engineers, U. S. A.

APPENDIX R.

ANNUAL REPORT OF CAPTAIN C. W. HOWELL, CORPS OF ENGINEERS, FOR THE FISCAL YEAR ENDING JUNE 30,

1874.

UNITED STATES ENGINEER OFFICE, New Orleans, La., October 5, 1874. GENERAL: I have the honor to submit the following reports of progress made in all works of river and harbor improvement under my charge, for the fiscal year ending June 30, 1874. Very respectfully, your obedient servant,

Brig. Gen. A. A. HUMPHREYS,

Chief of Engineers, U. S. A.

C. W. HOWELL, Captain of Engineers. U. S. A.

R 1.

IMPROVEMENT OF THE MOUTH OF THE MISSISSIPPI RIVER.

This work was continued throughout the fiscal year at Pass à Loutre.

DREDGING AND RESULTS.

A tabular statement is appended hereto giving detailed information regarding dredging, its results, and the conditions of winds, tides, currents, and stage of river attending it; also commercial statistics regarding the use of the channel made.

The following is a brief synopsis.

SOUTHWEST PASS.

Southwest Pass during the year retained its normal depth, and was much used for light-draught vessels.

Attempts in February and March to pass deep-draught vessels resulted in the usual blockade, which was maintained until a considerable fleet had been collected inside and outside the bar.

As, during this time, Pass à Loutre afforded a good channel, the parties controlling the fleet were finally forced to transfer it to Pass à Loutre, where it passed the bar with but little trouble, as it might have done a month previous.

Leaving Southwest Pass in its normal condition and improving Pass à Loutre permits a course of action that will greatly aid in preventing blockades, viz, the sending of all vessels not drawing more than 16 fleet to the former, and all deeper draught to the latter.

In this way a rush of vessels at particular seasons will be avoided at either pass, and greater care can be observed in selecting times and tides for crossing.

For the past few months this course has been partially followed, the local interests at Southwest Pass drawing to that place nearly all vessels of the lighter draughts, while the representatives, in New Orleans, of the commercial interests have insisted on having their deep-draught vessels taken to Pass à Loutre.

The result has been very satisfactory.

THE DREDGES.

Both dredge-boats were repaired during the year, and at its close both were in good working order and employed on the bar.

The McAlester will need but slight repair this fall to put her in condition to work throughout the year.

The Essayons will need greater repairs, and next year must be thoroughly overhauled.

The end-dock, steam-launch, and barge are in good order.

SURVEYS.

Surveys were continued as heretofore, but the field of survey greatly extended so as to cover the bars at Pass à Loutre, South Pass, and Southwest Pass several miles seaward of their crests, and the heads of the passes.

COMMERCIAL STATISTICS.

The following commercial statistics have been received from the office of the collector of the port of New Orleans:

PROJECT FOR THE YEAR ENDING JUNE 30, 1875.

The work will be continued as heretofore.

Both dredges will be kept at Pass à Loutre during the summer with but one crew.

In October the McAlester will be repaired and put on duty at the bar. The Essayons will then be brought up, repaired, and retained in port until the river rises. Afterward, both dredges will be kept at the bar. Should it prove advisable, one boat will be sent occasionally to Southwest Pass to keep the channel there in fair condition for the passage of light-draught vessels.

The work is not susceptible of permanent completion.

It is located in the collection-district of New Orleans and near the light-house at Pass à Loutre.

GENERAL REMARKS.

The recent legislation giving the Secretary of War authority to establish and enforce certain regulations governing the use of the pass improved, although regulations have not yet been drawn, has already had a beneficial effect. The facts on which it was based were not generally known, and the monopoly at which it struck was in a position to overawe its victims. This is now changed. The monopoly has been broken down, and in its place we have a very promising opposition in the towage-business.

Pilots, ship-masters, and ship-agents are resuming their proper authority, and, understanding the necessity for a system of laws, to govern their action, that looks to the general good rather than the aggrandizement of a corporation, are acting now very much as they should. For example-many pilots are taking greater pains to familiarize themselves with the channel; masters frequently wait for a high tide when their vessels are deeply laden; agents of steamers, when it is necessary, take tow-boats to insure crossing the bar without detention, and the towboat companies are willing to render such assistance without waiting for the vessel to get hard aground.

These are but a few indications of a growing, healthy, commercial sentiment, which, properly fostered, will make dredging as beneficial to commerce as it may be made, and place the efforts of the Government at their proper value.

The depths of channel given in this report, it should be observed, have been obtained by reducing the actual soundings to the plane of extreme low-tide for the year. This departure from the usual rule, adopted in hydrographic work of reducing to mean low-tide, was made for the purpose of inducing greater care on the part of persons using the channel, and has, I think, been of practical benefit.

I have not worked up my records from self-registering gauges so as to ascertain how far the plane of mean low-tide is above that of extreme low, but an inspection of the records indicates that the difference is between 8 and 10 inches.

Great care has been taken in making the soundings, men being employed who have, by constant practice, become expert and give the surface even where the bottom is softest.

Mushroom leads have been used, and their length not included in the measurement of the sounding-line-this length is about four inches. The lines were the best close-laid attainable in this market. These have been wetted, thoroughly shrunk and measured each time before using,