width of cross-section at the end of the jetties being that corresponding to the maintenance of the depths of 25 feet on the passes themselves.

The widths of all these bars have been taken from the Coast Survey map, and upon the same principle of measurements; on account of the smallness of the scale, the dimensions taken are not assumed to be strictly accurate. The width between jetty-heads at Pass à Loutre has been assumed to be 2,200 feet; though at this pass, on account of the abnormal state of things produced by its two mouths, this width apart for the jetties is not so certainly correct as at Southwest Pass and at South Pass. While the estimated annual advance of the bar at Southwest Pass and at Pass à Loutre may be said to be, and probably will be, the same, that for the South Pass is much greater than either.

I do not rely upon this method to furnish accurately the absolute, but it is evidently an approximation to the relative, advances of the bars, to which exception cannot justly be taken, and places the Southwest Pass, therefore, in an unfavorable light for improvement by jetties.

Sir Charles Hartley has said, and in this he has been cited repeatedly by the minority reports, that an outlet having a large discharge was at a disadvantage for improvement with another having a less discharge, on account of furnishing more material for the formation of the bar, and because the bar was usually farther out. This is correct as far as it goes, and it may be true, owing to the littoral current and other peculiarities, for the Danube, to which he particularly applied the principle. But its application to all rivers would lead to grave errors, because another general principle has not been considered, viz, the increased progress forward of a bar due to decreased width of discharge.

The effect of the application of the last rule upon the relative advantages of the Southwest Pass, Pass à Loutre, and South Pass has been to change the aspect of matters as to the choice of the passes for a permanent improvement. The discharge through the Southwest Pass is 34, through Pass à Loutre 234, and through South Pass only Too of the whole discharge of the Mississippi; and yet, cut and carve as you will the figures in the table above, the annual progress of South Bar, after the application of jetties, will not be reduced lower than 13 that of Southwest Bar.

6. An examination of the longitudinal section of the bars will lead to deductions of practical value.

The slopes of the bars interior to the crest give for each depth of water the inclination at which the erosive effect of the currents at that depth and the material of which the bar is composed hold each other in equilibrium. For Southwest Pass, at a depth of 25 feet, this inclination or rise is a little more than 1 foot per 1,000; for Pass à Loutre, the inclination is 1% feet per 1,000; and for South Pass at least 23 feet. The slopes of the bar at this pass are very irregular, in strong contrast with the other sections, and seem to indicate intermittent instead of the regular and constant action of the currents prevailing on the other bars.

The growth of the new bar beyond the jetty-beads will not be at a less inclination than those above given. Taking, then, the annual advance of Southwest Bar under the action of jetties at 1,000 feet, of Pass à Loutre 1,200 feet, and South Bar at only 1,500 feet, and the diminutions of depths in one year will be respectively 1 foot, 2.16 feet, and 33 feet; and hence the necessity for annual extensions of the jetties will be more urgent at South Bar than at any other.

A similar relation of slope in the bars will be found by considering the flare* of the mouths of the passes. This element would approximate to the real flare or spread of river-waters as they emerge from the jetty-heads, and give us the means of calculating that slope. It is not assumed that the results given in these methods of obtaining the rise of the bar as it advances into the Gulf are absolutely correct, the scale of the maps available for use being too reduced for that, even if the methods themselves were mathematically correct, which is not pretended to be said. But the results thus furnished will show the approximate relation between the bars in respect to their rise as they advance, and thus enable us to discriminate for or against a certain pass. The three passes having thus been relatively tested as to capacity and eligibility for improvement by jetties, the following results are obtained:

Southwest Pass is the most advantageous, except in the first outlay for jetties, these being about nine miles altogether in length; the advance and vertical rise of the bar-formation are less than in any other; the width between jetties of 3,900 feet, corresponding to the depth of 25 feet, gives it great advantages for permanence, as the necessity for converging these will not, as soon as in the other passes, create such obstruction to discharge that the flowing waters will abandon it for another pass.

Pass à Loutre: The jetties here would not differ materially in length from the preceding example, and the pass is rapidly approximating to Southwest Pass in eligibility; but at present the abnormal state of things arising from its bifurcation and the concavity of one of the jetties toward the currents renders it far inferior to the latter.

As to South Pass, the length of both jetties would be about four and two-thirds miles, but the extension and vertical rise of the bar-formation would be much greater than in the other cases. Its narrow width, between jetties of only 660 feet, would soon be consumed by the convergence necessary to keep up the scour.

The longitudinal section of its bar and bed, by its irregularities, indicates very clearly that the shoaling process is going on throughout, and that the pass at the present time is hanging between the conditions of a live pass and a stagnant ditch; to the last of which results it must arrive, if a revolution in the delta does not redeem it. And this most probable fate will be precipitated by applying the jetty system to its mouth. The shoal at the head of this pass is permanent, and results from the manner of discharge of the main trunk of the river. The application of works constructed to dissipate this bar would, in such a situation, be precarious, even if at all practicable. Some relief, however, may be possible by dredging or stirring.

The prominent physical features of the Danube are quite different from those of the Mississippi, and it is proper, in concluding this paper, to briefly glance at them. This river separates into two branches at Ismail Chatal, viz, the Kilia and the Toulcha; and the latter, after eleven miles, into two others, the St. George and Sulina branches.

*At the width of pass corresponding to 25 feet in depth.

The distances to the sea from Ismail by the Kilia is ninety-six miles; by the Sulina, seventy-eight miles; and by the St. George branch, ninety miles. The triangle thus formed, having Ismail as its apex, has a base of forty miles along the sea-coast, and a perpendicular of fifty-eight miles.

The first difference between the Danube and Mississippi is, that the former has divided into three long branches, each of which is an active stream; whereas the Mississippi has no branches, but pushes its trunk undivided to the Gulf.

At Isatktcha, fifteen miles above the division, the Danube is 50 feet deep and 1,700 feet wide. At Fort St. Philip, twenty miles above the head of the passes, the depth of the Mississippi is 151 and its width 2,360 feet. Its great depth constitutes another point of difference, which the Mississippi also maintains in its passes.

The average specific gravity of the matter held in suspension in the Danube is 2.5; that of the banks, 2.7; and for the Mississippi the latter element is 1.96. The specific gravity of materials on Southwest Bar. after it had been sifted from finer materials by the action of currents and waves, was 2.6, or about the same as that of the suspended matter in the Danube.

The proportion of silica in the material brought down by the latter river is very large-about 67 per cent.

It is not surprising if we read that at the Sulina mouth large warehouses have been erected on the banks alone as a foundation; that a crib, 30 by 15 feet, sunk off Sulina mouth, and loaded with 130 tons of stone, remained for nearly one week supported on only ten square yards of foundation, without the least settlenient, and that the driving of a single pile, 13 inches square, in the line of piers, to a depth of 16 feet, was a day's work for one driver.

Dunes of sand, 30 to 40 feet, are encountered at the Kilia mouths, and of smaller dimensions at those of the St. George.

What a contrast the above offers to the mud-shoals and banks at the mouths of the Mississippi, in which it is said that the weight of a man will send a pole down to a depth of 9 feet. Upon the surface of the bar, where the waves and currents sift out the finer materials, there is of course a deposit of more or less sand. From this brief analysis of physical differences between the two rivers some important deductions may be made. The strength and resistance of its banks is the cause, probably, why, having once separated into three branches, the Danube has been unable to reform its main trunk, or that the cross-section of either branch has not become so great as to convert the others into mere drains for overflow.

In the Mississippi, owing to the nature of its sedimentary deposits, the main trunk cannot be divided, because it has the power always to excavate its bed to the depth and width required to carry off its waters. No crevasse or artificial outlet has ever yet formed a branch to this river.

The same reasoning applies to the passes, which, perforated through the most yielding alluvions, will always suffice to discharge the river, and should one by some obstruction be impeded in its discharge, the others enlarge their cross-sections to supply the emergency.

It is this freedom to discharge under all circumstances which makes it a delicate operation to obstruct the flow of water through any one of the passes, under the plea of improving the navigation. The rapid extension of a pass by jetties, though under all circumstances prejudicial to the discharge through it, would not be so soon felt in its consequences

in the ordinary river; but at the Mississippi delta, such an operation might inure to the rapid deterioration or ruin of the pass, and this is the reason why the application of the jetty system to these mouths must sooner or later wear them out by forcing the waters into other channels. I send here sketches of the Balize bayou, which was the main entrance to the river at the first settlement of the country. These sketches are of 1722, 1724, and 1731. During this interval the pass shrunk in width to the comparative dimensions of a ditch. The rapidity of the change is the striking feature which could not have occurred except in the soil and the other conditions of the Mississippi passes.

The first indication of the change appeared to have been that the outer bar became deep, while the shoal lodged in the pass itself and its juncture with the Southeast Pass.

There are no recognized relics of the delta form, which have in former years occupied successively various positions in the lower portion of the Mississippi. Delta after delta has disappeared, and the question is, how does the trunk, after a certain elongation of the passes, impatient at the obstruction which their lengths offer to a free discharge of its waters, break through into the Gulf and commence a new formation? Such a revolution, impossible where the river empties into a shoal sea, is by no means so when the depth is great. On the other hand, after a certain elongation of the passes, do all the parts move gradually into the Gulf, the head of the passes and trunk, like the brush of a broom at the end of its staff, without breaking their connection? I am aware these questions cannot be answered, except that the delta was once at New Orleans, and is now in its present location. The present age of the delta is at least three hundred years, and the passes are rather long, and if the movement is not gradual, it must suddenly come. No observations, so far as I am aware, have been made with the view of noting any movement.

This is not mere speculation, though it may have that aspect; there is nevertheless a practical side to it. The outlets of the passes are moving at the average rate of 262 feet per annum, and the whole delta must in a certain period of years correspond to the same movement, either gradually or by a sudden burst.

Either view would have an important bearing upon the improvement of the passes by the means of fixed artificial works. Respectfully submitted.

JOHN NEWTON,

Lieut. Col. of Engineers, Bvt. Maj. Gen.

Brig. Gen. A. A. HUMPHREYS,
Chief of Engineers, U. S. A.

List of maps accompanying reports upon improvement of the mouths of the Mississippi.

1. Géologie pratique de la Louisiane, par R. Thomassy. Entrée du Mississippi en 1722, 1724, and 1731. [Map showing the changes which took place at the Balize during the above years.]

2. Map showing the supposed limits of the area in which the center line of the proposed canal would be located, according to Major Warren's idea.

3. Delta of the Mississippi River, from Captain Talcott's surve 1838, with comparative profiles of Southwest and South Passes. 4. Comparative profiles of the Southwest Pass and South Pass, Da of the Mississippi River, and of the Sulina branch of the Danube. 5. Sulina mouth of the Danube, from the "Minutes of Proceedi Institution of Civil Engineers, London, Vol. XXI, Session 1861-262. 6. Comparative chart of the surveys of 1838, 1867, and 1874, of Southwest and South Passes, Mississippi River, Louisiana.

R 16.

MOUTHS OF THE RHONE.

The following brief account of the application of the jetty system to the mouth of the Rhone, prepared in 1863, was recently communicated to the Chief of Engineers by Mons. E. Malézieux, Engineer-in-Chief i the Corps of Ponts et Chaussées, as affording accurate information of the subject.

[Translation.]

A decree of the 15th January, 1852, ordered the construction of the works for the amelioration of the mouths of the Rhone. The expend ture authorized by the decree was 1,500,000 francs, ($300,000.) The work executed for that purpose up to 31st December, 1862, co 1,464,253.40 francs, (substantially the amount appropriated.)

The works consisted of continuous embankments upon both banks d the Rhone, from the tower of St. Louis to the vicinity of the bar; tha on the left bank of the river had a total length of seven kilometres (22,966 feet,) and terminated 1,530 metres (5,020 feet) inside the cres of the bar. The embankment on the right had a total length of 6,50 metres, (21,326 feet,) and terminated 1,460 metres (4,790 feet) inside the bar-crest. The embankments are composed in part of earthen dike rising above the surface of water, and in part of jetties of stone that de not rise to the surface. The result of these works has been the con finement of the waters of the Rhone to a single channel running from west-northwest to south-southeast, which at the termination of the embankments has a width of only 400 metres, (1,312 feet.*)

When the concentration of all the waters in one channel was effected which was at the close of September, 1856, the ends of the jetties were 900 metres (2,953 feet) inside the bar, which was eroded, and from having had a depth upon it of 1.5 metres (5 feet) in July, 1852, was found in September, 1856, to have a depth of 4.15 metres, (13.5 feet.) But since then the bar has moved seaward, and the depth of water upon its crest has diminished, and it has now (1863) only a depth of 1.4 metres, (4.5 feet.)

Between June, 1852, and February, 1863, the bar moved 800 metres (2,625 feet) seaward, measured along the line of direction of the embank ments. Its mean annual advance since June, 1855, has been 74.35 metres (244 feet.f)

The variations in the depth of water upon the bar have always taken place at the end of the floods of the Rhone. Floods of no great height

The width of the Rhone at Arles, the head of the delta, is 600 feet.

The mean annual advance of all the bars or mouths between 1807 and 1846 was 21 metres, (76 feet.) Memoir of A. Surell, engineer of Ponts et Chaussées, in charge of Rhone works.