formation. It is now carried by the action of the jetties (which extend into deep water) iuto comparatively deep water, and adds some additional material to the deposit made by the suspended earthy matter.

In the case of the mouths of the Mississippi River, even at the mouth of the smallest pass, the quantity of both kinds of deposit-matter is enormous, and there is no littoral current to carry the suspended matter away. Even if there were at the mouths of the Mississippi a littoral current of the force of that existing at the Sulina mouth of the Danube, (the most careful observations have, however, failed to detect the existence of any at all,) it would be utterly impotent to cause any material modification of the bar-formations.

It may be remarked here that the distance which the currents of a delta river extends into a tideless or nearly tideless sea depends more on the volume of the river than the velocity of the current. The velocity of the current being the same in the one case with a small volume, and in the other with a large volume, in the first case the current will soon be neutralized, while in the other it will extend for miles into the sea before it is brought to rest.

From the foregoing it is apparent that the Sulina bar of the Danube has no resemblance to the bars at the mouths of the Mississippi River, and that what they have been dealing with in the improvement of the Sulina is a bar or shoal derived chiefly from the deposit of earthy matter held in suspension and not earthy matter pushed along the bottom of the bed of the Sulina.

A very important question connected with the jetty system is the rate at which the bar will advance under the influence of jetties. This, it seems to me, is not difficult of solution. The principles which should guide the application of this system are enunciated in that portion of the report of Humphreys and Abbot upon the Mississippi River, submitted August 5, 1861, which treats of the mouths of the river, especially the sections under the captions of "experimental theory of the formation of bars," and "recommendations for improving the navigation at the mouths."

The following is extracted from the latter section, pages 455 and 456; The development of the laws which govern the formation of the bars has removed all uncertainty as to the principles which should guide an attempt to deepen the channels over them. The erosive or excavating power of the current must be increased relatively to the depositing action. This may be done either by increasing the absolute velocity of the current over the bar, or by artificially aiding its action. To the first class of works belong the jetties and the closure of lateral outlets; to the latter, stirring up the bottom by suitable machinery, blasting, dragging the material seaward, and dredging by buckets. These plans are all correct in theory, and the selection from them should be governed by economical considerations.

If the excavating power and depositing action of the Southwest Pass had been equal when the yearly advance of the bar was 700 feet instead of 338 feet, the least depth upon it would have been 21 feet. This increase of excavating power may be obtained by constructing two converging jetties, beginning where the depth of 22 feet is found, and extended to that depth outside the crest of the bar, which would give them a length of about 2.5 miles. The experience gained in the progress of the work should determine where the convergence should cease and the parallelism begin. The erosive action should be aided by first dragging and scraping the hard portions of the bar. The depth of 21 feet thus obtained must be maintained by the annual extension of the jetties 700 feet into the Gulf, and the reduction of the mud-lumps by suitable machinery whenever they begin to appear.

But it appears to be desirable to go somewhat more into detail in this explanation. Accordingly, taking the Southwest Pass as a model, and taking the dimensions of the careful survey of 1838, we find that it has a mean width of 1,200 feet and a mean depth of about 60 feet. About seven miles before reaching the crest of the bar the channel begins to

widen and the depth to decrease, and they continue to do so until at the crest of the bar the width is 11,500 feet, and the mean depth, from having been 60 feet, is but 11.5 feet.

An addition of 338 feet is made to the bar every year along the whole line of the crest, 11,500 feet long. This is the annual extension into the Gulf. This addition or extension has the same mean depth of water on it as the crest, 11.5 feet. If we go back from the crest of the bar toward the point where the pass begins to widen, we shall find a depth of 21 feet in the channel-way, where it is about 6,000 feet wide.

The same bulk of earthy matter is, in a series of years, added to the bar annually, and if it be added to it on a line 6,000 feet long, instead of 11,500 feet long, the seaward length of the addition must be about twice as great, (the depth of water upon which this addition is made being substantially the same in each case;) that is, the bar, instead of being extended 338 feet into the Gulf annually, will be extended twice that distance, or about 700 feet.

If we refer to the channel where it is 25 feet deep, we find the width to be about 4,000 feet; and the mass of the annual addition to the bar being the same, the annual extension on a front of 4,000 feet, instead of being 338 feet, will be about 1,000 feet, and this will be about the annual extension of the bar for a depth of 25 feet if the jetties are suitably arranged for that depth. If they are at a greater distance apart, the depth will be less than 25 feet. If they are at a less distance apart, the depth will be greater, and, the addition to the bar being formed on a less front than 4,000 feet, will have a greater annual extension than the bar formed on that front. So that in applying jetties to permanently deepening the bar of the Southwest Pass to 25 feet, we must expect an annual extension of the bar of about 1,000 feet.

Examining the map of the bar, we find that the horizontal distance between the part of the channel (inside the crest) where the depth is 25 feet to the point in the channel (inside the crest) where the depth is 21 feet, is about 4,000 feet, and we have every reason to conclude, and not one reason for a contrary conclusion, that if the jetties are not extended after obtaining a depth of 25 feet, in four years' time the bar will have extended into the sea about 4,000 feet, and, following the law under which it has heretofore been formed, the depth on its crest will be 21 feet; that is, the bar-accretions will be made on a slope rising at the rate of 1 foot per every 1,000 feet of accretion.

The conclusion is inevitable: the jetties must be extended annually at the same rate that the bar is advancing, if we intend to maintain permanently the same depth upon the bar.

If the depth to be maintained is 27 feet at low water, or 28 feet at high water, it will be found by a similar process that the annual advance will not be less than 1,200 feet.

The jetties may be so arranged as to cause a greater depth than the one required, and thus obviate for a time the necessity of their aunual extension into the Gulf, but such an arrangement will entail a proportionately greater first cost in their construction. The final result as to cost and depth will be the same whether the jetties be converging or parallel, and the parallel has therefore been assumed as the model in this discussion.

Some engineers have adopted the opinion that the jetties, by increas ing the strength of the current largely, will carry the earthy matter forming the bar so far out and into such deep water that there will practically be no necessity for extending the jetties after the desired depth has been once obtained. This view is derived from the supposition

that the bar is formed by the check which the current of the river-water receives in entering the Gulf; which check, it is said, reduces its velocity so much that the earthy matter carried in suspension by the riverwater is dropped at once into the Gulf and forms the bar. This was the opinion usually held by engineers in former times, but was not based upon any measurement of the currents or careful observation upon them. It was known that the river-current was brought to rest in the sea, and it was assumed that, at the point where it apparently entered the sea, (that is, where its banks were salt water instead of earth,) a sudden and great reduction in the strength of the current took place, much greater than occurred at any other point of its prolongation into the sea. But those who have carefully examined the mouths of the Mississippi River, or who have examined the series of current observations made there under my direction, perceive that there is no material check to the river-current as it enters the Gulf, and that it requires exceedingly nice measurement to detect any change in this velocity over long distances. In fact, the current of the river is retarded at a very slow rate from the point where the pass begins to widen, seven miles inside the crest of the bar, until it is brought to rest, some twenty miles or more seaward of the crest, at high-water, and some ten miles or more at low-water, making the whole distance before it is neutralized twentyseven miles or more at high-water and seventeen or more in low-water. And along those distances of twenty-seven miles in high and seventeen in low water it drops the suspended earthy matter at a nearly uniform but slowly-decreasing rate.

These being incontrovertible facts, the questions next occur, where does the material come from that forms this great deposit which adds annually 338 feet to the bar of the Southwest Pass, with a depth upon it of 13 feet at low-water? and why is this material, wherever it may come from, deposited in juxtaposition to the old bar on the seaward side?

Two observed facts put together answer these questions clearly. The first, the ascertained fact, already mentioned, that throughout the whole course of the river there is a mass of earthy matter pushed along the bottom of the river, (not suspended in the water,) moving at a much slower rate than the current of the river. At the mouth of Red River, two hundred miles above New Orleans, this material was chiefly small gravel and coarse sand; not far below Red River, coarse sand and small balls of blue clay; still lower down, coarse sand; and in the vicinity of New Orleans, at all stages of the river, chiefly sand and earthy matter, the same kind of sediment as that found in suspension at that point, the sand being very fine. No coarse material passed this point of the river.

The second is the ascertained fact that, where the fresh-water current of the river meets the salt-water of the Gulf, the fresh water rises upon it, and creates a dead angle of salt-water on the seaward side of the bar; and when the earthy matter pushed along the bottom of the river arrives at this point, the fresh water having risen from it, there is no longer any pushing force to keep the earthy matter in motion. It remains in the still salt-water, forming an accretion to the bar. Its upper surface lies along the slope, on which the fresh water moves upward upon the saltwater, which repeated measurements upon the bar of the Southwest Pass prove to be (on that bar) a slope of one foot in a thousand. It can make no difference whether the river-current be moving at the rate of 4 feet, 3 feet, or 2 feet per second, when it reaches the point where it

rises on the salt-water, the matter pushed along the bottom will come to rest in the still salt-water substantially at the same point.

We have seen that no coarse material is carried or pushed by the river past New Orleans, the drifting material there being of the same character as the suspended matter. Fifteen miles below New Orleans a marked change takes place in the river; its course to the sea varies but little, and its cross-section becomes much more uniform than above, and, as a consequence, the suspended matter falls to the bottom in larger proportion than above New Orleans. The sedimentary matter thus dropped to and pushed along the bottom of the river during high-water to the point where the pass begins to widen, and thence to the outer crest of the bar, forms a part, but not the whole, of the annual accretion of the bar. That portion of the suspended sediment dropped in highwater on the seven square miles of the bar, and swept to its outer crest, forms another part of its annual accretion.

Respecting the character of the material composing this bar, George G. Meade, one of Captain Talcott's principal assistants, who had charge of that portion of the survey of 1838 comprising the Southwest and South Passes, says of the bar of the Southwest Pass:

*

The bar is composed of mud and sand, the matter held in suspension by the riverwater. Within and without the shoal the bottom is soft mud of a bluish and yellow tint, having a large proportion of alumine. Immediately on the shoal the bottom is harder, and has a greater proportion of saud.

Respecting the South Pass he states:

The bottom is generally sand, interspersed with spots of soft mud. The bottom on the bar is principally fine gray sand, mixed with a small proportion of mud. Without the shoal the soft yellow and blue mud of the passes is found. The character of the bar is sand, as it is of the passes and of the adjacent shoals.

Let us see what changes, if any, would take place in the amount of suspended earthy matter dropped between the point where the pass begins to widen and the crest of the bar, if jetties were constructed so as to give 28 feet water.

Half-way between the point where the pass begins to widen and the outer crest of the bar, we find (map of 1838 taken as the model) in the middle of the channel a depth of 28 feet at high-water for a width of 1,800 feet. Jetties properly constructed from this point to a similar depth outside the crest of the bar would give the required depth of channel-way.

It has already been pointed out that the greater part of the suspended earthy matter begins to fall regularly to the bottom as soon as the horizontal and vertical irregularities of the channel-way cease; and if the volume of discharge passes between straight jetties, of uniform distance apart, with a uniform cross-section throughout their length, we have the conditions favorable to the falling of the suspended matter to the bottom.

Now, all the earthy matter pushed along the bottom of the river above the point where the pass begins to widen, and all that dropped below that point for one-half the length of the bar, (where the jetties are supposed to begin,) will be pushed along the bottom between the jetties to the outer crest of the bar; and all the suspended earthy matter that drops to the bottom throughout the length of the jetties (one-half the length of the bar) will also be swept there. How much, it will be asked, would this last quantity (the suspended earthy matter dropped to the bottom throughout the length of the jetties) differ from the quantity dropped on the last or outer half of the bar if there were no jetties? The difference is indicated by the difference in their mean velo

cities, so far as the quantity of deposit is dependent on the mean velocities, and should be inversely as those velocities; that is, the quantity dropped on the same length would be between one fifth and one-sixth less between the jetties than on the lower half of the bar. Compared to the whole quantity dropped on the bar it would be about one-eighth less.

It has been recently stated by a civil engineer, in a pamphlet concerning the improvement of the mouths of the Mississippi River by jetties, that the amount of sedimentary matter carried in suspension by the Mississippi River is in exact proportion to the velocity of its current ; and that, as a given velocity of current will keep in suspension a corresponding quantity of solid matter at a less velocity, a certain portion of it will be dropped. To illustrate this he states that

When the Bonnet Carré crevasse occurred, the river below it (107 feet in depth) was shoaled up 31 feet, because the volume of water in the river, being lessened by the crevasse, was no longer sufficient to maintain the normal current in a channel large enough to carry the entire river, consequently the current below the crevasse slackened, and the excess of load was dropped in the channel until the bottom was filled up 31 feet with the deposit. This reduction of channel was sufficient to re-establish the current and prevent further deposit.

The first statement is in direct conflict with the results of the longcontinued measurements made upon the quantity of earthy matter held in suspension by the Mississippi River at Carollton, near New Orleans, and at Columbus, twenty miles below the mouth of the Ohio, one of the chief objects of which was to determine this very question, whether any relation existed between the velocity and quantity of earthy matter held in suspension. These results prove that the greatest velocity does not correspond to the greatest quantity of earthy matter held in suspension; on the contrary, at the time of the greatest velocity of the current at Carrollton the river held in suspension but little more sediment per cubic foot than when the velocity was least. When the quantity of earthy matter held in suspension was greatest the velocity was two feet per second less than the greatest velocity, the quantity of earthy matter in the one case being three times as great as in the other. We find at another time, when the velocity was one-half the greatest velocity, the quantity of earthy matter held in suspension was double in amount.

At Columbus we find similar conditions existing. At the time when the greatest quantity of earthy matter was held in suspension, the velocity was less than one-half the greatest velocity; and at the time of the greatest velocity the quantity of earthy matter in suspension was one-half the maximum quantity. Again, we find a time when the quantity of earthy matter in suspension was nearly the same as the maximum, the velocity being less than one-third of the greatest velocity. Again, we find the quantity of earthy matter in suspension the same, the velocity in the one case being 6.75 feet per second, and in the other 1.5 feet per second.

The following tables, illustrating what has just been said, have been prepared from the report on the Mississippi River. The figures given express the conditions existing not only on the one day noted, but on several successive days.

During the whole period of observation the river-bed remained unchanged. It will be noticed that even the maximum amount of sediment in the river-water is a very small quantity compared to the mass of water, it being by weight in the proportion of one ounce of fine earth to six hundred and eighty ounces of water; and by volume one cubic inch of earthy matter to one thousand three hundred and sixty cubic inches of water.