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in consequence of their having used piles of inadequate length in the formation of the lower portion of the line, the depth of water on this portion varying from eight to more than nine feet. In repairing the breach they have adopted piles of a much greater length and driven them much deeper into the bed of the Pass. By applying this remedy, and observing similar precautions on other portions of their work still to be done, they feel sanguine no such disaster will again occur. The contractors had now two pile-driving boats in operation, one worked by steam, employed in repairing the breach at the lower end of the line, and the other by manual power, at the upper end of the same line. The length of the line as repaired and extended on the 21st ultimo was about the same as heretofore reported, viz: sixteen hundred feet.
I desired of the contractors a designation of the point at which they proposed to cross the tidal bar and enter the open gulf with the new channel; and in reply was informed that they had fixed on no such. point, nor did they intend to do so until the changes and currents produced by their work should indicate the best position and direction of the channel in question.
TRIANGULATIONS AND SOUNDINGS.
I assumed as a prime base a line 1,000 feet long, carefully measured along the line of piles, the direction being N. 40° E. or S. 40° W. Signal poles bearing red jacks at their tops were erected at the extremities of this base. At each extremity the angles made by lines drawn to the light-house were carefully observed by the use of a box sextant, graduated, by means of the vernier attached, to minutes of a degree.
The distance from the lower end of the assumed base to the lighthouse was carefully computed, and assumed as a secondary or main base for all subsequent triangulations.
The triangulation was extended upwards about two and a half miles to the observatory at the pilot's station, on the easterly side of the Pass, and to the telegraph station on the westerly side, and downward to a high pole and signal on an extensive mud lump of some fifty or sixty acres, called Stake island; also to a large metallic buoy, called the can buoy, situated at the bifurcation of two distinct main channels leading across the tidal bar, through one or the other of which all vessels entering or leaving the Pass must be conveyed.
In addition to the points of triangulation above mentioned, four other points were established and marked by signal poles, surmounted by red flags or jacks, set in mud lumps, rising some eight to ten feet above the surface of the water, and presenting areas varying from about one-eighth to three-fourths of an acre, respectively.
Two of these signals were located at the distance of about two miles apart, on a line crossing all the channels of the Pass, nearly at right angles with the general direction of the currents of the Pass, and about one mile below the lower extremity of the primary base.
The other two signals were placed at the distance of about two and a half miles apart, on a line nearly parallel to that of the two signals
just before mentioned, and at the distance of about three-quarters of a mile below them.
The line last mentioned is nearly coincident in position and direction with those of the crest of the tidal bar, and crosses the main navigable channels leading across the bar.
The selection of the four points indicated as above was made in conformity to the presentation of suitable mud lumps rising above the surface of the water in positions most favorable for lines of soundings, leading across all the channels intended to be affected by the work of Messrs. Craig & Rightor in the Southwest Pass.
The position and extent of the several lines having been determined, soundings are to be made and repeated as often, at least, as once a week on all the lines leading across the channels, for the purpose of showing any changes that may take place in the positions, widths, and depths of the channels.
The manner provided for taking and applying the lines to be sounded provides for the casting of the lead at stated intervals of time, (viz: once in every minute or half minute,) and for noting the depth at every sounding in feet and parts.
With the data thus obtained, (the length of the lines having been previously determined,) a submarine section on each line sounded is to be prepared and delineated, the line sounded being regarded as the base, and the depths of soundings as ordinates, depending therefrom at equal distances asunder, corresponding to the number of soundings made along the line. A waved line, drawn through the lower extremities of the ordinates, will show the inequalities of the bed of the Pass, its channel, &c., at the time and place of each set of soundings. This operation, being repeated once a week on every line proper to be sounded, and corresponding delineations being prepared therefor on every occasion of sounding, will clearly exhibit the nature and extent of the changes that occur from week to week in the depths and positions of the channels.
The operations detailed as above, contemplate the preparation of a sectional diagram for each set of observations, the diagrams being numbered 1, 2, 3, 4, &c., in the order of the preparation, for each line of the triangulation upon which the sounding may have been made, each series of diagrams being referred to its appropriate line of soundings, as marked and designated on the plot of triangulations.
In accordance with the foregoing representations, instructions have been furnished to Wm. Johnson, esq., who has been employed to direct and supervise the operations above considered; a copy of which instructions is as follows:
"SOUTHWEST PASS, April 18, 1857.
"SIR: You are desired to remain in the supervision of the operations of Messrs. Craig and Rightor, and of the results produced thereby so far as they relate to the accomplishment of the objects specified in their contract for the removal of obstructions from this Pass.
"In the discharge of the duties claiming your attention you will observe the following details, viz:
"1st. You will make such observations as are necessary to test the
accuracy of the triangulations already made by us conjointly, carefully rectifying such errors as may be detected therein.
"2d. You will repeat the soundings already made on the several lines sounded by us, and continue the repetition as often at least as once every week, the weather permitting.
"3d. You will plot each line of soundings as above, and all other lines of soundings you may find occasion to make, in the manner explained to you, viz: in separate slips of paper, designated and numbered as directed. The plot of each line will be attended by ordinates, depths, and a waving line, indicating the variable depths of the river channel, after the manner exemplified in our late drawings.
"4th. In all your soundings you will ascertain, as nearly as practicable, the elevation of the surface water above ordinary low water at the time of sounding, and make due allowance therefor in the depth of the soundings.
"5th. You will establish one or more bench-marks on a level of ordinary low water, in the following manner, viz: Fix one or more tide-gauges at suitable points; assume a point about an inch below extreme low water and mark it with 0 or zero; divide the gauge-rod into inches and quarters and number the inches, from zero downward, 1, 2, 3, 4, &c.; observe and note the inches and parts on the rod at extreme ebb tide daily for one entire lunation; add the recorded observations together for one lunation and divide their sum by the number of observations, and the quotient will indicate the bench-mark level for the lunation.
"Repeat the observations during a series of lunations, and divide the sum of the series (for one year if practicable, or six months if otherwise,) by the number of the series, and the quotient will indicate the true level of the bench-mark, which may be fixed by driving a stake at the margin of the water at that level till the head of the stake coincides exactly with the surface of the water.
"6th. Keep a daily register of the observations made upon the tide gauge, from which the level of the bench-mark (or ordinary low water,) may be deduced with accuracy.
"7th. Keep a diary of all your proceedings relating to the triangulations, soundings, tidal observations, &c., &c., prepared with neatness and accuracy, to be submitted in the form of "field notes" at the expiration of your period of service.
8th. You are desired to report in general terms your proceedings in all respects as often as once in a month, on or about the end of every month, by letters, to my address at Louisville, Kentucky, unless otherwise directed.
"9th. Whenever Messrs. Craig & Rightor shall give notice that their work in the Southwest Pass is ready for final inspection (or fix the date at which it will be ready for final inspection,) you are desired to apprise me thereof by telegraphic despatch if the result of your previous observations and surveys should render it certain or highly probable that the work is actually ready, or will be ready for final inspection according to contract.
10th. It is understood that the work will be ready for final inspection when a straight channel, 300 feet wide and 20 feet deep, clear of
all obstructions, shall have been formed entirely across the tidal bar, from the secondary base surveyed by us to the open gulf.
"11th. In general you are desired to ascertain and report all perceptible changes in the bed of the Pass below the secondary base above mentioned, and especially in the positions, widths, and depths of the channels below that line.
"12th. You are authorized to purchase such articles of stationery, and to hire such boats, hands, tools, &c., as are necessary to the performance of the duties assigned you, with the understanding that vouchers duly executed in duplicates must be taken and rendered for all such expenditures.
"13th. Preparations having been made for the adjustment of all accounts likely to be incurred during the current quarter, you are desired to forward to the undersigned monthly estimates for each month of the succeeding quarter, in order that proper remittances may be forwarded for defraying the expenses of each month of the quarter last mentioned, (third quarter of 1857.)
"14th. Your attention is particularly requested to the observance of each and every item of the foregoing instructions, by means of which I shall be kept adequately apprised of the progress of the work committed to your supervision.
"In conclusion, I would observe that your experience as an engineer, and your familiarity with nautical affairs, inspire the belief that you will be able to discharge the duties confided to your charge in a manner creditable to yourself and beneficial to the public service.
"Respectfully, sir, your obedient servant,
"S. H. LONG, "Lieutenant Colonel T. E.
"Captain Wм. JOHNSON, Southwest Pass."
Having accomplished my inspections, triangulations, and other arrangements, as above, I communicated the following notice to Messrs. Craig & Rightor, the contractors for the improvement of the Southwest Pass:
"SOUTHWEST PASS, April 20, 1857. "GENTLEMEN: The bearer, Captain Wm. Johnson, having been employed to inspect your work for the improvement of the Southwest Pass, and to report its progress, and the effects produced thereby, in so far as relates to the removal of obstructions from the main navigable channel and to the formation of a new channel, 300 feet wide and 20 feet deep, as stipulated by contract, you are desired to receive him in the capacity above intimated during my absence, and to regard him as my agent and representative in all matters relating to the fulfilment of your contract till I return again to the Pass.
"Whenever the work shall have been so far advanced as to enable you with certainty, and to the satisfaction of Captain Johnson, to fix a date at which it will be ready for final inspection and acceptance, agreeably to the contract, you are desired to apprise him of such date, that I may be seasonably notified thereof by him and return again to
this place for a personal inspection and acceptance of the work, if executed in conformity to the terms of the contract. Respectfully, gentlemen, your obedient servant,
"Messrs. CRAIG & RIGHTOR,
"Contractors, Southwest Pass."
"S. H. LONG, "Lt. Col. Corps Top. Engs.
In conclusion, I take leave to present a further exposition of my views in relation to the formation and character of the tidal bar, and, for this purpose, shall adopt the following hypothesis, based on the present aspect of things and upon occurrences known to have taken place in former times.
The tidal bar is known to be composed of sedimentary matter brought down and deposited by the waters of the Mississippi, and consisting of earthy and vegetable materials, exceedingly comminuted, unctuous, and adhesive.
The points at which the most copious depositions have been made are those at which the currents of the river are merged into the still waters of the gulf. The deposits brought down in each successive year are made in advance of those of the preceding year, thus contributing to the annual advancement of the tidal bar into the gulf.
The deposits, when first made, as before intimated, are fine, unctuous, and impalpable, and constitute a semi-liquid mass of earthy, mineral, and vegetable particles, which, when brought into contact and combination with the salt water of the gulf, are subjected to chemical and mechanical changes, the former of which have never been adequately investigated or explained.
With respect to the mechanical changes, or those produced by the combined agency of the river currents and gulf billows, they may be accounted for in part, if not altogether, in the following manner:
The annual amount of deposit is distributed along the outer margin of the tidal bar in accumulations more or less abundant, according to the quanties of fresh muddy water conveyed across the crest line of the bar at or near the points of deposition.
The deposits having been made, the gulf billows begin to exercise a mechanical action upon them, which results in the formation of a crust or indurated covering upon their surface, which continues to increase in thickness and solidity till, at length, the covering of the mass becomes firm and unyielding, and, at the same time, impervious to the gasses that are formed by the decomposition of the vegetable matter contained in the mass of deposits confined beneath the crust or covering.
In this way annual deposits, accumulations, &c., are constantly occurring, in proof of which, borings through the alluvial formations of the Mississippi delta, from its surface to great depths below, have developed a succession of stratifications of mud and indurated clay of different degrees of hardness, alternating with each other to the full extent hitherto penetrated.
The decomposition of the vegetable matter probably occurs annually, or, perhaps, less frequently after the subsidence of the spring