REPORT ON THE COAST SURVEY

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To such as have never attentively reflected upon this important subject, it may not readily occur in how great a variety of modes the knowledge gathered in a properly executed survey of the coast is instrumental in preserving the seaman from the disasters to which he is liable. There is no difficulty, indeed, in understanding how, if the practicable channels are traced out and marked by buoys, and if the natural landmarks visible at a distance are accurately mapped down, with their bearings as observed from point to point, and if artificial beacons are erected on the headlands, or on the reefs or shoals, a navigator with the chart before him may be able, in clear weather and by day, to pick his way in safety by the eye alone. There is no difficulty in understanding how he may, even on clear nights, be sufficiently secure in depending upon the guidance of the beacon-lights of which the survey has fixed the most advantageous localities. But it is not in the broad light of day, nor yet when the heavens are serene by night, and the landmarks and beacons are easily discernible, that ocean disasters occur most frequently, or take the navigator most usually by surprise. The moments by far the most perilous are those in which the solitary voyager is, as it were, walled out from the world by an impenetrable barrier of mist, limiting the range of his vision, in every direction, to a distance often not greater than his own vessel’s length.

In such a situation of perplexity, helpless as it may appear, the charts of a perfect coast survey furnish a gauranty of safety, which, though it exacts of the mariner a more untiring vigilance, is yet no less to be relied on than the guidance of visible beacons. The cast of the lead informs of the water beneath him, and of the nature of the materials which form the bottom encountered by the plummet. A few successive casts indicate to him the degree of inclination and the direction of the slope. With these elements to guide his judgment, he finds his place upon the chart without difficulty, and traces the line of his progress step by step. And it is the description of information here indicated which is most valuable of all to sailors, since this is the only species which is available when they are in their greatest peril. Yet it is obvious, without argument, that it is a kind of information which can only be perfectly gathered by patient, persevering and long-continued labor; and which, in consequence of the changes continually going forward in the bed of the sea, can only be preserved in its accuracy and its value, by frequent and careful revision.

Another mode in which a scrutinizing examination of the coast contributes to the safety of navigation is, by the discovery of sheltered bays or inlets to which vessels may resort in stress of weather, and where they may lie in safety till the winds and waves recover their tranquillity. Such harbors of refuge, as they have been not unaptly called, have been found to exist in many parts of our extended coast, having been quite unknown even to the sailors who frequent most the neighboring seas, until the searching operations of the Coast Survey had brought them to light. Commerce having failed, for whatever causes, to appropriate these harbors to its uses, whether on account of their inconvenient situation in reference to the interior of the country, or on account of the wildness or sterility of the surrounding territory, or simply for the reason that their capabilities had been overlooked; all the benefits which have flowed from their discovery, all the lives which they have rescued from destruction, and all the wealth which they have snatched from the jaws of the deep, are undeniably to be passed, wholly and entirely, to the credit of this great public enterprise. And were it possible to strike a balance between the amount of outlay, on the one hand, by which results of such value and importance have been accomplished, and the amounts of material wealth, on the other, which they have redeemed from the waves, it would unquestionably appear that the sum-total of all the appropriations which have been made from the public treasury for the prosecution of the work from the beginning, have been amply more than overpaid by the benefits which have crowned some of these, its subordinate and least conspicuous achievements.

But though the leading object contemplated in a survey of the coast is to provide for the security of navigation, and thus to subserve the interests of commerce and of humanity, still there are other important ends incidentally but at the same time necessarily promoted in the conduct of its operations, which are too important to be disregarded, and which must be taken into the account, if we would comprehend the full measure of the benefits which such a work is capable of conferring upon mankind. These will be more particularly exhibited in the detailed account which will presently be given, of the results which have been already brought out, in the prosecution of our own Coast Survey. It is sufficient here to state in brief, that they embrace the accurate determination of favorable bases for the institution of trigonometrical inland surveys, which may lead to the construction of scientifically exact maps of the territory of the several States; of observations on the phenomena of terrestrial magnetism, and the laws which govern their variations; of investigations of the difficult problems connected with the irregularities of the tides and of the winds; of the exploration of the Gulf Stream and other ocean currents; of the study of the distribution of submarine deposits by these currents, and the formation of such deposits near the points of discharge of the numerous rivers which intersect the coast; of the examination and settlement of questions relating to suspected or probable changes of the relative level of land and sea, and other points of the highest interest to geological science; of the determination of the sites most advantageous for the construction of national works of defence, and many other matters of economical or scientific interest, less general in their character.

METHODS OF PRACTICE EMPLOYED IN THE CONDUCT OF A GEODETIC AND HYDROGRAPHIC SURVEY.

Such being the diversified and valuable ends, for the attainment of which so numerous and elaborate and extensive hydrographic surveys have been set on foot in modern times, along the coast lines of our own and of foreign countries, it is consistent with the design of this report to sketch in outline the methods of practice which have been employed in the actual execution of works so magnificent, so truly national in character, so incalculably useful, and so honorable to the governments which have undertaken them, and of whose wisdom they are among the most splendid and enduring moments. The information here embodied would be unnecessary and out of place, were it to be addressed to geometers or physicists only: but if the committee rightly interpret the purpose of the Association in their appointment, it is a part of their duty to prepare such an account of the great public work referred to them for review, as may be proper to lay before the reading public at large.

GEODESY.— With the simple processes involved in ordinary field surveys, every one may be presumed to be sufficiently familiar to enable him on a slight consideration to perceive their insufficiency to determine correctly any considerable portion of the earth’s surface, or of any long line of inland or sea-coast boundary. And whoever has reached this point will find no great difficulty in understanding the general principles on which the peculiar methods adapted to such extensive works are founded. In an ordinary survey, the surface of the earth is regarded as a plane, and every field is considered to be apart of this plane, bounded by horizontal lines. Now as the earth is not plane upon its surface, but spherical, it is obvious that there is error even in the smallest survey executed according to the ordinary rules. And it is no less obvious that if two fields, side by side, be independently surveyed, the two planes supposed to represent them will not be mutually extensions of each other; but will be slightly inclined to each other, the angle of their inclination being equal to the amount of the earth’s curvature between their middle points. In the case we have just supposed, indeed, this angle would be so very slight as to be altogether imperceptible: but should we compare two fields separated from each other by the distance of a hundred miles, we should find their inclination to be so great, that the plane of one of them extended in the direction of the other, instead of coïnciding with it, as it would do if the earth’s general surface were itself a plane, would pass above it at the height of a mile and a quarter.

Again, in defining te boundaries of fields, it is necessary to refer them to certain fixed lines of direction, certain cardinal points of the horizon, or, as they are commonly called, points of the compass. Now if the earth were a plane, all the lines which we call north and south would be parallel to each other; and so far as the limited space comprehended within a field or a farm is concerned, such lines are actually sufficiently near to parallelism for every practical purpose. But since the earth is spherical, all such lines, wherever drawn, must intersect each other at last if extend to the poles; and they must consequently be more or less convergent in every other latitude. The extensive surveys of the public lands, therefore, executed by the government surveyors according to the ordinary methods, exhibit but a loose approximation to correctness; and they have only been preserved from intolerable inaccuracy, by dividing the territory to be surveyed into zones limited by parallels of latitude at moderate distances from each other; and resting each successive zone upon a new base. To illustrate, by specification, the magnitude of the error involved in this mode of surveying, when pursued over long lines or extended surfaces, it may be observed that such a survey carried from the shore of the Gulf of Mexico to the northern boundary of the state of Mississippi, would be erroneous to the extent of four miles in every hundred.

But in a survey designed to represent the features of a coast with such exactness as to assure safety to the navigator who depends on the information it affords him, there must be no question of miles or even of fractions of miles of error; the rigorous demand is, that there shall be no sensible error at all. And therefore it is that the methods adopted must recognize, in the first instance, the fact that the earth is a sphere. And all the lines and surfaces involved in the various geometrical and trigonometrical operations, must be reduced to conform to this fundamental requisition.

But even this does not represent the entire complication of the task. The results which would be obtained by the strict application of the processes of spherical geometry and trigonometry to the glove of the earth considered as perfect in figure, though immensely nearer to the truth than any that could be reached by plane trigonometry alone, would still fail to exhibit the points of the earth’s surface as nature has presented them. For the earth after al is not a perfect sphere; but is larger in equatorial than in polar diameter by about twenty-six miles. It is this deviation from the truly globular shape which makes of the earth what is called an oblate spheroid; and this must still be allowed for, before a survey like that which we are considering can possess the accuracy that either science requires, or the safety of navigation demands.

To a survey of this kind is applied the term, geodetic. Having indicated the nature of the geometrical principles upon which its determinations rest, it may be proper to give a brief account of the instrumental methods which are employed in the field, in gathering the data for these determinations. The first step, which is to serve as the basis of all subsequent operations is, to measure with extreme accuracy, in a convenient and level spot, a base line of several miles in length. And when we speak of accuracy, it is to be observed that the word as here use is to be received in a much more several rigorous sense than is the case in its ordinary applications. By the methods in practice in the American Coast Survey, a line ten miles long may be measured with no greater error than a fraction of an inch.

After this first very delicate but mainly mechanical process has been completed, the next thing to be done is to fix upon some prominently conspicuous and conveniently distant point, visible from both extremities of the base, and forming with them, by imaginary aerial lines, a triangle of large dimensions. All the angles of this triangle are then very carefully measured with instruments of the nicest accuracy; and from the values thus obtained and the known length of the measured base, the length of the unknown aerial sides is determine by trigonometrical computation.

Each of these unknown sides having become now known, may in its turn be made the base of a new aerial triangle; its extremities furnishing two of the angular points, and some new station still more distant, and falling conveniently in with the plan of the survey, supplying the third. Thus, by the continued repetition of the process here described, a chain of triangles is constructed step by step, which follows generally the windings of the coast, but extends also as far inland as may be necessary to obtain for the triangles the dimensions most favorable to accuracy. For it need hardly be remarked that, if there be minute errors committed in the instrumental parts of the work, as is doubtless unavoidable even when instruments possess the extremest delicacy which art in its highest refinements has been able to secure, these, though singly imperceptible, may grow by repetition and accumulation, until error actually sensible begins to vitiate the results. On this account, therefore, it is desirable that the instrumental measurements should be as few as possible: and accordingly, in this principal chain of distances, the triangulation points are chosen as far from each other as the nature of the country will admit. It is a consequent necessity that the network of triangulation, in order that it may fulfil every important condition, should occupy a band of considerable breadth along the coast. Indeed, some of the lines which form the sides of these aerial triangles, are not less than seventy miles in length: yet it is entirely demonstrable that the accumulated total of all the error with which they are vitiated, does not exceed a single foot.

As a check upon errors of this description, and as a means of testing the precision with which the instrumental and numerical processes have been conducted along an extended line of triangulation, new bases, called bases of verification, are occasionally measured, often some hundreds of miles distant from the original base; and the length as trigonometrically computed through the whole chain, is compared with that which is found by actual measurement. So close has, in many instances, been the agreement between results obtained by methods so widely different, as almost to exceed belief. A base of verification measured in France, in connection with the triangulation carried on through that kingdom for the determination of the length of a degree of the meridian, differed less than a foot from the value found for the same base, by computation through a chain of triangles, four hundred miles long. A similar base upwards of seven miles in length, measured on Salisbury Plain, in England, showed less than a single inch of disagreement with the result deduced by calculation from an original base at the distance of one hundred miles. The American Survey furnishes an example no less remarkable. A base five and a half miles long on an island in Chesapeake bay, as actually measured, and as determined through a triangulation of more than three hundred miles, presents results discordant to the extent of only four inches.

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