REPORT ON THE COAST SURVEY

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The zenith sector of the Coast Survey is constructed on the plan designed by Mr. Airy, the Astronomer Royal of England, and is similar to the one recently used in the British Ordnance Survey, but possessing some improvements in the details suggested by the use of the latter. It is an admirable instrument, perfect in design, and in point of accuracy compares well with a zenith telescope of equal power: but its ponderousness and the greater labor attendant on its use make it less eligible for observation in the field. Its capabilities, however, are much greater than those of the zenith telescope, inasmuch as it measures absolute zenith distances. It has been used with great success at six of the principal stations of the survey, and will be found described and figured in one of the forthcoming volumes of the Coast Survey Records and Results.

IRREGULARITIES OF THE EARTH’S FIGURE.— When the latitudes observed at different stations are referred to one another, by means of geodetic differences of latitude, there are discovered certain discrepancies, much larger than could result from the probable residual errors of observation– discrepancies, therefore, which can only be ascribed to local irregularities in the figure and density of the earth, and which have been called station errors. They are similar to those which are caused by the proximity of mountains; but they occur where the external configuration of the earth’s crust would not indicate the probability of their existence. Prof. Bache recognized this source of error as early as the year 1844; and as a consequence, he determined to carry a continuous series of astronomical stations through the primary triangulation, in order to arrive as nearly as possible at the true figure of the portion of the earth’s surface on which he was engaged.

The elements of the figure of the earth used in these comparisons are those obtained by Bessel in his last discussion of the results of ten measures of arcs. It is an interesting fact, which it is here in place to mention, that the observed difference of latitude of the extremities of the arc at present comprehended under the connected triangulation of the American Coast Survey, agrees very closely with the geodetic difference as computed from Bessel’s elements. This arc extends from Maine to Maryland, over about five degrees of latitude and six and a half of longitude; in intermediate portions of its length, the deviation from the mean ellipsoid reaches three seconds of arc and over. The laws which govern these irregularities are still the subject of investigation. In the eastern portion of the survey, where the earth’s crust has been more disturbed, the average amount of station error is between one and two seconds of arc; but it amounts to only about a half a second in the sections south of the Delaware, where the diluvium rests apparently undisturbed.

OBSERVATIONS OF AZIMUTH. — Observations of azimuth are made at all the latitude stations. The instruments used are the same as those employed in the measurement of the angles in the primary triangulation. The meridian is determined by observations made on the pole star or other close circumpolar stars, principally when the object is near its greatest elongations. The motion of the star in azimuth, at these points of its revolution, being almost imperceptible, the result will be practically independent of the time. Moreover, if observations be taken within forty-five minutes on either side of the elongation, they can be reduced to that position by a most simple formula. The pole-star, however, even at its culminations, moves so slowly in azimuth, that the liability to error in pointing is found to be no greater at those periods than at the elongations; and Mr. Bache has finally pursued sometimes the plan of observing the star at equal intervals on both sides of the meridian before and after the culmination. When this method is employed, the reduction is simplified into a mere taking of means: and it may here be noticed, that in these as in all analogous processes, it has been the aim of the Superintendent so to arrange the methods of observing, as to diminish as far as practicable the labor and liability to error of the computations. The azimuths are liable to irregularities similar to those which have been described as affecting the latitudes: and a comparison of azimuths observe at different stations, extreme and intermediate, has manifested the existence of station errors among them, which can only, like the former, be referred to local variations of level.

OBSERVATIONS OF TIME AND LONGITUDE.— Observations of local time are made in connection with whose of latitude and azimuth: the instruments employed being generally portable transmit instruments of from twenty-six to forty-eight inches focal length, constructed by Simms of London and Wurdemann of Washington. For time-keepers, chronometers by the best makers are used, except for telegraphic determinations of longitude, when astronomical clocks are employed.

In the plan of reorganization of the survey, in 1843, it was “urged, and deemed essentially necessary, that the difference of longitude between some main points of the survey, and the meridian of any or all of the European observations, be ascertained immediately.” In order to carry into effect this important injunction, Prof. Bache has availed himself of every means science could afford. All the observations obtainable of eclipses, occultations and moon culminations made in this country previously to 1844, together with the simultaneous observations made abroad, so far as they could be found, have been collected and reduced: and similar observations have since been continued at Cambridge, Philadelphia, Washington, Charleston and Cincinnati. All these stations being connected by the electric telegraph, their differences of longitude have been determined with great accuracy; and all the results are referred to Cambridge as a common station of reference.

The difference of longitude between Cambridge and Liverpool has also been determined by means of large numbers of chronometers carried repeatedly between the two stations on the Cunard steamships. These chronometric expeditions, in the words of Mr. W.C. Bond, director of the Harvard University Observatory, “for the magnitude and completeness of their equipments, have not been equalled by any of the similar undertakings of European governments. Even the ‘Expedition chronometrique’ of Struve was on a scale much less extensive.” The voyages were continued through a number of successive years. The first great special expedition took place in 1849, and the most recent in 1855. In the latter the effect of temperature on the rate of the chronometers formed a subject of special investigation. For each instrument the effect of temperature on its rate was ascertained by experiment, and the average temperature during each trip was kept account of by means of a thermometric chronometer, constructed like the others, but with undivided balance, so that its daily rate was affected by six seconds for a change in temperature of 1° Fahr. Fifty-two chronometers were employed in this expedition, and were transported six times between Cambridge and Liverpool; giving nearly 300 individual longitude determinations, with a probable error of the mean result of two-tenths of a second of time. Yet this result differs by more than a second of time from that of the expedition of the preceding years, giving 4h44m 31s.8 for the longitude between Greenwich and Cambridge, while the former expeditions had given 302.6. the results by the astronomical methods also differ, eclipses and occultations giving 29s.5, and moon-culminations 28s.5. The results by the latter method are doubtless liable to large constant errors. Indeed, each method appears to be affected by sources of constant error which no accumulation of results will culminate, and a greater approach to the truth must be sought by multiplying methods as far as practicable.

It is to be hoped that a submarine telegraph will ere long afford another, and perhaps very accurate method of verifying those results, and determining the longitude within the narrowest limits of error.

The differences of longitude between Cambridge and the principal stations of the survey in other sections are determined by the aid of the electric telegraph, wherever this has been established. In this method, which is by far the most accurate for determining difference of longitude, the Coast Survey has taken the lead, and has brought it to a great state of perfection, which subsequent operations of a similar nature executed in Europe have not yet reached. The idea of comparing the local time of different places by means of the electric telegraph is sufficiently obvious, and dates from the conception of the telegraph itself; but the refined methods by which the intervention of human senses and operations, and the consequent liabilities to error, are, in the greatest possible degree, avoided, and by which the time of transmission is measured and eliminated from the longitude, have been the result of careful study and long experience. The method of recording observations of time on a chronographic register, by means of a galvanic circuit, known in Europe as the American method, originated in the Coast Survey with the first attempts to determine longitude by means of the electro-magnetic telegraph. The chronographic record is made on a cylinder, revolving with nearly uniform velocity, covered with a sheet of paper, upon which a pen traces a line, interrupted or deflected for an instant, through the agency of an electro-magnet, every time the pendulum of the clock passes the vertical, and, in doing so, interrupts a galvanic circuit. Either cylinder or pen are at the same time slowly moving lengthwise; so that the line formed is a long spiral, which is thus graduated into spaces corresponding to seconds of time, and described with uniform velocity. When any instant of time is to be recorded, the observer strikes a finger key, which also breaks the galvanic circuit, and causes a similar mark to be made on the record; the position of which, in reference to the adjacent seconds marks, can be read off with great precision. In the chronographs employed in the Coast Survey, a second is generally represented by from one-half to three-quarters of an inch, the cylinder being regulated so as to make one revolution in half a minute. By an ingenious arrangement in the clock, the break for every sixty seconds or minute is omitted, and every five minutes two breaks are omitted. By this means, a whole sheet may be read off without any other note than the time of beginning and ending.

The method of determining longitudes by means of the electric telegraph is substantially, and in brief, as follows:– A transit instrument, astronomical clock, and chronograph, is mounted at each station. After suitable observations for instrumental corrections at each station, which are recorded only at the place of observation, the clock at the eastern station is first put in connection with the circuit, so as to write on the chronographs at both stations. A number of stars, culminating near the zenith of the two stations, are selected by the observers. As they appear first upon the eastern meridian, their transit is recorded by the observer striking the finger key upon the chronographic registers at both stations. After an interval of time equivalent to the difference of longitude between the two places, which is measured by the clock, the same stars appear on the western meridian, and the observer at that station records this transit precisely as the other had done; and the difference of the two records of time is the measure of the difference of longitude.

It will be observed that these records have been obtained at both stations; and a little reflection will show that if there be any sensible interval of time consumed in the transmission of the signals, the difference of longitude obtained from the record at the eastern station will be too great by that interval, and that at the western station will be too small by the same amount. The mean result will give the longitude free from this error, and the difference measure the time of transmission of the signals through the whole circuit.

Ten stars are generally exchanged with the eastern clock in the circuit, and, after the first five, the transit instrument is reversed, so as to eliminate any residual error in the correction for collimation. The western clock is next put on, and the same operation repeated with ten other zenith stars. Not only is the result improved by the accumulation of individual results, but the advantage is gained, that the interval is measured by another clock, and the time of transmission eliminated in the inverse order of effects. The transits of the stars are generally recorded over fifteen wires. After the exchange of signals by the second clock is completed, local observations for instrumental corrections are again made, which conclude the night’s work. These operations are repeated on at least three different nights; after which the observers and instruments exchange places, so as to eliminate the possible errors arising from causes connected with their individual peculiarities.

By the perfect and admirable method just sketched, we are able to measure arcs of longitude with the same degree of accuracy with which arcs of latitude have heretofore been ascertained; and a new element has thus been introduced into Geodesy. Since the general introduction of the electric telegraph and the development of the American method of longitudes, it has been applied to many of the older European geodetic surveys; and in general a very full acknowledgment has been made of their indebtedness to American science by the eminent geometer having charge of such works, with the single exception of Mr. Le Verrier, who has made use of the earlier methods of the Coast Survey, since abandoned for others more perfect, without any mention of their origin, in his report to the Academy of Sciences. It was one of the earliest discoveries resulting form the telegraphic determinations of longitude, that the time of transmission of signals between stations several hundred miles apart is quite sensible, and that it appears in a great degree to depend on the distance. On the telegraph lines used in this country, where large iron wire is employed, and the circuit is completed through the earth, the rate of transmission has been found to be from 11,000 to 20,000 miles per second. Whether this actually was the velocity of the galvanic current, or whether it rather measures the time during which the current must be established in order to produce the effects that we observe, is, for the present, an unsolved question.

The telegraphic determinations of longitude have been extended from Washington northward to Philadelphia, New York, Cambridge, Bangor, and Halifax, and southward to Petersburg, Wilmington, Charleston, Savannah, Mobile, and New Orleans.

Great credit is due to the public spirit of the telegraph companies, who have extended every facility to the operations of the Coast Survey, and have given the use of the line, after the working hours, free of charge.

In sections of the coast to which the telegraph has not yet penetrated, such as Florida, Texas, and the Pacific Coast, the longitudes of cardinal points are determined by observations of moon culminations, and by chronometers. Corresponding observations of moon culminations are made at several American observatories, at the expense of the Coast Survey; and most valuable aid is derived from the series of meridian observations of the moon made at Greenwich, which are always most promptly obtained through the kindness of the Astronomer Royal. The manner in which the reductions are made to keep pace with the observations at the latter observatory is admirable, and worthy of general limitation.

The chronometric determination of longitude between Savannah, and Fernandina, in Florida, the details of which have been communicated to the American Association at the Montreal meeting, may serve in plan of execution and mode of discussion as a model for operations of a similar character.

OBSERVATIONS ON THE MAGNETIC ELEMENTS.— Observations of magnetic declination, dip, and intensity are made at all points where it is desirable for purposes of navigation that the declination should be known; and also at many stations of the primary triangulation, generally those where astronomical observations are likewise made.

The instruments employed are similar to those used in the British magnetic surveys; being declinometers and horizontal force magnetometers on the plans of Gauss and Weber, Lloyd and Lamont, by Jones, of London; and dip circles by Barrow and Gambey, provided with Lloyd needles for total intensity. In this manner are obtained not only the variations of the compass required for the charts, but important contributions also to our knowledge of the earth’s magnetism, which have already born fruit in the construction of a magnetic chart for the United States. This subject, one of great scientific importance, will be found referred to more at length in the second part of this report.

DETERMINATIONS OF LEVEL.— The heights of the trigonometrical stations above the level of the sea are determined, generally, by observations of reciprocal zenith distances, which observations are frequently checked by direct levellings from the ocean. Interesting comparisons are in progress between the results obtained, and those which are furnished by barometric and thermometric measurements.

PLANE-TABLE TOPOGRAPHY.– The topographical survey is executed with the plane-table. The outlines of the shore, the irregularities of the surface, the forms and dimensions of hills, forests, streams, rocks, meadows, towns, and villages, are all represented by certain conventional modes of drawing. The topographical maps are generally surveyed on a scale of 1/10000 of the natural dimensions. In localities where a great amount of detail is to be represented, such as large cities and their vicinity, a scale of 1/5000 is employed, while on flat and thinly settled ranges of the coast a scale of 1/20000 is used. The extent of ground represented upon a single topographical sheet depends upon the scale; on a scale of 1/1000, or about 6 inches to the mile, a quare foot of the drawing represents about 4 square miles of the surface of the earth. The size of the sheets is generally 30 inches by 48.

On the sheets drawn in the field, the irregularities of the surface,– the hills and depressions,– are indicated by curves, or contour-lines, representing the intersections, with the natural surface, of a system of horizontal planes, one above another, at equal distances apart, measured vertically. In the reduced drawings for the engraved charts, the spaces between these horizontal lines are filled up with hachures, the darkness of the shade being made proportional to the steepness of the slope, according to a system somewhat modified from that of Lehmann. Scales of shades have been printed, showing the distance between the hachures and the strength of stroke adapted to different scales of the maps. The absence, in general, of very abrupt slopes, and the frequency of gentle ones, rendered necessary the modification of the Lehmann system. The topographical survey is carried as far inland as is required for purposes of navigation, and for the defence of the coast.

THE HYDROGRAPHIC SURVEY.– Next in order, and based upon the positions furnished by the triangulation and topographical survey, comes the hydrography. This department of the survey supplies the results which are of most universal interest to the public, and of which the practical utility is the most easy to be understood. By a thorough system of hydrographic survey, the configuration of the bottom of the ocean is mapped out, new channels are discovered, hidden dangers are detected, the situations of rocks and shoals which were but imperfectly known are determined with accuracy, the peculiarities of the tides and of the direction and velocity of tidal currents are made apparent, and the most suitable positions are determined for the planting of buoys and the erection of beacons and lighthouses.

Soundings are taken with such frequency as to exhibit accurately the variations of depth of water; the number of casts of the lead depending on the degree of irregularity of the bottom, or its greater or less declivity. The casts are determined in position relatively to the trigonometrical stations, generally by means of angles measured with the sextant at the place of sounding, and sometimes by the use of two theodolites on shore. In soundings off the coast, and in cases where extreme accuracy of position is necessary, as in the determination of sunken rocks, the latter is the method preferred. The position of the sounding vessel is not, in practice, determined at every sounding, but at every sixth or tenth, according to frequency; and the intermediate soundings, being taken at equal intervals of time, are laid down at equal intervals between the positions determined by angles.

The soundings extend from the shore, and from the head of tide-water generally, as far seaward as the exigencies of navigation require. One of their principal uses being to assist the mariner in determining his position, it is desirable that the record should exhibit not merely the depth, but the character also of the bottom. The sounding leads are, accordingly, provided with an apparatus designed to bring up along with it a portion of the sedimentary material which it encounters; and specimens of the varieties of this material from different localities are preserved among the collections of the Coast Survey, and the respective characteristics of the bottom are indicated on the charts.

Microscopic examinations of many of these specimens have shown them to be crowded with organisms, of which the number and the genera vary with the depth; and hence it is not impossible that science may discover, in the laws governing the distribution of microscopic forms in the bottom of the ocean, an additional means of assisting the mariner to determine his position at sea.

Various improvements in the apparatus for bringing up these specimens of the bottom have been introduced in the course of the Survey, invented by naval officers while prosecuting these researches: among them may be mentioned Stellwagen’s attachment to the lead,– a metallic cup, with a circular leather valve, closed by the pressure of the water in being drawn up,– a contrivance in common use, and answering very well the purpose intended, in moderate depths. For greater depths, Craven’s specimen-box, with a metallic hinged valve or lid, and Sand’s specimen-box, with a sliding valve closed by the action of a spiral spring, are used with success. For soundings exceeding 100 fathoms in depth, Massey’s self-registering sounding machine is used in connection with the lead and line, with several improvements suggested by its use.

The hydrographic sheets are drawn on scales varying from 1/5000 to 1/40000, the more usual scales being those of 1/10000 and 1/20000.
OBSERVATIONS ON THE TIDES.— In order to reduce all the soundings to the depth at mean low-water, the level of which is assumed for the plane of reference, observations of the tides of sufficiently long duration for this purpose are made by the hydrographic parties as part of the programme of their work. These are in connection with the system of more long-continued observations made at selected points of the coast, where tidal registers have been kept up with great attention and perseverance, for the purpose of ascertaining, upon the basis of extensive observation, the complicated laws governing the tides in the different seas which wash our shores. The observations have been made half-hourly, at a number of stations, for various periods of from one to three years. At intermediate stations, half-hourly observations during two lunations are found to be sufficient to determine the constants.

A self-registering tide-gauge is much used, by which a continuous curve representing the successive changes in the height of water is traced on paper moved by clock-work, by a pencil actuated by the rising and falling of a float in a vertical box, to which the tide has free access. The superintendent engages personally in the discussion of this most interesting subject; and he has made several communications to the Association to whom this committee owe their appointment, on the nature of these researches, and the results attained, which are to be found among the published transactions.

This subject is the more important from the fact that the tides of the Atlantic, the Pacific, and the Mexican Gulf coasts of the United States present three distinct types. The diurnal tide, which is moderately large on the Atlantic coast, and produces a diurnal inequality in height and time which is perceptible without being excessive, is very great on the Pacific; and in the Gulf of Mexico its predominance is so decided that the semi-diurnal tide is almost obliterated.

In addition to these investigations, which relate to the general phenomena of the tides, careful observations are made upon the direction and velocity of the currents which the tides produce. The information thus obtained is of no less importance to the mariner, than a knowledge of the amount of rise and fall of the water. It is accordingly embodied in the charts, along with those other matters of varied information which have been enumerated as essential to the safety of navigation; and thus constitutes the final contribution to that fund of positive knowledge of the sea and its bed, of its changes and the law that govern them, on which the commerce of the country so largely depends for its safety.

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