ing a base with the duplex or any good bar apparatus is nearly three times as great as that required for tape measurement.

Time, Latitude, Longitude, and Azimuth.— The time determinations as made by the Survey are those required in connection with observaItions of latitude, longitude, and azimuth.

Time is determined by observations of the sun or stars with instruments which measure their altitude or indicate their meridian passage. Sextants and altazimuth instruments are generally used for approximate determinations, and transit instruments adjusted to the meridian for refined work. With the transits, chronographs are frequently used for recording the observations.

Latitude.-Determinations of an inferior grade are made with sextants or altazimuth instruments by observations on the sun or stars, but determinations of the first order are made by observations on the stars with zenith telescopes. In the latter method, the observation of from fifteen to twenty pairs of stars on three nights is considered necessary for a good determination.

Longitude. The most accurate results in the determination of longitude are obtained by the telegraphic method, by which the differences of time between the positions in the field and some first class astronomical observatory are determined by the exchange of time signals. The instrumental equipment for this purpose consists of a transit, a break circuit chronometer, a chronograph, and a set of telegraphic instruments at each station. For work of primary importance, the local time is determined in the usual manner at the two places on the same nights for six nights, and a comparison of the chronometers made on each night over the telegraph line by means of chronometer signals. The transmission time of the electric current is derived by sending signals in both directions. The personal equation of the observers is eliminated by the interchange of observers after half the work is completed. In longitude work of the first order, observations on twenty stars each night are considered necessary for an accurate determination of the local time at the two stations, the transits being equipped with Repsold self-registering micrometer eye-piece for the purpose of eliminating the personal equation of the observers.

In cases where the telegraphic method cannot be employed, longitudes are determined by observations of the eclipses of the satellites of Jupiter, of solar eclipses, of moon culminations, of occultation of stars, and by transporting chronometers. The results obtained by these methods are of varying degrees of accuracy depending upon the inherent errors of the methods themselves.

Azimuth.-Astronomic observations of azimuth are made to ascertain the directions of the lines established by a survey relative to the true meridian. Azimuths differ in grade according to the purposes for which they are required. For exploration work and reconnaissance, or for magnetic observations, theodolites with 3 to 6 inch horizontal circles are used to observe on the sun or on Polaris. For tracing meridian lines, or for tertiary triangulations, theodolites with 6 to 10 inch horizontal circles are used to observe on circumpolar stars. For primary tri

VOL. 5 30

Terrestrial Magnetism.-The complete magnetic work embraces the determinations of the three magnetic elements: the "declination" or "variation" of the compass, the "dip" of the magnetic needle, and the "intensity" of the magnetic force.

As it is impossible to represent the earth's magnetic condition for more than a definite moment of time, it is of great importance to provide for a continuous record of the innumerable fluctuations of the magnetic needle. In the work of the Survey, this is accomplished by the establishment of certain base stations equipped with sensitive magnetic instruments which make a continuous record, day and night, of the variations of the magnetic forces. At the present time these stations are situated at Cheltenham, Maryland; Baldwin, Kansas; Sitka, Alaska; Vieques, Porto Rico; and near Honolulu, Hawaii. From their records the magnetic charts are brought up to date and surveyors and mariners are provided with the exact amount of variation or change at any point between any two given dates. In addition to this work, true meridian lines are being established near the various county seats throughout the country for the purpose of enabling the surveyors to test and verify their compasses.

Hydrography. The various processes which comprise the hydrographic work, and by means of which the shape of the submerged portions of the earth's surface is determined and subsequently delineated on the maps in the form of channele banks, and shoals, may be summarized as follows: (1) by triangulation certain points are established on the land from which the positions of the soundings subsequently taken are determined; and (2) by a topographical survey the positions of the shore lines, wharf lines, rocks that show above water, and the limits of dry shoals and banks are obtained. The data thus obtained is plotted on a map based on a suitable projection, the scale of which depends upon the minuteness of detail with which the submerged features are to be delineated. A scale of 1.000 is considered as well adapted for the survey of most harbors.

Such a map shows the geographic positions of the various points such as church spires, chimneys, peculiarly shaped rocks and trees, and signals built over triangulation stations, selected as suitable to observe upon in locating the positions of the surroundings.

When a sufficient number of such signals and objects is available, and the tide gauge or staff has been erected at some point in the vicinity of the work, the lines of soundings are run by a boat and the soundings taken with a leadline.

A sounding party usually consists of the officer in charge, two observers with sextants, a recorder with watch or clock and record book, the leadsman with his leadline, and four oarsmen.

The method of operation is as follows: Beginning at a point, the position of which relative

to the shore is noted by the recorder, the observers measure the angles between three signal points on shore and read the angles measured; the leadsman gets a cast of the lead and calls out the number of feet or fathoms; and the recorder records all of these together with the time when the boat commenced to move along the line of soundings. Once started the boat continues without stopping to the end of the line. In the meantime other pairs of angles are taken by the observers at three or four minute intervals, or as often as necessary, each set of angles locating the position of the boat at the instant of observation. A final set of angles is taken at the end of the line, and the boat is then moved to position at the beginning of a new line of soundings. When the depths change rapidly the soundings are taken as frequently as possible, the time being noted to seconds; but, when the depths are nearly uniform, indicating a comparatively level bottom, the soundings are preferably taken at equal intervals of time. When practicable the lines of soundings are run on ranges, i. e., the boat being kept on the same line with two objects on shore. The correction for the difference of level due to the rise and fall of the tide during the sounding operations is obtained from the readings of the tide gauge, such readings usually being made at five or ten minute intervals by an observer specially detailed for that purpose, and all the soundings reduced to the common plane of mean low water, the plane of reference adopted by the Survey for its charts of the Atlantic and Gulf Coasts. In smooth, shoal water, it is required that the reduced soundings on lines crossing each other shall not differ more than 12 per cent. of the depth. In deep-sea work, offshore, the tidal correction is not applied to the soundings.

At the end of each day's work the results are graphically transferred to the map, every position of the boat corresponding to any pair of angles measured on any three points on shore being plotted by means of a three-armed protractor. The successive positions of the boat being thus located on the map, the various soundings are easily and accurately spaced between them from the number and time intervals shown in the record.

The methods described above are employed to develop the slopes of the bottom, but in regions like the coasts of Maine and Alaska, which are characterized by many isolated rocks and ledges on the bottom, or like the coasts of Florida, Porto Rico, and the Philippines, fringed with coral reefs and numerous coral heads, supplemental special examinations are made where soundings on the regular lines shoaler than the surrounding depths indicate the existence of yet shoaler water. Such examinations are greatly facilitated by the use of the "channel sweep." This device is attached under the vessel and gives it any desired draft of water up to 6 fathoms. By its use many rocks are discovered which the leadline fails to develop. It is also used to verify channels that have been marked out through reefs, or areas of broken ground.

In the work of deep-sea sounding, the leadsman is replaced by a sounding machine, the leadline by a fine steel piano wire, coiled on a drum, and the lead by a solid spherical shot weighing about 100 pounds. While sounding, the ship remains in sight of land, her position

being determined in a manner similar to that explained above in the case of the boat. The depth is obtained by recording the number of revolutions of the drum, the shot being automatically detached from the wire when it reaches the bottom.

Topography.-The principal method employed by the Survey in the execution of its topographic work is that which requires the use of the plane table and stadia. All the necessary operations for producing the topographic features of the map are executed by those instruments in the field with the country as a model. In special cases other instruments are employed as auxili aries, but in general the plane table alone fulfills all the requirements. On account of the rapidity and comparative accuracy with which the results are obtained by the graphic solution of the three point problem by the plane table, thus enabling the topographer to ascertain his position at an unknown point, together with the effective use of the stadia, this method is peculiarly fitted for delineating coast topography, usually characterized by such features as outlying islands and ledges, inaccessible rocky bluffs, and large marshy areas intersected by numerous streams.

Tides and Tidal Currents.-The vertical motion of the water surface is called the "tide," and the horizontal motion of the water itself is called the "tidal current" or the "tidal stream." The latter is oftentimes of much greater importance to navigation than the former, and on account of the many difficulties which have to be surmounted in obtaining sufficient information to enable predictions relative to their direction and velocity at any particular time and place, they require very careful consideration. Tidal observations are made either upon a simple graduated staff, or else by means of a self-registering gauge. The latter is always employed in places where a long series of observations is desired. It consists of a simple but ingenious combination of a clock mechanism with a recording apparatus in which a pencil, controlled in a lateral position by the rise and fall of the water, traces an undulatory curve upon a long strip of paper which moves slowly at right angles to the motion of the pencil and is reguated in its progress by the clock which controls the apparatus.

Leveling-The work of leveling is executed by two principal methods: (1) by the use of precise spirit levels; and (2) by vertical angulation.

The precise level used by the Survey differs from other instruments of this class in the following particulars: The telescope and level are irreversible; the Y's are absent; the level is rigidly fastened to the telescope, and is placed closer to its center line by being countersunk into the barrel of the latter; the use of an alloy of nickel and iron, having a small coefficient of expansion, in the construction of the telescope and the adjacent parts; the protection of the level vial and the middle of the telescope from sudden and unequal variations of temperature by incasing them in an outer tube, and the provision of an arrangement by the use of which the observer is enabled to see. without changing his position, the level bubble with his left eye at nearly the same instant the rod is read through the telescope with his right eye.

In the execution of the field work, a line of precise levels is always run twice, usually in

seasonal, the laws of which are comparatively unknown, brings the accuracy of the results obtained by the method considerably below that of spirit leveling. Tests for accuracy in connection with precise leveling work indicate that vertical angulation gives results that are correct to within one inch to the mile.

Coast Pilots.-See COAST PILOTS.
Chart Publications.-See CHART.

The accompanying map is a portion of a "Sailing Chart" which as published embraces the Atlantic coast of the United States between Cape Cod and Cape Hatteras. It shows only the outline of the continent, the seacoast lights, and the special geographic information useful for the purposes for which the map is intended, namely, a guide to navigation in close proximity to the coastline.

Gravity-The gravity work is now accomplished by the method and instruments suggested by Dr. T. C. Mendenhall, Superintendent of the Survey, in 1890, which permit of the accomplishment of the work with an ease and precision far superior to any of the methods previously used. See GRAVITY.

Geodesy.-See GEODESY; GEODETIC ARCS. For the latest information on the different subjects consult the following publications of the Survey Map Reductions, etc.: Appendix 20, Report for 1860; Magnetic Work, etc.: Appendices 8 of 1881, 1 of 1895, 5 and 6 of 1902, and 3 of 1904; Gravity: Appendices 12 of 1893 and 6 of 1897; Tides, etc.: Appendices 7, 8, and 9 of 1897; Signals in Triangulation, etc.: Appendices 10 of 1882 and 4 of 1903; Triangulation: Appendix 9 of 1882; Plane Table Manual: Appendix 7 of 1905; Some Instruments: Appendix 8 of 1894: New Level: Appendix 6 of 1900; Astronomical Work: Appendices 7 of 1897-8, and 8 of 1904; Hydrography: Appendix 6 of 1905.

Coast Defense, systematic protection of a country against hostile attack along its coastlines. In providing such defense a nation will consider not only the safety of its territory, but also the security of its commercial interests. In any system of coast defense a good navy is the most important feature; and so essential is it considered, that all other means are regarded as adjuncts or auxiliaries of the navy. Along a well-defended coast, in suitable places, are stations or points of support where is stored the requisite material for building, equipping, repairing, and supplying naval vessels, and where provision is made for furnishing men when additional force is needed. Forts are built in places where the coast artillery may co-operate with the navy in obstructing the advance of an enemy intending to capture a city or to invade the country; where their guns may command the entrance to a harbor or other approach by water; wherever they may cripple the enemy's attack on the defensive fleet, leaving it free to attack the enemy in turn; where forts may assist each other, and co-operate in repelling an invasion or preventing a blockade or a bombardment; where minor channels of approach may be closed or guarded, thus enabling the navy to give entire attention to the main channel, etc. Torpedoboats, harbor-mines, the searchlight, which illuminates the harbor and permits detection of the enemy's manoeuvres, are all valuable aids for the forts. The unfortified coast, as well as the

land approaches to cities, must be defended in time of war by whatever means are at command.

The guns for coast defense change with the places and purposes for which they are to be used. To pierce the side armor of battleships, the flat trajectory, high-power guns are used; for projectiles intended to fall on the decks of an enemy's ships, the high-angle guns (mortars or howitzers) are employed. The number and size of guns vary with the estimated number and calibres of those which may be brought to bear by an enemy. From the depth and length of the channel may be determined how many and what classes of ships the enemy can operate in it, their armament, etc., and from these data the number and calibres of guns needed may be calculated.

The difficulty of obtaining naval supplies after war has been declared makes it necessary to store them during times of peace. Besides quantities of supplies at points of support for the navy, important harbors are generally equipped with torpedo storehouses, cable-tanks, mining casements, etc., for use in emergencies.

The tactics of coast defense have changed greatly since the beginning of the last decade of the 19th century. Besides defense against attack upon fortified places preparation is also necessary for resisting assaults upon unfortified coasts. The length of the United States coast-line makes its secure fortification a matter of great difficulty and expense.

Methods of reconnaisance are important for defense as well as for attack. The defense seeks to veil all batteries, and even in times of peace a journey along a fortified coast might not discover many forts or other means of defense. The enemy, in preparation for attack, or in action, seeks to discover the nature and strength of the defenses; and the defense in turn uses all possible methods of ascertaining the number and classes of the enemy's ships, the probable line of attack, and whatever else may be discoverable in regard to his strength and purposes. Pictures of every important_warship in the world are studied by certain officers of every navy, and so close are some of these studies that a war vessel may be recognized by such an officer in any position in which he sees it. For the important work of reconnaisance observation stations are fully equipped for the coast artillery, photographs, drawings, descriptions of warships, telescopes, etc., for use in procuring information of the enemy, being fully provided.

In times of peace a country may, to its peril, neglect its coast defense, as was shown by the condition of the defenses of the United States at the beginning of the war with Spain. European nations have for years divided their tidewater regions into naval zones, and have assigned to each zone reserves, torpedo divisions, lighthouse establishments, coast-guard services, and signal stations.

In France the naval authority is made paramount, and all naval affairs are in charge of the minister of marine, assisted by a chief of staff. Not only the mobile defenses, but the coast fortresses, submarine mines, and the fort batteries are under control of the minister or his aid. In June, 1903, the general board of the United States navy made public the details of a plan for its reorganization with a general staff