countries, including chronometric longitude determinations in 1849 and 1850, between the United States and Europe; trans-Atlantic longitude determinations by means of the cable in 1866 and in 1870; telegraphic longitude determinations between Greenwich, England, and Brest, France, in 1872; the Labrador eclipse expedition of 1860, and the solar eclipse expedition to Spain in 1870; the naval eclipse expedition of 1889 to the west coast of Africa; the transit of Venus expeditions of 1874 and 1882; gravity observations made in New Zealand, New South Wales, British India, Japan, and the Hawaiian Islands in 1882.

CHAPTER II

ACTIVITIES

Hydrography consists of determining the configuration of the bottom of the sea, lakes, and rivers, and the character of rivers, tidal and ocean currents, and their effects in producing progressive or periodic changes in the configuration of the bottom. As an activity of the Coast and Geodetic Survey, it consists of examining waters of the sea coasts as far as tide water, sounding them, and preparing nautical charts and other information for the use of navigators and for use in military defense, in the planning of harbor improvements, and in the study of oceanography. This work consists of topographic and hydrographic surveys and physical hydrographic observation studies.

Hydrographic Work. The activities of the Coast and Geodetic Survey in hydrography and topography are confined to the Atlantic, Pacific, and Gulf coasts and the coasts of the island possessions, and the rivers to the head of tide water and ship navigation of the United States. The Corps of Engineers of the War Department surveys the Great Lakes and issues charts for their navigation, and the Hydrographic Office of the Navy Department duplicates nautical charts issued by other nations and publishes charts based on surveys by the U. S. Navy of coasts not under the jurisdiction of the United States. This division of the hydrographic work of the government is required by existing law.

The chief object of the hydrographic work is to furnish navigators with such information as to the depth and configuration of the bottom, and, of the movement of currents and tides, as will enable them to lay a safe course and to verify their position at sea. The nautical charts are as essential to navigation as the compass or the engines of a vessel. These charts for all navigable tidal waters under the jurisdiction of the United States are produced only by the Coast and Geodetic Survey.

Topographic Surveys. Topography is an important feature of the work of charting the coasts, as it shows the relation of land to water for purposes of navigation and harbor and waterway improvement.

In addition to the work of triangulation and leveling for the establishment of reference marks described under the head of geodetic work, the Coast and Geodetic Survey conducts topographic operations which are mostly restricted to a narrow margin, rarely more than three or four miles wide along the coast and surrounding harbors, bays, and rivers to the head of tide water. Topographic operations of other areas in the United States come within the sphere of work of the Geological Survey of the Department of the Interior.

The topographic work of the Coast and Geodetic Survey consists of mapping the natural and artificial features of the coast, by indicating by means of symbols the "shore lines" or boundry between land and water, and "contour lines," which are lines of equal elevation above a selected plane such as the sea level; locating the rocks that show above water; indicating the character of the shores bordering on the water and their prominent and characteristic features; and locating points along the shores determined by triangulation, such as church spires, chimneys, conspicuous rocks and trees, and specially built signals, such as are suitable for observation from a boat or vessel while sounding.

Hydrographic Surveys. The hydrographic surveys consist of determining the depth of the water and outlining the channels, banks, and shoals, and locating all dangers and aids to navigation, and charting this information for the use of navigators.

When soundings are made it is necessary to have some means of determining the exact position of the vessel at the time of such measurement. The first need of the hydrographer therefor, in making his survey is a projection or outline map showing the meridians and parallels, the shore line, the character of the shore, and the land features visible from the water, prominent objects, spires, houses, artificial signals, capes, headlands, and outlying rocks and islands visible from the shore.

Soundings must be preceded by triangulation and topographic surveys, and accompanied by tidal observations; and if offshore,

by astronomical, magnetic and current observations in addition. The triangulation fixes the position of the trigonometric points on land by which the location of the soundings is determined, and the topographic survey provides the delineation of the shore, locates the rocks that show above the water, and the limits of dry shores and banks.

The points determined by triangulation are plotted on the projection, or outline map, in their proper geographic positions. These consist of prominent objects, such as church spires, chimneys, conspicuous rocks or trees, and specially built signals, all of which are of a suitable character to observe upon from a boat or vessel while sounding. Not until a sufficient number of these signals or other objects have been located and a tide gauge or tide staff has been erected at some suitable point, can any accurate and complete hydrographic survey be made.

To make these soundings, power launches and ships are mostly used. The lines of soundings are run in accordance with such a scheme as will best develop the contour of the bottom and its characteristic features. It is generally a rectangular system of lines. In a close survey the lines are spaced twenty yards apart with soundings ten yards apart in each line. When practicable the lines of soundings are run in ranges, that is, with the boat in the same straight line with two fixed objects on shore. The sounding lines are closely parallel to one another, the boat moves slowly and the soundings are taken as rapidly as the depths permit.

There are three different classes of hydrographic surveys:

1. Inshore hydrography, used in bays, estuaries, etc., and along coasts as far offshore as the surveyor can see the shore objects necessary for locating his position.

2. Offshore hydrography, when the surveying vessel starts at a known point situated at the limit of visibility of objects on shore, proceeds seaward the necessary distance, and returns shoreward until the shores can again be seen and the position located.

3. Wire-drag surveys in areas where the soundings indicate the possible existence of pinnacle rocks, boulders, or coral reefs undetected by the lead.

The necessary

combinations of these three classes of work constitute a complete hydrographic survey.

Hydrographic surveys are made by the Coast and Geodetic Survey in what are designated as inland waters, inshore waters, and offshore areas.

On the Atlantic coast, inland waters include shoal bays, sounds, and tidal rivers that can be used for through traffic by boats drawing not more than ten and in most cases not more than four feet. Such waters are usually connected with the sea by shoal inlets and with other inside waters, or with each other by canals, cuts, or dredged channels.

Inshore waters include all areas of moderate depth on the outside coast, from a depth curve arbitrarily adopted outside of which no dangerous shoals are believed to exist, to the heads of navigation of all bays, sounds, and tidal rivers that have a navigable connection with the sea.

Offshore areas extend from the outer limit adopted for inshore areas to a depth curve generally fixed at one hundred fathoms on the Atlantic coast, which for most of the coast is near the edge of the continental shelf.

The Pacific coast is generally mountainous, with comparatively few harbors or inside waterways and with comparatively deep water close to the shore. The purpose of the surveys here is to meet the needs of vessels approaching from seaward and of coasting vessels which keep to a few comparatively narrow tracks to insure up-to-date charts of the various harbors and to make soundings offshore to develop fishing banks that are known to exist. Up to the depth of about twenty fathoms the soundings are made by the lead line. This is a line marked in fathoms and feet, to which is attached a piece of lead about two inches in diameter and ten inches long. As the sounding boat moves along on a straight line the leadsman casts the lead ahead of the boat, reading the depth from the line as the lead strikes the bottom and the line becomes vertical. The point where each sounding was taken is accurately located by observers in the boat who, with sextants, measure the angles between known objects on shore.

For soundings betwen twenty and thirty-five fathoms, a method often used is that of dropping the lead near the bow and reading the depth as the lead comes vertical under the leadsman stationed on a platform aft. Various methods are used for carrying the lead forward and automatically releasing it.