The compass shows the mariner his course, but not how far he may have been drifted from it by wind or current. It is the province of nautical astronomy to tell him from time to time his exact position by latitude and longi tude. Nautical astronomy was greatly ad vanced by Prince Henry the Navigator, a prince of Portugal, who founded an observa tory and school at Sagres in 1420, employed the best Italian mapmakers and pilots, and inspired the heroic age of Portuguese and Spanish ex ploration, which took its course toward the Atlantic because Europe, whose face was turned toward Asia up to the time of the clos ing of the routes to the East through the cap ture of Consantinople by the Turks in 1453, now began to transfer the centre of her civili zation and commercial activity to the Great Western Ocean. The great works of the an cient geographers were rescued from the ex piring Byzantine Empire and reproduced in the West, and, after the lapse of a thousand years, maps were once more constructed on the principles of latitude and longitude initiated by Ptolemy. It was into such a state of, the progress of knowledge of the coasts of the world that the generation of Columbus, Gama, and Magellan was introduced — the generation that gave America, the route to India, and the circumnavigation of the globe. The revolution that had been effected in the map of the world between the sailing of Columbus and the year 1600 may be realized by comparison of Behaim's famous globe of 1492— the year of the sailing of Columbus—with Hakluyt's map of 1399. Behaim's globe had no America, and the Indian Ocean was inclosed to the south by an eastern extension of South Africa : it was essentially the same as Ptolemy's of 13 centuries earlier. But, Hakluyt's map, a century later, shows the modern world, except for the omission of Australia which had recently been reached by the Dutch in those voyages from their East Indies' colonies as a result of which its western coast was explored and represented in the map by Ortelius, in 1570, as New Holland.
In the 16th century the art of drawing up sea charts found its greatest cultivation among the Spaniards and Portuguese. The govern ments of Spain and Portugal endeavored to have charts constructed with the greatest pos sible accuracy. Portugal created a commis sion, of which Martin Behaim was a member, for the greater development of this science. In Spain, Amerigo Vespucci was appointed Chief Pilot in 1508, and soon became the head of a hydrographic bureau for the execution of charts and kindred purposes. It was ordered at this time °that henceforth all navigators sail ing towards known or unknown parts of India, who should discover new regions, islands, har bors or bays, affording some interest for the general chart, should, on their return to Europe, report the same to the Chief Pilot.° Such was the importance attached to the proper construction of these charts, that for eigners were summoned to correct or complete these documents. It was for this purpose that the celebrated English navigator, Sebastian Cabot, repaired to Spain in 1512.
In the course of his voyages, Cabot care fully observed the changes in the variation of the compass, and he was instrumental in bring ing it to pass that, after the early part of the 16th century, care was taken to introduce this datum when drawing charts. Charts were fur ther improved toward the middle of the 16th century by the use of the Mercator projection, which has rendered signal services to naviga tion, and by a more accurate outline of the sea coasts and a more precise representation of the position of each of the points: the progress thus made was in direct relation to the im provement of the methods of determining lati tude and longitude, and the improvement of nautical instruments. The soundings which could be ascertained with the appliances then in existence soon were noted in charts at im portant points of the coasts, and hydrographic signs, resembling those at present in use, began to be employed to show reefs and rocky bottom by means of crosses, and sand banks and shal lows by means of masses of dots. The period which opens with the voyages of Captain Cook, in the second half of the 18th century, marks the beginning of the more precise observations in hydrography. After these voyages, the chart of the Pacific, until then almost a blank, dif fered but little from that of the present, in general superficial outlines; and the reduction of Australia approximately to the proportions now accepted, and its separation from New Zealand and from the Terra Australis Incog nita were among his great achievements. It
was reserved to the United States Exploring Expedition under Charles Wilkes to discover the Antarctic Continent in the first days of the year 1840, and to chart 1,500 miles of its coast line; and also to contribute extensively to the records of geography and terrestrial physics with which the splendid set of sea voyages by our own and all other maritime countries have stocked the hydrographic offices established by the various nations during the last one hun dred years or so.
By the labors of these institutions, new de tails have been introduced for determining position, based upon astronomical and geodetic observations; the survey of coastlines no longer made by the compass, but by trigono metrical measurement; the meteorological ele ment is introduced in the graphical representa tion of the seas, hydrographic signs guide the seaman, reliable soundings inform him of the depth of water in the approach to nearly every coast, the currents are delineated, and the local tides are indicated.
Modern Practice.— The two branches of surveying on which hydrography is most de pendent, and which it is most convenient to have precede it, and on which its accuracy largely depends, are triangulation and topog raphy. The triangulation fixes the position of the trigonometrical points on land by means of which the location of the soundings is deter mined. The .topography provides the delinea tion of the shore, locates the rocks that show above water, and the limits of dry shores and banks. These data are placed in their proper relative position on an outline map whose scale will depend upon the minuteness with which the submerged features are to be mapped, and this in turn will largely depend upon the char acteristics of the area and its importance to navigation or other commercial purpose. A scale of 1-10,000 (which means that 10,000 feet on the surface of the earth are represented by one foot on the projection) is well adapted for the survey of most harbors. The points de termined by triangulation are plotted on the outline map or projection in their proper geographic positions. These points consist of prominent objects, such as church spires, chim neys, conspicuous rocks or trees and specially built signals, all of which are of a suitable kind to be observed from a boat or vessel while sounding. When 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 in the vicinity of the work, the next step in the hydrographic survey is making the soundings for the depth of water. Power launches and ships are now more often used than rowboats in running sounding lines. The lines of soundings are run in accordance with such a plan as will best develop the con tour of the bottom and its characteristic features; generally a rectangular system of lines is used, and where the two systems of lines -cross each other the soundings must agree within prescribed limits varying with the depth of water. When beginning a line of soundings, two observers, with sextant in hand, the recorder, with a watch or clock and record book, and the leadsmen, with his sounding line, take their respective positions. The ob servers measure with their sextants the two angles between three shore signals and read the angles measured; the leadsmen gets a cast of the lead and announces the depth of water in feet or fathoms and feet, and the recorder records all these with the time the boat begins to move. The boat does not stop again until the end of the line is reached. The position of the boat is determined by the observation of pairs of angles at equal intervals of time (two to four minutes), or when there is a change in the direction of the line, or a change of speed of the boat or sudden change in depth of water, and at the end of the line. Where the depths are changing rapidly the soundings are taken by the leadsmen as frequently as possible, and the time of each sounding may be noted to sec onds; but where the bottom is comparatively level the soundings are preferably taken at equal intervals of time. The pairs of angles taken for position of the sounding boat are plotted on the projection or outline map by means of a three-arm protractor, thus each sounding is laid down in its proper position. When practicable the lines of soundings are run on ranges—that is, with the boat in the same straight line with two fixed objects on shore.