Читать книгу Nautical Charts - G. R. Putnam - Страница 5

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Need of maps. Maps are useful and necessary for many purposes. Only by means of a correct map or globe can a clear idea of the geography of a region be given. An attempt to convey the same information by a written description would in comparison be both cumbersome and obscure. Even by passing over an extensive region a man unaided by instruments will obtain only a rather crude notion of the relations, which he could clearly see on a good map. The importance among the human arts of the making of maps is indicated by the references to them in very early historical records, and by the skill in map drawing shown by some of the primitive peoples of to-day. This skill exists particularly among races whose mode of life gives them a wide horizon, as for instance the Eskimos. An interesting instance of this was the case of Joe, an Eskimo guide, who, in 1898, before the surveys of the Yukon delta were made, drew a map of the Yukon mouths with much more complete information than any previously available.

Without attempting to enumerate in detail the special uses for maps, in the broader sense they may be said to be essential for commercial, engineering, military, scientific, educational, and political purposes.

Early geography and map making. The oldest map known is a plan of gold mines in Nubia, drawn on a papyrus. This is of the thirteenth century B.C., and was found in Egypt.

In the earliest historic times men believed the earth to be a flat surface of nearly circular outline, a natural inference for those with limited outlook and communication. Later the idea was introduced of the ocean as a river bounding the earth disk. The spherical theory of the earth was, however, early accepted by learned men, and was demonstrated by Aristotle (384 to 322 B.C.), who used as proofs the earth's shadow on the moon, and the change in the visibility of the stars in traveling north or south. Crates constructed a terrestrial globe in the second century B.C.

There is no Greek or Latin map extant of earlier date than the time of Ptolemy, but there are references showing that maps were in use. One of the first of such passages in Greek literature is the interesting comment of Herodotus written in the fifth century B.C., "but I laugh when I see many who already have drawn the circuits of the earth, without any right understanding thereof. Thus they draw Oceanus flowing round the earth, which is circular, as though turned by a lathe, and they make Asia equal to Europe."

A map of the world was drawn by Anaximander, 560 B.C. A hundred years later Democritus drew a map having an oblong shape, and taught that the width of the world from east to west was one and a half times its extent from north to south, a conclusion based on his travels eastward as far as India. This theory, which was for a time accepted, has left an enduring mark in the words longitude and latitude, originally signifying the length and the breadth of the earth.

The first application of astronomy to geography was made by Pytheas, who about 326 B.C. obtained the latitude of Marseilles by an observation of the altitude of the sun. Dicearchus in 310 B.C. determined the first parallel of latitude by noting places where on the same day the sun cast shadows of equal length from pillars of equal height. Eratosthenes (276 to 196 B.C.) was the first to compute the circumference of the earth from observations of the altitude of the sun at Alexandria and at Syene in Upper Egypt and an estimation of the distance between these two places. Ptolemy, a Greek of Alexandria, in the years from 127 to 151 A.D. wrote extensively on geographic subjects, and collected into systematic form all geographic knowledge then existing; he was the greatest geographer of early history.

In the ten centuries which followed, part of the early advance in this science was obscured, and the theory that the earth was a flat disk surrounded by the sea again became prevalent. The voyages of discovery of the middle ages, however, led to a rapid development of geographic knowledge.

The flattening of the spherical earth was not suspected until in 1672 a clock regulated to beat seconds at Paris, when taken to Cayenne near the equator was found to lose two and one-half minutes a day. Newton proved that this was due to the fact that the earth is an oblate spheroid. In 1735 accurate measurements were undertaken to determine the size and shape of the earth. The equatorial diameter has been found to be 7926.6 miles and the polar diameter 7899.6 miles, the difference, or 27 statute miles, being the amount of the flattening at the poles.

The first sailing directions. The early Greek and Roman writers do not allude to charts or maps intended especially for the use of seafarers. There are, however, extant several peripli or descriptions of the coast. Some of these appear certainly to have been intended for use as nautical guides, corresponding to the modern sailing directions. It is probable that they were explanatory of or accompanied by coast charts, now lost. They are of interest therefore as being probably the first compilations for the guidance of seamen. One of the earliest, written apparently in the fifth and fourth centuries B.C., is entitled "Scylax of Caryanda, his circumnavigation of the sea of the inhabited part of Europe and Asia and Libya." It contains a systematic description of the coasts of the Mediterranean, Black Sea, and part of the west coast of Africa. The following are some extracts which indicate the character of the work. It is to be noted that no bearings are given, and that distances are usually stated by day's sail: Africa is referred to as Libya.

"Europe. I shall begin from the Pillars of Hercules in Europe and continue to the Pillars of Hercules in Libya, and as far as the land of the great Ethiopians. The Pillars of Hercules are opposite each other, and are distant from each other by one day's sail.... From Thonis the voyage to Pharos, a desert island (good harborage but no drinking water), is 150 stadia. In Pharos are many harbors. But ships water at the Marian mere, for it is drinkable.... From Chersonesus is one day's sail; but from Naustathmus to the harbor of Cyrene, 100 stadia. But from the harbor to Cyrene, 80 stadia; for Cyrene is inland. These harbors are always fit for putting into. And there are other refuges at little islands, and anchorages and many beaches, in the district between.... After the isthmus is Carthage, a city of the Phœnicians, and a harbor. Sailing along from Hermæa it is half a day to Carthage. There are islands off the Hermæan cape, Pontia island and Cosyrus. From Hermæa to Cosyrus is a day's sail. Beyond the Hermæan cape, towards the rising sun, are three islands belonging to this shore, inhabited by Carthaginians; the city and harbor of Melite, the city of Gaulus, and Lampas; this has two or three towers.... The sailing along Libya from the Canopic mouth in Egypt to the Pillars of Hercules ... takes 74 days if one coast round the bays.... From the cape of Hermæa extend great reefs, that is, from Libya towards Europe, not rising above the sea; it washes over them at times.... From Thymiateria one sails to cape Soloes, which juts far into the sea. But all this district of Libya is very famous and very sacred.... This whole coasting from the Pillars of Hercules to Cerne Island takes twelve days. The parts beyond the isle of Cerne are no longer navigable because of shoals, mud, and sea-weed. This sea-weed has the width of a palm, and is sharp towards the points, so as to prick."

That there were many other similar writings in the following centuries is shown by the following quotation from Marcianus, in a preface to sailing directions written in the fifth century A.D.: "This I write after having gone through many sailing directions, and spent much time on their examination. For it behooves all who are men of education, to scrutinise such attempts at learning in this subject, so as neither rashly to believe the things that are said, nor incredulously to set their private opinions against the careful decisions of others."

The oldest extant sailing directions of the middle ages bear date 1306 to 1320.

Development of chart making. The application of the compass to nautical use in the twelfth century A.D. had a marked effect in encouraging voyages of exploration, and therefore indirectly on chart making. The following, written toward the close of the twelfth century, is the first known mention of the use of the compass in Europe: "The sailors, moreover, as they sail over the sea, when in cloudy weather they cannot longer profit by the light of the sun, or when the world is wrapped in the darkness of the shades of night, and they are ignorant to what part of the horizon the prow is directed, place the needle over the magnet, which is whirled round in a circle, until, when the motion ceases, the point of it (the needle) looks to the north." The nautical compass of that time appears to have consisted of a magnetized needle, floated in a vessel of water by a cork or reed, and having no index nor compass card. Peregrinus in 1269 made notable improvements in the compass, including a pivot suspension for the needle, a graduation, a lubber line, and an azimuth bar for sighting on the sun or other object.

Nautical charts are known to have been in use since the thirteenth century A.D., but the earliest extant of which the date can be fixed is Vesconte's loxodromic chart of 1311.

The loxodromic charts first appeared in Italy, and were so called from the fact that they were crossed by loxodromes (or rhumb lines) radiating from a number of crossing points distributed regularly over the map. Compass roses carefully drawn were later added at these crossing points, the first appearing on a chart of 1375. The earliest known mention of the variation of the compass from true north was on the first voyage of Columbus, who discovered this important fact in 1492, and as a consequence his "seamen were terrified and dismayed." Before that time it was assumed in Europe that the compass pointed "true to the north pole." The apparent failure to detect the variation earlier was doubtless to some extent due to its small amount at that time along the Mediterranean. The earlier charts showed both lines and compass roses apparently oriented with the true meridian, though there is some evidence to indicate that they were actually oriented with the magnetic meridian, the designer not recognizing any difference. The variation of the compass was first marked on a map in 1532 and on a printed chart in 1595, but the placing of magnetic compasses on charts did not become customary until about fifty years ago. These early charts were drawn on parchment, using bright colors. They were copied by hand, one from another, with gradual variations. They had no projections, and the draftsmen evidently had no idea of the sphericity of the earth. Islands and points were usually exaggerated; shallows were indicated, but no soundings; no information was given as to the interior of the countries; a scale of distances was nearly always provided.

Charts were first printed about 1477, and are known to have been engraved on copper by 1560.

The maps of Ptolemy were ruled with degree lines, but no chart was so provided until 1427; by 1500, however, most charts were graduated. Before this date it is not known on what projection the charts were constructed. On the first graduated charts the degree lines were equidistant parallel straight lines cutting each other at right angles and thus dividing the chart into equal squares or rectangles. These were known as "plain charts." This square projection had little to commend it save simplicity of construction, as in higher latitudes it gave neither directions nor distances correctly. The difficulties of its use in navigation were early recognized, and nautical works contained chapters on "sailing by the plain chart, and the uncertainties thereof."

The example of early chart making shown in Fig. 2 is of great interest as being the earliest extant chart which includes America. This chart was drawn on ox-hide in 1500 by Juan de la Cosa, who accompanied Columbus on his first voyage as master of his flagship, and on his second voyage as cartographer. The chart, of which only a portion is shown here, purports to cover the entire world; it joins Asia and America as one continent, the Pacific Ocean being then still unknown.

FIG. 2. CHART OF NORTH ATLANTIC OCEAN, BY JUAN DE LA COSA, 1500. EARLIEST EXTANT CHART SHOWING AMERICA.

Fig. 2 enlarged (652 kB)

Gerhard Krämer, a Flemish map-maker, better known by his Latin name of Mercator, in 1569 published his famous Universal Map. In this map the meridians and parallels were still straight lines intersecting at right angles, but the distances between the parallels were increased with increasing latitude in such proportion that a rhumb line, or line cutting the meridians at a constant angle, would appear on the map as a straight line. Mercator never explained the construction of his chart, and as the above condition was not accurately carried out, it is thought that the chart was drawn by comparing a terrestrial globe with a "plain chart." After examination of a mercator chart in 1590, Edward Wright developed the correct principles on which such a chart should be constructed, and published in 1599 his treatise "The Correction of Certain Errors in Navigation." It took nearly a century to bring this chart into use, and even in the middle of the eighteenth century nautical writers complain that "some prefer the plain chart."

The Arcano del Mare, 1646, was the first marine atlas in which all the maps were drawn on the mercator projection.

In the sixteenth, seventeenth and eighteenth centuries charts and sailing directions were often bound together in large volumes. These usually had quaint titles, not overburdened with modesty, of which the following is an example: "The Lightning Columne, or Sea-Mirrour, containing the Sea-Coasts of the Northern, Eastern, and Western Navigation. Setting forth in divers necessaire Sea-Cards, all the Ports, Rivers, Bayes, Roads, Depths, and Sands. Very curiously placed on its due Polus height furnished. With the Discoveries of the chief Countries and on what Cours and Distance they lay one from another. Never there to fore so Clearly laid open, and here and there very diligently bettered and augmented for the use of all Seamen. As also the situation of the Northerly Countries, as Islands, the Strate Davids, the Isle of Jan Mayen, Bears Island, Old Greenland, Spitsbergen and Nova Zembla. Adorneth with many Sea-Cards and Discoveries. Gathered out of the Experiences and practice of divers Pilots and Lovers of the famous Art of Navigation. Where unto is added a brief Instruction of the Art of Navigation, together with New Tables of the Sun's Declination, with a new Almanach. At Amsterdam. Printed by Casparus Loots-Man, Bookseller in the Loots-Man, upon the Water. Anno 1697. With Previlege for fiftheen years."

In 1633 a cartographer was appointed to the States-General of Holland, and it was his duty to correct the charts from the ships' logs. The Dutch at an early date made important progress in publishing charts. In 1720 there was established in Paris by order of the king, a central chart office ("dépôt des cartes et plans, journaux et mémoires concernant la navigation"), and in 1737 the first charts were published by this office. Detailed surveys of the coast of France were commenced in 1816.

In 1740 "the commissioners for the discovery of longitude at sea" were authorized by Parliament to expend money on the survey of the coasts of Great Britain, this commission having been created in 1713. Various rewards were offered by this commission, including one of £10,000, for the discovery of a method of determining the longitude within 60 miles, an interesting side light on the uncertainties of navigation at that time. Compensated timepieces, which have been so important a factor in improving navigation, were invented by Harrison about 1761.

FIG. 3. LOXODROMIC CHART OF NORTH ATLANTIC OCEAN, 1565. A PLAIN CHART WITH LATITUDE DEGREES OF EQUAL LENGTH.

In 1795, by an Order in Council, a Hydrographical Office was established in London, "to take charge and custody of such plans and charts as then were, or should thereafter be, deposited in the Admiralty, and to select and compile such information as might appear to be requisite for the purpose of improving navigation." This office had at first one assistant and one draftsman. Before that time many charts of a private or semiofficial character had been published; the catalogue of the East India Company in 1786 included 347 charts.

In 1807 the Congress of the United States authorized the President "to cause a survey to be taken of the coasts of the United States, in which shall be designated the islands and shoals, with the roads or places of anchorage, within twenty leagues of any part of the shores of the United States; and also the respective courses and distances between the principal capes, or headlands, together with such other matters as he may deem proper for completing an accurate chart of every part of the coasts within the extent aforesaid." This law was the origin of the present United States Coast and Geodetic Survey, now under the Department of Commerce and Labor.

In 1841 a systematic survey of the Great Lakes was commenced; this is the Survey of the Northern and Northwestern Lakes, briefly known as the Lake Survey, conducted under the Corps of Engineers.

In 1866 the United States Hydrographic Office was established under the Navy Department "for the improvement of the means for navigating safely the vessels of the Navy, and of the mercantile marine, by providing under the authority of the Secretary of the Navy, accurate and cheap nautical charts, sailing directions, navigators, and manuals of instructions for the use of all vessels of the United States, and for the benefit and use of navigation generally."

Systematic surveying and chart making date back little more than a century, and most of the information shown on modern charts has been gathered in that time. At present all the principal maritime nations of the world have made, or are extending, careful surveys of their own coasts.

Several of the countries have added valuable contributions in the examination of other regions and oceanic areas beyond their borders. The maritime and colonial interests of Great Britain impelled that nation to carry on extensive surveys along coasts whose inhabitants were not prepared to do this work in the earlier days; the British have made surveys along the coasts of Asia and Africa and a part of South America, and the resulting charts have been a very important and not sufficiently known contribution to commercial intercourse among the nations, as well as to geography.

The Dutch, French, Spanish, and other European governments have made nautical surveys in various parts of the world, largely in connection with their own colonies, and in recent years much useful work has been done by vessels of the German government. The United States has also beyond its own territory made valuable additions to hydrographic knowledge in the work of officers of the Navy in a number of oceanic exploring expeditions, and surveys on the coasts of Mexico and in the West Indies, and in the explorations of Fish Commission vessels.

FIG. 4. EARLY CHART OF NEW YORK HARBOR, 1737.

Extension of maritime surveys. Of the total area of the earth's surface, 51,886,000 square miles is land and 145,054,000 square miles is sea. The oceans thus occupy nearly three-fourths of the whole surface, affording highways open to the nations. To conduct international commerce by water the ships of one country must enter the ports of another. Thus both on the open sea and in the harbors there is an interest, common to seamen of all nationalities, in the advance of marine surveys and in the publication of charts.

To keep the coasts properly charted, as well as lighted and buoyed, is an obligation devolving on modern nations, not only for the benefit of their own commerce but for that of other countries.

As shown below, only a small part of the coast line of the world is thoroughly surveyed. In the extensive ocean areas which are dotted with islands or reefs, a large amount of work is required for their sufficient charting, although many doubtful areas have been cleared up in recent years. Even the parts that are known to be of depths so great as to be free from navigational dangers should be sounded over sufficiently to develop the general configuration of the ocean bottom.

Through international understanding a thorough exploration of all the water area of the globe and the coasts may in time be effected, and the many doubtful spots which still disfigure the charts may be either eliminated or definitely located.

Present state of progress of hydrographic surveys. A comparatively small proportion of the coasts of the world can be considered as completely surveyed at the present time, and even such regions require much additional revision. In the class of more thoroughly surveyed coasts should be included the Atlantic and most of the Pacific coast of the United States, Porto Rico, nearly all the coasts of Europe, Algeria, and portions of the coasts of Japan, the Philippine Islands, and India.

A large part of the world's coasts has been surveyed incompletely, but sufficiently well to permit the publication of navigational charts. This is the condition as respects most of southeastern Alaska and some other portions of the Alaskan coast, British Columbia, most of Mexico, Central America, the West Indies, Brazil and parts of Chile, the Hawaiian Islands, China, Malay Peninsula, Siam, the Dutch East Indies, Australia, New Zealand, Persia, Arabia, most of Africa, Iceland, northern Scandinavia, and Finland.

Another considerable portion of the coasts has not been surveyed, but has been covered by explorations which have been embodied in nautical charts of varied degrees of incompleteness. In this class are the north coast and considerable portions of the south and west coasts of Alaska, the Aleutian Islands, Siberia, most of the oceanic groups in the Pacific, the northern coasts of Europe and North America, Greenland, the west coast of South America, Venezuela, and Argentina.

Only a very small proportion of the total length of coasts is now entirely unexplored, and such portions are confined to the polar regions.

Chart publications of various nations. There are about eighteen nations publishing navigational charts, and adding to the information on which charts are based. Many of these nations republish to some extent the charts prepared by the others. Great Britain has kept up a series of charts covering all parts of the world and practically including in some form all information published elsewhere. This series now (1908) includes 3725 different charts, of which the annual issue is about 600,000 copies. France (1906) publishes 2948 different charts.

In the United States, charts are published by the Coast and Geodetic Survey for the coasts and tidal waters of the main country and the insular possessions, by the Hydrographic Office for oceanic areas and foreign coasts, and by the Lake Survey for the Great Lakes. The total number of different charts issued by these bureaus is about 2300, and the total annual issue is about 225,000 copies.

Systems in use on various charts.

Longitude. The first chart of New York, published by the Coast Survey in 1844, was referred to the City Hall of New York as the initial longitude, and some years ago it was the prevailing custom for each nation to use a local initial longitude. While this satisfied local pride it led to much geographical and navigational confusion. Happily the charts of all countries are now referred to Greenwich, with the following exceptions:

France refers to Paris, which is 2° 20´ 15´´ E. of Greenwich.

Spain refers to San Fernando, which is 6° 12´ 20´´ W. of Greenwich.

Portugal refers to Lisbon, which is 9° 08´ 24´´ W. of Greenwich.

Units for depths. The English fathom or foot is used for depths on the charts of Great Britain, the United States, and Japan. Russia uses the sajene of seven English feet. On the modern charts of practically all the other countries the meter is used, though on older charts various units are found.

In the first group feet are ordinarily found only on large scale or local charts of areas with moderate depths, and the other charts are in fathoms, except that on the earlier charts of the Coast and Geodetic Survey feet were used on a sanded surface inside of the three-fathom curve and fathoms on the white surface outside of that curve. Heights are stated in feet on the charts of the first group.

Plane of reference. As the depth of water varies with the tide, it is necessary for charting purposes to adopt some standard plane to which the soundings are referred. Practically all countries have adopted for this purpose a low stage of the tide, as this is obviously on the side of safety; in most cases an extreme low water is used, so that the actual depths will seldom, owing to the tide, be less than those shown on the chart. The definite reference planes used on the American charts will be mentioned later.

On nearly all charts heights are referred to mean high water, doubtless owing to this being the visible limit of the land at high tide. On topographic maps of the interior, the heights are referred to mean sea level, which plane is of course lower than the preceding by one-half the range of tide.

Symbols on charts. Fair uniformity as to general principles, with differences as to details in carrying them out, exists on the various series of charts regarding their general arrangement and the more important symbols, such as in the shading of land to distinguish from water, the use of depth curves, the representation of hills by shade or contour, the indication of shoals and dangers, and of lighthouses and buoys.

Desirability of uniformity in charts. Ships engaged in international commerce must enter foreign ports. As the information is constantly changing and charts are being corrected or improved, it is sometimes desirable for the navigator to consult the local foreign charts, and it may often be necessary for him to carry in his chart room the charts of several different countries. There are therefore important advantages in international uniformity in chart publication.

There should be a common initial longitude, and as the longitude of Greenwich has been so extensively adopted, it appears quite probable that its use may some day become universal.

A common unit for soundings and heights would be very desirable, but the fact that a large group of nations has united on the metric system, while a small group with great commercial interests retains another system, makes the attainment of uniformity difficult.

Substantial agreement as to the use of symbols on charts, particularly such as represent aids or dangers to navigation, would be desirable and doubtless feasible.

Privately published charts. Many of the earlier charts were prepared and published by private enterprise, and such charts are still published, as, for instance, the so-called "blue-back" charts printed in London. These charts have usually differed from those published by the various governments either in representing the main features in a very bold manner with little detail or in including a considerable area with many plans on a single large sheet backed for permanency. An objection to the latter is that the durability together with the high price tends to keep an old chart in use long after it is out of date. It would be financially difficult for a private firm to give the service that a government does in the matter of correcting the charts and issuing new editions, and this is an important consideration in the selection of charts.

Purpose of charts. The main purpose of charts is to furnish graphical guides to aid in taking a vessel safely from one port to another; they are maps for the use of navigators. An experienced mariner may be able to steer his vessel over a familiar course without charts, but this does not make their publication less necessary. Even such an expert pilot doubtless studied the charts in the first place; the uncertainties of the sea and the changes of information are such that his vessel's equipment should include the latest charts, and safety requires their examination. The passengers and the merchants who intrust their lives or their goods to the sea are largely dependent upon the correctness of the charts.

Besides their main purpose charts fill many other needs, among which are; for preliminary planning of harbor improvements and various engineering works, for defensive works and other military uses, for the fishing interests, and for general information as to the coastal regions. Charts will furnish much of interest and instruction to the traveler by sea and the dweller near the coast, who will learn to read them. Passenger steamers should more often for the interest of their patrons display charts of the waters traversed. No written or verbal description can give as clear an idea of geographical features and relations as a good map or chart.

As the charts are revised from time to time, a comparison of editions at different dates furnishes a record of the changes wrought by nature or man, and this is especially useful in studying the action in many harbor and river entrances, as well as for historical purposes.

Requirements for charts. As charts are maps of the water areas, including the adjoining land, and intended primarily for the use of mariners, they differ in important respects from topographic maps or general maps, even such as include the water areas. The main requirements for charts are these; correct and complete information, early publication of new data, clear and intelligible representation of the information, convenient arrangement as navigational instruments, and high standard of publication.

The special and sometimes difficult conditions under which charts must be used on shipboard call for good judgment throughout their preparation. Even the paper on which they are printed is of importance, in order that they may be sufficiently durable and suitable for plotting.

Information given on charts. It is evident that it is impossible to represent on a chart of any practicable scale all the features that exist on the corresponding area of the earth's surface. It is essential, therefore, that a selection be made of the classes of facts that are to be shown, as well as of the detail that is to be used for each class. The practical utility of the chart depends largely on the good judgment used in this selection. In the information shown, charts differ from maps principally in representing by soundings and curves the configuration of the bottom of the water area, and in showing ordinarily the topographic features only in the vicinity of the coast line.

The convenience of mariners should govern in the selection and arrangement of the information to be shown on charts, though they may be made useful for other purposes so long as this convenience is not lessened. The needs and preferences of navigators alone, however, differ so much that a reasonable chart must be somewhat of a compromise between conflicting views. For certain classes of navigation a boldly drawn chart showing only the dangers and a few other soundings and some landmarks might be useful. For other maritime purposes a more detailed chart would be valuable. The first, however, would fail to give facts often demanded in the navigational use of the chart, and the second if carried to an extreme would make a chart difficult to use.