HYDROGRAPHY. Hydrography has been viewed by past generations of writers as comprehending all those branches of science which pertain to the waters of the earth's sur face. In the present generation, however, the description and knowledge of the oceans is em braced in oceanography; the description of the river systems, their function as agents for the sculpture of the land, and the conditions under which their courses are modified, and the de scription of the lakes, and the processes of their creation and extinction are viewed as sub jects of physical geography.; the treatment of rivers from the point of view of their manner of flow, their floods, and their employment as sources of power is designated as river hydrau lics; and the motions and oscillations of lakes and other bodies of fresh water, including their physical, chemical, meteorological and biological description, constitute the science of limnol ogy. Hydrography, in its definite application, comprehends those operations and investiga tions which relate to the representation and charting of the depths, shoals and shore lines of oceans, lakes, rivers, harbors and other bodies of water, mainly in the interests of navi gation and for the purpose of indicating the hidden dangers to be avoided and the channels where safety is to be sought in the guidance of shipping. It is not only necessary to designate theexact position and extent of dangers to navigation, but also to indicate the undulations and characteristics of the bottom which, being charted, frequently enable the navigator, by casts of the lead or sounding-plummet, to rec ognize his position, or the course he is follow ing, and which, at night, are often the only gtudes to an anchorage.
History.— There came a time in the history of the world when commerce was pushed into the sea. There was an age in which, by two world-old high roads stretching across Persia, the slow caravans passed and repassed bearing silks and spices and merchandise from Central Asia and India to Syria and the West. Then came the sweeping hordes from the northeast to interpose a barbaric wedge which practically shut the main gates of the Eastern trade; and Europe began to find the way to India and China in ships. Long before the rise of Greek civilization, and long before the oldest Greek and Hebrew records, Phoeniciannavigators, having emigrated from the shores of the Ery thrman Sea or Indian Ocean, were making voy ages over the Mediterranean Sea and through the Pillars of Hercules or the Strait of Gibral tar into the Atlantic Ocean. And, after the Phoenicians, the Carthagenians, animated by the same spirit of commercial enterprise, con tinued to conduct voyages which might rival those of much later centuries. These experi ences must necessarily have produced sixty° knowledge of the configuration of the coasts with which the Phoenicians and Carthagenians were familiar and of the depths of the border ing waters, but this knowledge, whatever its ex tent, was not transmitted to us, and it remained to later centuries to provide those foundations of knowledge upon which the superstructure of hydrography has been raised.
Although the Greeks must be regarded as the founders of scientific geography, they are not known to have undertaken voyages of dis covery before the 4th century a.c., when they sent an expedition to the North Sea under the direction of the illustrious astronomer and mathematician, Pytheas, who, at that distant period, had determined the latitude of seilles with such exactitude that 20 centuries afterward Gassendi found it correct to within a few seconds. There is no doubt that, before this voyage, the chart of the seas to the west of Europe was almost a blank, and that, down to the time of Strabo, it retained the form given to it by Pytheas. Hydrography is also indebted to the solicitude shown for the sciences by the Ptolemies, the successors of Alexander in Egypt. Timosthenes, a native of Phodes, who was admiral of the Egyptian fleet under Ptol emy Philadelphus, drew up a treatise "Concern ing Ports) which may be regarded as the precursor of the modern sailing directions issued by the hydrographic offices of the mari time nations. Although the Romans carried
their arms throughout nearly all the world known to the ancients, including in their rule a great extent of coast bordering on the At lantic, they never organized any voyages of discovery into this outer sea, after the manner of the Carthagenians and Greeks.. It was near the beginning of the Christian era when the celebrated Grecian geographer, Strabo, sug gested that besides the world known to the Greeks and Romans, other continents or other worlds might yet be discovered inhabited by different races of men. Strabo does not appear to have been acquainted with Hippalus, an tian navigator, living about the same time, who provedthe regular alternations in the di rection of the monsoons of the Indian Ocean, and profited by the discovery to open up a route across the high seas between the shores of the Red Sea and India. Coast routes, followed up to this time, were abandoned, and a fresh im petus was given to voyages in oriental waters. Nevertheless, 150 years afterward, Ptolemy, whose great authority gave his views a scien tific stamp, rejected the hypothesis of an ocean extending to the east of the Asiatic continent and regarded these great land masses as ex tending indefinitely toward the north and east. His maps, in which he accomplished so great an advance in projection by and longi tude, show also that he shared the view of many of his predecessors in representing the Indian Ocean as a sea enclosed by an exten sion of Africa eastward to China. This South ern Ethiopia remained on maps down to the time of the second voyage of Captain Cook. Two principal views prevailed among the an cients regarding the distribution of coasts and seas. The school that may be called Homeric — to which Eratosthenes and Strabo belonged — considered the three continents of the. Old World as forming a single island surrounded by the ocean. On the other hand, the ad herents of what may be called the Ptolemaic school — to which Hipparchus belonged — did not admit the extension of the sea around the known world. They considered the Atlantic and Indian oceans to be great enclosed seas like the Mediterranean; they held that the extreme points of the known lands toward the east and the west approached so nearly to each other that a ship, parting from the west, might easily reach the eastern extremity, which they re garded as greatly extended. This error was perpetuated, thanks to the influence of Ptolemy, and led directly, 14 centuries afterward, to the discovery of the New World by Columbus. When the barbarians invaded and overran Europe in the 4th and 5th centuries, the maps and other scientific works of ancient civiliza tion were destroyed or lost sight of for centu nes afterward. In the Middle Ages, the Arabs extended their voyages as far as China in the East, and, in the West, they were acquainted with the whole of southern Europe and of northern Africa. It is to their intercourse in the 10th and 11th centuries that the knowledge leading to the introduction of the mariner's compass is believed to have been conveyed from China to The ancients, who made so many excellent circumnavigations of the Med iterranean, never constructed general or coast charts. The marine compass-charts of the Middle Ages therefore demonstrate a great progress in knowledge in relation to coast charts which were based on rounds of angles taken from successive positions by means of the compass. The compass-charts, or porta lani, a name applied both to the charts and the accompanying sailing directions, made their appearance in Italy in the 13th century. The most remarkable of these compass-charts is known as the Catalan chart. It is a map of .the world in six sheets, dated 1375, and is at once a planisphere and a marine chart. It indicates in particular the new islands discovered in the Atlantic Ocean, and shows much progress in the representation of the Indian Seas. It is on this chart that India appears for the first time as a peninsula, and the Indian Ocean is no longer an inclosed sea as had been previously represented.