Page:The American Cyclopædia (1879) Volume IX.djvu/121

HYDROGRAPHY 113 home coasts, and their surveying vessels fre- quent all parts of the globe, and penetrate seas hitherto almost unknown, mapping the limits of harbors, determining with precision the geographical position of headlands and en- trances, and of rocks, shoals, and sands, many of them hitherto unknown. In this science England is far in advance of all other nations. Not content with a most complete and admira- ble survey of her own coasts, she has extended her work to all of her possessions and to the coasts of foreign nations. Many eminent sur- veyors are numbered among her naval officers ; but it is probable that few have done so much or displayed so much zeal and devotion to the science as the late Admiral Beaufort, so long at the head of the hydrographic office of the admiralty. His surveys were sometimes ac- tually carried on at his own expense. Much importance is attached to the results expected from the scientific cruise of the British ship Challenger, which at the present time (1874) is engaged in a voyage around the world, probably the most important of its kind ever undertaken. She carries a large number of men familiar with almost all the branches of science and art, whose labors, it is hoped, will be productive of much information in natural science and in marine surveying and deep-sea dredging. Although surpassed by England in the number and completeness of her foreign surveys, the hydrographic work on our own coasts is unequalled for accuracy and rapidity of execution. Under the charge of the coast survey of the United States it has progressed in company with the trigonometrical and topo- graphical work of that service ; and it is safe to assert that the completed charts of the coast and the various harbors stand alone in the an- nals of surveying for beauty of execution, ac- curacy, and completeness of detail. A large corps of skilled professional hydrographers are constantly employed prosecuting the survey* of the numerous harbors on the Atlantic, Pa- cific, and gulf coasts; and others are engaged in deep-sea explorations along the course of the Gulf stream, in the gulf of Mexico, and on the coasts of California and Oregon. These deep-sea expeditions have been especially use- ful in determining the routes suitable for sub- marine cables, several of which have been laid over lines previously sounded and surveyed by officers belonging to the coast survey. One of the most successful hydrographic expeditions of modern times was that undertaken between 1851 and 1853 under the auspices of the coast survey of the United States, by Lieut, (now Rear Admiral) James Alden of the navy, in the schooner Ewing and steamer Active. More than 1,300 m. of the Pacific coast was ex- plored, from lat. 32 30' to 48 20' N., and the geographical positions of all the prominent headlands and of the entrances to the har- bors were determined by astronomical observa- tions, from the southern boundary of the Uni- ted States to the strait of Fuca; lines of soundings were carried along the coast through- out its entire length, and hydrographic recon- noissances made of most of the harbors, with ac- curate views of the different entrances and of prominent points on the coast ; and subsequent careful detailed surveys, based upon accurate geodetic determinations, have failed to change the results of this work in any important par- ticular. The immediate result of this recon- noissance was the publication of a chart of the Pacific coast for the use of mariners, and sub- sequently of a marine directory, which has since been elaborated and published as a " Coast Pilot of the Pacific Coast of the United States." The method of hydrographic survey- ing, as now practised both in this country and in Europe, is as follows : 1. Reconnaissance, as, for instance, the hydrographic survey of a har- bor on a foreign coast, or any place where ac- curate geodetic information cannot be obtained. The hydrographer, obliged himself to make all the determinations of points on shore and the outlines of the coast, applies the principles of geodesy and topography, but of course in a com- paratively rude manner. A base line may be measured, if on land, in the ordinary way ; but if the working ground is so far from shore as to render points on shore useless (as is sometimes the case in surveys of shoals off a low and flat coast), or if the coast is occupied by an enemy, a base line is sometimes measured by anchoring a boat at each end of it, and noting the interval between the flash of a gun fired from one boat and the report as heard at the other. But this very rude method is only ad- missible where no other is possible. Where the surface to be surveyed is small, good re- sults have been obtained from a base line mea- sured by a cord, the two ends being marked either by boats or buoys. Signals are erected at each end of the base line and on prominent points along the shore, the latter being deter- mined by horizontal angles measured from each end of the base line. Not only the angle be- tween each end of the base and each signal is measured, but the angles between the differ- ent signals themselves ; and the triangles thus formed are either computed by trigonometry or platted by intersections upon the chart. The latitude and longitude of some prominent points are also determined. The outlines of the coast or harbor are drawn between inter- mediate points determined by horizontal angles, and the chart is then ready for platting the sounding lines. Next, a tide gauge is erected. This is generally a plain staff, graduated to half feet; and by continuous observations of the rise and fall of the tides, and of the times of high and low water, the hydrographer ob- tains an approximate establishment for the port, and also the means of correcting his soundings for the rise of the tide, which is called "reducing them to the level of low water." The shore line having been rudely determined, and such natural and artificial features mapped as may be considered neces-