Page:The American Cyclopædia (1879) Volume IV.djvu/779

 COAST SURVEY 763 from Dunkirk to the island of Formentera in the Mediterranean. Several arcs have also been measured in Germany ; and in fact there is hardly a civilized nation that has not con- tributed its share to our knowledge of this subject. It was in the first attempts to mea- sure the French arc that the science of geodesy originated. In the United States, an arc of the meridian has been measured by the coast survey, between the years 1844 and 1867. It extends from Nantucket in Massachusetts to Farmington in Maine, and is 3 23' in length. A second arc is in process of measurement be- tween Ocracoke inlet in North Carolina and the head of Chesapeake bay. The northern part, which extends over Chesapeake bay, was completed in 1871, and is 2 40' in length. The total extent of the whole arc when completed will be about 4 32'. Other arcs will be mea- sured as the geodetic work advances. Varia- tion of the Compass. It is well known that every navigator determines his course and di- rection by means of an instrument called the mariner's compass, which depends solely upon the direction which a small needle or bar of steel assumes when magnetized and left free to move in any direction. This direction is nearly north and south ; and it was formerly supposed that the north end of the needle pointed di- rectly to the north pole of the earth, to which ft was attracted by some powerful agent in the polar regions. Continued observation and study of the subject, however, disclosed the fact that this was not true. The real direction of the needle at any point between the equator and the poles is inclined to the horizon, and also to the true meridian ; or, in other words, the needle has a dip and a variation or decli- nation. At Key West, for instance, in 1858, the dip of the needle was 55 ; at Cape Hatte- ras, 68; at New York, 73; at Halifax, 76; while the declination was 5 E. at Key West, and 6 W. at New York ; and between these places there is a line where there is no variation from the true meridian. This line passes near Wilmington, N. C., Charlotteville, Va., and Pittsburgh, Penn. Going westward from this line, the declination of the north end of the needle to the east of the true meridian increases rapidly until the Pacific coast is reached, when it is from 15 to 20 E. It is found also that the direction at any one place is not constant, but that it changes not only during each day, but from year to year. In surveyors' and mariners' compasses the dip of the needle is overcome by suspending it from a point above its centre of gravity; but the declina- tion still exists, and gives rise to what is popu- larly called the variation of the needle. Sur- veyors on land have generally the means of as- certaining the variation by astronomical obser- vations, by which very great errors in their maps may be avoided ; but the common mari- ner has no such resource. In sailing from the gulf of Mexico to Portland, Maine, his compass would pass through all points from 5 E. to 13 W. How is he to know this, unless it be put down on his chart? And how will a chart for one year answer for another, when the varia- tion is constantly changing? It is plain that the laws which govern these changes must be found out, so that the changes may be predict-* ed. The determination of these laws, and the actual direction of the needle at various points of the coast at particular times, have occupied the attention of the superintendents of the coast survey. It is impossible to explain here the delicate and laborious nature of the obser- vations necessary for determining the direction and intensity of the magnetic force. The re- sults published by the coast survey have been obtained by comparing the observations of many years with the deductions of theory, and may be said to exceed in importance and use- fulness all that had previously been done in this country. From the results of observations made not only on the Atlantic and Pacific coasts, but also in the interior of the continent, a map has been constructed giving the lines across the continent along which the dip is the same, and along which the declination is the same, and corresponding lines for the intensity. From such results the laws are obtained for predicting future changes. From these facts the importance of a full investigation of a subject of such vital interest to navigation may be ap- preciated. As it forms a part of the duties of the coast survey to furnish the mariner with all the information possible on this subject, it ought not to be neglected even in a brief account of the work. Office Wort;. The ob- servations and charts made in the field are at the close of each season's work sent to the coast survey office in Washington, where they are combined to form the charts. The trian- gulation and astronomical observations are subjected to a double computation, first by the observers, and next by the computers in the office, in order to insure perfect accuracy and faithfulness. The topographical and hydro- graphical sheets are combined, and reduced drawings are made on scales suitable for pub- lication. A connected series of charts of the whole coast is projected on a scale of ^.Vw Besides these, local charts of harbors and bays are published on larger scales, adapted to the importance of the locality and amount of detail to be shown. General coast charts are also published on a scale of TW.Tmr- When the reduced charts are drawn and verified, they pass into the hands of the engraving division of the office, where they are executed on copper. When the plates are completed, electrotype copies are taken of them, which serve for printing the charts, the original plates being preserved in the archives. The electrotyping establishment of the coast survey is very extensive, and one of the most successful in the world. It was in practical operation several years before the process came into general use, and considerable ad- vances were made in the art at this establish-