Page:The American Cyclopædia (1879) Volume IV.djvu/775

 COAST SURVEY 759 sured with theodolites more portable than the one above mentioned, the circles varying from 6 to 12 inches in diameter according to the dis- tances to be observed. These triangulations determine the relative positions of prominent points, at distances averaging from two to five miles, and thus form a skeleton map, in which the details of configuration of shore line and surface remain to be supplied by the topogra- pher. If the figure of the earth were precisely known, it would suffice, in order to fix the sit- uation of the whole network of triangles on the surface of the earth, to determine the latitude and longitude of some one point, and the direc- tion with reference to the true meridian of one of its lines, i. e., its azimuth. But, owing to the irregularities of the earth's figure, it is found necessary to repeat the determinations very frequently, by which means the work is checked and any accumulation of error avoid- ed. Latitude. All the known methods of determining latitude have been used in the coast survey, for the purpose of ascertaining their relative merits. Large vertical circles or repeating theodolites were first used for ob- serving double altitudes ; but these made way for more perfect and larger instruments, the zenith sector, the prime vertical transit, and the zenith telescope or equal altitude instru- ment. The first mentioned of these instru- ments, constructed according to the plan of Prof. Airy, gives very satisfactory results; but it is rather heavy for transportation, and the observations are laborious. The method of de- termining latitudes by observations of transits of stars over the prime vertical, first used by Bessel, is admirable in theory, but in practice a great loss of time and labor is often caused by clouds. The zenith telescope, or equal altitude instrument, was first applied to the determina- tion of latitudes by Capt. A. Talcott, a former officer of the United States army ; it has been remodelled and specially adapted to the pur- pose in the coast survey, where it has become a favorite instrument on account of the great number and accuracy of the results that can be obtained in a given time, and the facility with which it is used. A large number of observa- vations (from 150 to 200) are made at each sta- tion, and the mean is taken as the result, the uncertainty of which is thus reduced to a small fraction of a second. Mr. George Davidson has devised an instrument, now in successful use both on the Atlantic and Pacific coasts, which combines in a simple and beautiful manner the principles of both the zenith telescope and the transit instrument, so that observations for lon- gitude and latitude may be made by the one in- strument at the same station. This instrument gives general satisfaction for its compactness, delicacy, and accuracy, and will doubtless soon be in general use. Longitude. For the pur- pose of determining the longitude of all points in the coast survey, with reference to Green- wich and other European observatories, it is only necessary to ascertain that of one point, the difference of which from all others is known by the triangulation, and by differences of lon- gitude obtained by means of the electric tele- graph. The observatory of Cambridge, Mass., has been used as such a point of reference. Its longitude from Greenwich has been determined by astronomical observations of eclipses and occultations of stars by the moon, and of moon culminations, made not only there, but at other observatories in the United States, the longi- tude of which from Cambridge has been deter- mined by electric telegraph. Besides the as- tronomical methods, a chronometric method has also been used. A large number of chro- nometers have been transported repeatedly and in different years between Cambridge and Liverpool, for the purpose of comparing the time of the observatories at those places. The results of these expeditions, in which special regard was had to the effect of temperature on the rates of the chronometers, show a pretty ' close agreement in different years, but differ from those by astronomical observations by about two seconds of time. The telegraphic determinations of longitude have been ex- tended from Washington northward to Phila- delphia, New York, Albany, Cambridge, Ban- gor, Calais, and Halifax ; southward to Peters- burg, Wilmington, Charleston, Savannah, Pen- sacola, Mobile, and New Orleans; and west- ward to Cincinnati, St. Louis, Salt Lake City, and San Francisco. Most successful results have also been obtained from the telegraphic campaigns between Heart's Content and Valen- tia over the British Atlantic cable ; and equally good determinations have been secured between Havre, France, and Duxbury, Mass., over the French submarine cable. The results have been entirely satisfactory, and have given the difference of longitude between Greenwich and Cambridge within the smallest limits of error. Through these methods the longitude of San Francisco on the Pacific coast is as accurately known as that of Cambridge or Greenwich. In the. telegraphic method, which is by far the most accurate for determining difference of longitude, the coast survey has taken the lead, and has brought it to a state of perfection which subsequent operations of a similar na- ture executed in Europe have not yet reached. The idea of comparing the local time of differ- ent places by means of the electric telegraph is sufficiently obvious, and dates from the con- ception of the telegraph itself. But the re- fined methods by which the intervention of human senses and operations, and the conse- quent liabilities to error, are in the greatest possible degree avoided, and by which the time of transmission is measured and eliminated from the longitude, have been the result of careful study and long experience. The first experiments on the velocity of galvanic signals were made in 1848. They have since been frequently repeated ; and the velocity is found to vary from 10,000 to 20,000 miles per second, according to the size and kind of wire em-