Page:The National geographic magazine (IA nationalgeograph21890nati).pdf/25

 reliance was placed were decidedly in error. The result from the chronometric expedition in 1855 previously referred to differing over a second of time.

In constructing charts for use at sea, the accurate determination of latitude and longitude is of the utmost importance. The navigator starting on a voyage must know the exact position of his destination as well as the location of the dangers to be avoided. He must know the error and rate of his chronometer when he sets out, but as the rate is not constant he should have some means of re-rating it at any place where he may stop. If the longitude of this place is well determined, the operation of obtaining the error and rate is an easy one, and may save his vessel from loss.

Surveys, of coasts or countries most have well established starting points, and while the latitude of a place is comparatively easy to determine, the longitude, except when the telegraphic method is used, is attended with more of less uncertainty.

In 1878, Commodore R. H. Wyman, U. S. N. Hydrographer to the Bureau of Navigation, organized by permission of the Navy Department, an expedition for the telegraphic determination of longitude in the West Indies and Central America. The submarine cables of the West India and Panama Telegraph Co. had just been completed, extending from Key West through Havana and Santiago de Cuba, south to Jamaica and Aspinwall, and east through the Virgin and Windward Islands to the northeast coast of South America, thus affording admirable facilities for the accurate determination of many points. It had long been known that the longitudes of various points in the West Indies and in Central and South America, did not harmonize, there having been no systematic attempt to determine them with relation to each other or to a common base. Longitudes in the western part of the Caribbean Sea depended upon the position of the Morro lighthouse at Havana, which had been determined by occultations. Further to the eastward, positions depended upon that of Fort Christian of St. Thomas. This in its turn depended upon the observatory of Major Lang in the Island of Santa Cruz about forty miles distant. This position depended upon numerous observations of moon culminations and occultations. Martinique and Guadeloupe in the Windward Islands had been surveyed by French officers who based their positions upon longitudes derived from moon culminations. The absolute determination of these starting points would of course fix all points derived from them.