TELEGRAPHIC DETERMINATION OF LONGITUDE. 427 



reference to the meridian of some other place; being measured by- 

 time, is determined by the comparison of the local time with the 

 time at some other place, the longitude of which is known. 



Discrepancies in the results of observations for the determination 

 of longitudes seem unavoidable with most of the metliods usually em- 

 ployed, such as transportation of chronometers from place to place, 

 observations of the relative positions of the moon and stars, and ob- 

 servations of occultations and eclipses. 



Until the completion of telegraphic connection between this coun- 

 try and England, the exact longitude of the Washington Observatory 

 W'ls quite uncertain. A great many transfers of chronometers across 

 the Atlantic had been eftected by the Coast Survey at a great expen- 

 diture of labor and money. Yet the result of the latest expeditions 

 diftered from that deduced by Prof. Newcomb from moon-culminations 

 by more than three and a half seconds of time, equal to nearly a mile, 

 the final telegraphic determination lying between the two results. 



In other parts of the world, however, the discrepancies are much 

 greater. On the southern shore of the Caribbean Sea, an uncertainty 

 of five or six miles exists with regard to many positions, and some of 

 the islands in the Pacific Ocean have had longitudes assigned them by 

 different surveyors within the last fifty years differing by as much as 

 twenty-seven miles. 



Where, however, chronometers have to be carried only a short 

 distance from an established meridian, the results are much more ac- 

 curate. In 1852, the longitude of Key West was measured by Coast- 

 Survey ofiicers from Savannah (previously established by telegraph 

 from Washington), and was found to be 81 48' 30."7. In 18V3, by 

 telegraph, Washington to Key West 81 48' 27."2. It will be seen 

 that tlie difference between these results is only 3. "5, equal to about 

 100 yards, and that the statement lately published in Appletons' 

 JouEXAL, that the recent telegraphic determination showed the for- 

 mer position to be several miles in error, is incorrect. 



Of late years the establishment of telegraphic connection between 

 so many points of the earth's surface, both by submarine cable and 

 by overland lines, has added to the modes of determining longitudes 

 anotlier, by far the most simple, elegant, and accurate. 



This method can, however, only be used between places having 

 telegraphic communication with each other ; bvit the exact determina- 

 tion of these meridians renders easy the correction of errors in the 

 longitude of neighboring places. 



The establishment of differences of geographical longitude by the 

 electric telegraph and of geographical latitudes by the zenith tele- 

 scope constitute tw^o of the most important improvements in practi- 

 cal astronomy of modern times, and both have had their origin in the 

 United States. To the skillful and indefatigable astronomers of the 

 Coast Survey and those of the corps of United States Engineers are 



