Page:Popular Science Monthly Volume 7.djvu/443

Rh 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 methods usually employed, such as transportation of chronometers from place to place, observations of the relative positions of the moon and stars, and observations of occultations and eclipses.

Until the completion of telegraphic connection between this country and England, the exact longitude of the Washington Observatory was quite uncertain. A great many transfers of chronometers across the Atlantic had been effected by the Coast Survey at a great expenditure of labor and money. Yet the result of the latest expeditions differed 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 accurate. In 1852, the longitude of Key West was measured by Coast Survey officers from Savannah (previously established by telegraph from Washington), and was found to be 81° 48' 30."7. In 1873, by telegraph, Washington to Key West 81° 48' 27."2. It will be seen that the difference between these results is only 3. "5, equal to about 100 yards, and that the statement lately published in that the recent telegraphic determination showed the former 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 another, by far the most simple, elegant, and accurate.

This method can, however, only be used between places having telegraphic communication with each other; but the exact determination 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 telescope constitute two of the most important improvements in practical 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