CONTENTS.
On the Telegraphic Determinations of Longitude by the Bureau of Navigation: Lieut. J. A. Norris, U. S. N.
Reports of the Vice-Presidents:
Geography of the Land: Herbert G. Ogden
Geography of the Air:A. W. Greely, Chief Signal Officer, U. S. A.
Annual Report of the Treasurer
Annual Report of the Secretary
National Geographic Society:
Abstract of Minutes
Officers for 1890
Members of the Society
Published April, 1890.
THE
NATIONAL GEOGRAPHIC MAGAZINE.
ON THE TELEGRAPHIC DETERMINATIONS OF LONGITUDE BY THE BUREAU OFNAVIGATION.
The following definitions are given by Chauvenet in his Spherical andPractical Astronomy.
"The longitude of a point on the earth's surface is the angle at thePole included between the meridian of that point and some assumed firstmeridian. The difference of longitude between any two points is theangle included between their meridians." To describe the practicalmethods of obtaining this difference or angle, by means of the electrictelegraph both overland and submarine, and especially those employed bythe expeditions sent out by the Navy department, is the object of thispaper.
Before the invention of the telegraph various methods more or lessaccurate in their results were employed, and are still in use where thetelegraph is not available. The one most used and giving the bestresults was that in which a number of chronometers were transportedback and forth between two places the difference of whose longitudeswas required. "For," as the author quoted above says, "thedetermination of an absolute longitude from the first meridian or of adifference of longitude in general, resolves itself into thedetermination of the difference of the time reckoned at the twomeridians at the same absolute instant." If a chronometer be regulatedto the time at any place A, and then transported to a second placeB, and the local time at B, be determined at any instant, and atthat instant the time at A, as shown by the chronometer is noted, thedifference of the times is at once known, and that is the difference oflongitude required. The principal objection to this plan is that thebest chronometers vary. If the variations were constant and regular,and the chronometer always gained or lost a fixed amount for the sameinterval of time, this objection would disappear. But the variation isnot constant, the rate of gain or loss, even in the best instruments