Page:The National geographic magazine (IA nationalgeograph21890nati).pdf/21



Vol. II.

following definitions are given by Chauvenet in his Spherical and Practical Austronomy.

"The longitude of a point on the earth's surface is the angle at the Pole included between the meridian of that point and some assumed first meridian. The difference of longitude between any two points is the angle included between their meridians." To describe the practical methods of obtaining this difference or angle, by means of the electric telegraph both overland and submarine, and especially those employed by the expeditions sent out by the Navy department, is the object of this paper.

Before the invention of the telegraph various methods more or accurate in their results were employed, and are still in use where the telegraph is not available. The one most used and giving the best results was that in which a number of chronometers were transported back and forth between two places the difference of whose longitudes was required. "For," as the author quoted above says, "the determination of an absolute longitude from the first meridian or of a difference of longitude in general, resolves itself into the determination of the difference