Page:Popular Science Monthly Volume 7.djvu/444

428 due the introduction and perfection of the instruments and methods now employed, which make the results so accurate and the work so simple. "Among the very earliest of the astronomers to introduce this method of measurement was the lamented Captain J. M, Gilliss, U. S. N., who determined in this way the difference of longitude between Santiago and Valparaiso, Chili."

As soon as the Atlantic cable was laid, in 1866, the Superintendent of the Coast Survey took advantage of the opportunity to establish, by way of Newfoundland and Ireland, the difference between the meridians of the British Islands and those of the United States.

In 1869-'70, a similar determination was made by different observers through the French cable from Duxbury, Massachusetts, to Brest. Again, in 1872, the measurement was made through the same cable, using the island of St. Pierre, in the Gulf of St. Lawrence, as an intermediate station.

The exquisite accuracy of the results of these measurements is demonstrated by their accordance. Referring them to the station of the New York City Hall, the resulting longitudes are as follows:

The instruments in common use for making observations to ascertain the difference of longitude between two stations are, at each of the stations, a transit instrument, a break-circuit sidereal chronometer, and an electric chronograph; with the usual telegraphic sending and receiving instruments.

The transit instrument is a telescope, capable of being mounted accurately and firmly in an exact north-and-south line, so that the precise local time may be determined by the passage of well-known stars across the meridian.

The chronometer is adjusted to keep sidereal time and is furnished with an attachment by which the mechanism breaks an electric circuit every second.

Chronographs for the automatic registering of the exact time of any occurrence are constructed in various forms. Those generally used by astronomers in this country consist of a train of wheel-work driven by a weight, and causing a cylinder covered with a sheet of paper to make exactly one revolution in a minute.

A little carriage, to which a pen of peculiar construction is attached, moves upon wheels along the cylinder in the direction of its length, about one-tenth of an inch for each revolution of the cylinder, so that the pen records a perpetual spiral. The pen is so mounted as to have a slight lateral movement, and is so attached to an electro-magnet that, when the electric circuit in which it is placed is broken every second by the chronometer, which, with a small battery, is included in