428 THE POPULAR SCIENCE MONTHLY. 



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 tliis 

 raetliod of measurement was the lamented Captain J. M, Gilliss, 

 U. S. N., who determined in this way the difierence of longitude be- 

 tween Santiago and Valparaiso, Chili." 



As soon as the Atlantic cahle was laid, in 1866, the Superintendent 

 of the Coast Survey took advantage of the opportunity to establish, 

 by way of Newfoundland and Ireland, the difierence between the me- 

 ridians of the British Islands and those of the United States. 



In 18G9-'70, a similar determination was made by different observ- 

 ers 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 inter- 

 mediate 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 : 



1866 west of Greenwich i"" 56" 1'.71 equal to 74 0' 25".65 

 1870 " " 4" 56 1'.70 " 74 0' 25".50 



1872 " " 4" 56 P.67 " 74 0' 25".05 



The instruments in common use for making observations to ascer- 

 tain 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 cir- 

 cuit 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 clironometer, which, with a small battery, is included in 



