AN accurate comparison between two astronomical cloeks, situated at a great distance from each other, can be made by overland telegraphy, by means of which the time signals given by both clocks are recorded, side by side, on an electric chronograph at each station. The ordinary electric chronograph is provided with two pens, or writing points, operated by electro-magnets, so that each pen makes a distinctive mark on the same unifor)ly rotating cylinder or moving strip of paper, when the circuit of its electro"magnet is made or broken. Usually one pen marks the time by the local clock, while the other records signals made by the observer, on the passage of a star over the wires of the transit instrument or the occurrence of any other event. For the determination of longitude, the electro-magnet of the second pen is connected with the clock at the distant station. In order to determine the difference of longitude between two stations by this method, it is necessary to transmit from eaeh station to the other currents ( Contiim·led on page SO/,)