Time Signal.—Many observatories send out time signals either daily, hourly, or sometimes continuously every second, or every other second, to various parts of the country for the purpose of giving accurate time to all sorts of industries. They are sent over the telegraph lines, the wires being permanently run into the observatories for the purpose, and the signals are generally sent automatically by a distributing clock which is kept as near the exact times as possible. An electric current passes through the clock and is broken or closed regularly by a toothed wheel on the second hand arbor of the clock. Perhaps the best known set of time signals is that sent out by the Naval Observatory at Wash ington. It is as follows: three or four minutes before noon, whenever the telegraph companies switch in the loops to the observatory, the clock begins to send out make-circuit signals every sec ond over the various lines, the minutes being indicated by leaving out the sec onds 55, 56, 57, 58, and 59 in each, and the half minutes by leaving out the 29th second of each. The click following such a one-second gap then always indicates the beginning of a half-minute, and the first following a gap of five seconds indi cates the beginning of a minute, except at the exact noon. Just before this there is a gap of 10 seconds, and then exactly at noon the circuit closes and remains closed for just a whole second, the be ginning of the mark indicating exact noon. The closing for a whole second is in order to make sure that that particu lar mark goes through all the telegraph lines, for the particular signal is made to do a great many things at different places, such as the dropping of time balls and it is more important that this partic ular second be distinctly sent than any of the others. After the break at the close of the noon signal the telegraph companies quickly switch out the loops to the observatory, and the lines imme iately resume their normal work. In the city of Washington this particular noon signal drops a time ball on the top of the State, War, and Navy Department building, and it also automatically cor rects, by setting forward or back ex actly to 0 hours 0 minutes 0 seconds, all the clocks in the department build ings of the government, no matter how much they may have gained or lost since the preceding noon.
Local, Universal and Standard Time.— Local mean time is that indicated by the transits of the sun at any particular meridian, and of course this differs for places of different longitudes on the sur face of the earth. In fact, the differ ence of longitude between two places is simply the difference of their local times, and the accurate determination of this difference is one of the most common kinds of astronomical work, especially in the principal observatory of any country. The operation consists simply in making the most accurate determination possible of the clock-corrections at each observa tory and then comparing the clocks by telegraphic time signa's with each other. But if this local time is used as the standard time at every place it cause: the greatest confusion to people travel ing from one place to another and to the railroads connecting them. England
early adopted the time of the Greenwich meridian for the whole country, and like wise France that of Paris. In the United States uniformity was more diffi cult on account of the great difference of longitude of its different parts. San Francisco time being nearly four hours slower than the local time in the E. part of Maine. But an excellent compromise was brought about in 1883, principally through the influence of the railroads, pushed on by a few scientific men, and it resulted in the present system of standard time throughouut the country. This is based on Greenwich time, and differs from it at any place by some whole number of hours, the minutes and seconds being the same over the whole country, and exactly the same as those of Greenwich. The E. part of the country uses time five hours slower than that of Greenwich, or Greenwich five hour time; i. e., when it is standard noon in the E. part of the United States it is 5 P. al. at Greenwich. In the Mississippi valley they use Greenwich six-hour time. The dividing line is not an arbitrary one, but is settled by the railroads, generally where they find it most convenient to change at the end of divisions of the roads. The Pennsylvania roads change at Pittsburgh. The cities and towns along the roads adopt the time of the nearest railroad, and no inconvenience results even where the difference of standard from local time is something more than half an hour. The only in convenience is in cities like Pittsburgh, where two different hours are used by different railroad systems centering there. But the trouble is infinitely less than it was when there were more than 70 different standards of railroad time in the country. Out on the plains they use Greenwich seven-hour time, and on the Pacific slope Greenwich eight-hour time. These four standards are commonly called the Eastern, Central, Mountain, and Pacific time, respectively. See DAY LIGHT SAVING.
While the above is not an ideal sys tem, it is about the best that can be used at the present time. If the people at large can ever be educated up to the standard of dissociating entirely 12 o'clock from noon, then we can adopt a universal time for all parts of the earth. This is already done by many scientific men, especially astronomers, meteorol ogists, and those having any thing to do with terrestrial magnetism, it being nec essary in all these matters to have a common universal time for use, and Greenwich time, either mean or civil, as defined above, is almost universally adopted for these purposes. Such a time is also a necessity in international tele graphic communication, and, once adopt ed, would be found of universal con venience. Its introduction would come about much easier if at the same time the 12 or 24-hour division of the day could be abandoned, and the decimal di vision of a universal day come into gen eral use. This, however, involves the abandonment of all existing time pieces and the purchase of new ones showing the decimal division of the day, and as this involves a large financial outlay it will be very difficult of introduction.