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POPULAR ASTRONOMY
two planes, or through the two transit instruments, at the same time. The interval of time will depend entirely on the inclination of the meridian at these two places. If, then, we erect a transit instrument in one place, and another transit instrument at another place, and compare the times at which the same star passes the two transit instruments, we have the means of seeing how much the planes of the meridian are inclined. Its makes no difference whether we suppose the earth to turn round so as to bring the plane of PL to pass through S, or suppose the star to turn round the earth, so as to make S pass through the plane of PL; the result is just the same. Now there are various ways[1] in which the comparison
- ↑ Since the delivery of these Lectures, the comparison of clock-times, by means of the Electric Telegraph, has been applied to the determination of the difference of longitudes, first in America, and subsequently in Europe. The mode of procedure is as follows: suppose it is desired to ascertain the difference of longitudes of Paris and Greenwich. At the moment when a star crosses the meridian of Paris the clock-time is observed; and simultaneously the observer, by means of the Electric Telegraph, transmits a signal to Greenwich; the time at which the signal arrives at the latter place is noted by an observer there. If electricity travelled instantaneously from the one place to the other, the difference in hours, minutes, and seconds, between the times at which the signal was despatched and received, would be exactly the difference between Paris and Greenwich time; but, the velocity of the electric fluid being finite, the quantity in question is less than the difference between Paris and Greenwich time, by the time occupied in the transmission of the signal: less, because Greenwich time is behind Paris time. Again, when the star reaches the meridian of Greenwich, a signal is in like manner sent to Paris by the observer: the difference in hours, minutes, and seconds, between the times of its being sent and received is now greater than the difference between Paris and Greenwich time by the time occupied in the transmission of the signal: greater, because Paris time is before Greenwich time. The mean between
