There are still other methods, depending upon measurements of the moon's position by observations of its altitude or azimuth. In all such cases, however, every error of observation entails a vastly greater error in the final re sults. Lunar methods (excepting occultations) are only used when better ones are unavailable.
Finding Longitude by Mechanical Meth ods.— 1. By the Chronometer; which is simply an accurate watch that has been set to indicate Greenwich time before the ship leaves port. In order to find the longitude by the chronom eter, the sailor has to determine its (error" upon local time by an observation of the alti tude of the sun when near the prime vertical. If the chronometer indicates true Greenwich time, the error deduced from the observation will be the longitude. Usually, however, the indication of the chronometer face requires correction for the rate and run of the chro nometer since leaving port. 2. The Telegraph. —When we wish to find the longitude between two stations connected by telegraph, the process is as follows: the observers at both stations proceed to determine their own local time by a series of star observations with the transit in strument. Then, at a time agreed upon, the observer at the first station (switches his clock* into the telegraphic circuit, so that its beats are communicated along the line and received upon the chronograph of the second, or say the Western station. After the Eastern clock has
thus sent its signals, it is switched out of the circuit, and the Western observer now switches his clock into the circuit, and its beats are re ceived upon the East chronograph. The oper ation is closed by another series of star obser vations. We now have upon each chronograph sheet an accurate comparison of the two clocks, showing the amount by which the West ern clock is slow of the Eastern. If the trans mission of electric signals were instantaneous, the difference shown upon the two chronograph sheets would exactly agree. Practically, how ever, there will always be a small discrepancy amounting to twice the time occupied in the transmission of the signals; but, the mean of the two differences will be the true difference of longitude of the places after the proper cor rections have been applied. Care must be taken to determine with accuracy and eliminate the personal equations of the observers. See LATITUDE.