In the present advanced state of knowledge it may be useless to dwell upon the importance to navigation, as well as to general geography, of correct information relative to the latitudes and longitudes of the principal places on the surface of our globe. The ease with which the situation of a place
on
the meridian is obtained, for general purposes, is well known, and the comparative difficulty of ascertaining the distance, east or west,
from
a given meridian is equally so, particularly where that meridian is a quarter of the globe distant, which is the case as relates to India. Having, however, one point correctly determined, the situations of others, at moderate distances from it, may be come at with greater facility; either by chronometers, by correspondent observations, or, where places are on the same continent, by actual survey. One of the best methods of determining the position of a point, thus distant from the first meridian, is by eclipses of the satellites of Jupiter. Correspondent observations of eclipses of the sun, of the moon, or of occultations, happen but seldom, and the method by the moon's transit requires, that the position of that luminary should be correctly set down in the Tables; or, in the case of correspondent transits, that the instruments at both places should be most accurately placed in the meridian, and the transits taken with the least possible error of observation; as only a very small error in the Tables, or in the observed place of the moon, may produce a considerable one in the result. But eclipses of the satellites of Jupiter occur often, and correspondent ones with those taken at Greenwich, are not
very
unfrequent, even in this distant part of the globe. The observations taken at Greenwich also show the difference or error of the Tables, and consequently, the error of the longitude deduced from them. Errors also which may arise from a difference in the powers of the telescopes, and in the eyes of observers, as well as from a general difference in the state of the atmosphere, may be counterbalanced by taking a series of these eclipses, consisting of immersions as well as emersions.