88 ASTRONOMY. | 
meridian difference of two places, since it is equal to the difference of time 
given by two clocks keeping true time, at the respective places of obser- 
vation. This difference expressed in hours, minutes, and seconds, is con- 
verted into degrees, minutes, &c., by being multiplied by 15, and then will 
be equal to the above-mentioned angle formed by the two meridian circles 
at the pole, or the arc of the equator contained between these circles. 
The measurement of the distance of two meridian circles, however 
involves greater difficulties than the determination of the altitude of the 
pole. The meridian difference of two places is determined by the time 
required by a star, in the course of its apparent daily revolution, to pass 
from the meridian of the eastward place to that of the western. Should 
this star be the sun, we can only compare the times which the clocks in the 
two places show at one and the same moment. For since at each place the 
clocks are set 0, or 12, at the time the sun passes his meridian, these, if 
they keep accurate time, must always differ by the same number of hours, 
minutes, and seconds. There is therefore only needed some means by 
which the clocks and instruments used at the different places of observation 
may be compared with each other, and this may be done in two ways; first, 
by carrying one clock to the other, without changing their rate in the least ; 
or, secondly, by observing the clock time at which some phenomenon visible 
from both places, and determinable to the seconds, takes place. The 
difference in time obtained by either of these methods is then the desired 
meridian difference, or the difference in longitude, of the two places. We 
may also remark that the first method consists in the employment and 
application of portable clocks or chronometers. ‘The second, however, con- 
sists In the observation, a, of eclipses of the moon and of Jupiter's satellites ; 
b, of occultations ; c, of eclipses of the sun; d, of corresponding culminations 
of the moon and neighboring fixed stars; e, of lunar distances; and f, of 
gunpowder or other artificial signals. It is not our place here, to go over 
these different methods separately and circumstantially, nor to explain the 
mode of calculating the meridian difference desired. It must suffice to 
examine a little more closely one, and indeed a very simple method, namely, 
to show how from observations of the eclipses of one of Jupiter’s satellites 
in two places, the difference of their geographical longitude can be obtained. 
Let, for instance, in pl. 10, fig. 1, E be the place of one observer, V that of 
another, both points being on the surface of the earth. Let both observers 
note by their clocks the time of their place at which the eclipse of any one 
of Jupiter’s satellites takes place, that is, the time when the satellite begins 
to enter the cone of shadow ending at z. We will now assume that the time 
at E is 8 o’clock and the time at V is 10 o'clock, the difference of these 
two periods amounts to 2 hours 30°, therefore V is 30° east of EK. 
Unfortunately, the moments (beginning and end) of an eclipse of one of 
Jupiter’s satellites cannot be so readily determined as that of the moon, on 
account of the unequal illumination and magnifying power of the different 
telescopes, as also the different acuteness of vision of different observers. 
Consequently, the meridian difference deduced will not be accurate, or at 
any rate not reliable ; while the other methods admit of greater precision. 
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