OBSERVATIONS FOR TIME AND LONGITUDE, 
197 
In the following:— 
JR indicates right ascension of the heavenly body, 
5| „ the moon’s bright limb. 
T' „ approximate longitude in time, 
T „ longitude in time. 
C „ the difference of JR. 
B „ the mean of the second differences of JR. 
The Observation:— Having the instrument set in the plane’of the 
meridian, proceed as follows:— 
From the list of ff Moom Culminating Stars/’ given in the ' Nautical 
Almanac/ select the fetar whose transit you intend to observe, and calcu¬ 
late the local mean time of its meridian passage in the manner shown on 
p. 140. Take from the f Nautical Almanac/ page IV., the moon’s meridian 
passage (upper), and from this subtract the time of the moon’s semi¬ 
diameter passing the meridian, before full moon , but add it after full 
moon , the result will be the mean time of transit of the moon’s bright 
limb; but if the meridian of place of observation is at any great distance 
from the meridian of Greenwich, or any other meridian, from which the 
difference of the longitude is to be found, then it will be necessary to 
correct this in the manner shown in the explanation of page IV., given at 
the end of the f Nautical Almanac.’ All this should be done some time 
before the transits are to be observed. 
If the instrument is fitted, as it should be, for taking transits, it will 
have four wires, one horizontal and three vertical, in the place of the 
usual web, and the exact time of the contact of both the moon’s bright limb 
and the star must be observed at each of the three vertical wires, and the 
means taken as the true time of observed transit. Be sure to be ready 
at the instrument some time before the first object comes to the meridian, 
and make a note of the difference between the declination of the moon 
and the star* as when the moon transits before the star, it will only be 
necessary to move the vertical circle by that amount to ensure the star 
coming into the middle of the field, but if the star transits first, its alti¬ 
tude must be computed beforehand, and for this the latitude must be 
known, thus:—Add together the complement of the latitude of the place 
of observation and the declination of the star, when they are of the same 
name, or taking their difference when of contrary names ; the altitude to 
be reckoned from the south point of the horizon when the latitude is 
