OBSERVATIONS for time and longitude. 195 
Longitude by Moon Culminating Stars. 
The observation can be taken with the transit theodolite, which must, 
however, be accurately set up in the plane of the meridian. This can be 
done by either of the following methods :— 
By Meridian Passage of the Pole Star .—Find the mean time of the 
meridian passage of the pole star in the manner shown on p. 140. 
Level the instrument, and if this be carefully done the line of collimation 
will move in a plane perpendicular to the horizon, and will pass through 
the zenith, then by making it also pass through the celestial pole, and 
clamping the horizontal plates when it is in that position, the movements 
of the telescope will be restricted to the plane of the meridian. This is 
done by turning the telescope on to the pole star, and covering it with 
the point of intersection of the telescope wires at the time (previously 
ascertained) of its upper or lower, culmination, and then firmly clamping 
the horizontal plates. The meridian line should now be laid out to the 
north and south of the observer by sending a man with a lantern and a 
staff in both directions, and making him drive the staff into the ground 
at the spot where the observer sees the lantern in a central position on 
the cross wires of the telescope. 
By High and Low Stars— -This method is accurate, and will be found 
convenient when the pole star cannot be observed. Having placed the 
instrument approximately in the meridian, choose two stars differing 
considerably in declination, and but little in right ascension. Note care¬ 
fully the time that each star passes the central wire; take the difference 
of these times, to which apply the rate of the watch, due for the interval, 
and convert this into a sidereal interval by Table XXXI., or by the 
f Nautical Almanac ’ table of time equivalents. Take from the f Nautical 
Almanac ’ the apparent right ascensions of the stars, and subtract the 
less from the greater. If this difference agrees exactly with the sidereal 
interval obtained by the watch, the telescope will move in the meridian, 
but when the transit of the high star has been observed first, and this is 
not the case, and the interval shown by the watch is less than the differ¬ 
ence of the stars’ right ascensions, the telescope must be moved to the 
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