146 
HINTS TO TfeAVELLEM 
it has passed it by a like space of time. As the sun or star will be rising 
very slowly, the observations should be taken with deliberation, at about 
minute intervals. Should the sky become overcast, the observations on 
either side of the meridian can easily be reduced to the meridian altitude, 
and this circumstance adds considerably to the value of this class of 
observation, as the meridian altitude may be lost. 
A latitude obtained by either the meridian or circum-meridian altitudes 
of the sun, or stars, which are all on one side of the zenith, i.e. all either 
to the north or south of the observer, is liable to considerable inaccuracy 
from the existence of instrumental errors. 
To get a more certain result it is necessary to determine the latitude 
from the mean of results of observation of north and south stars, by 
which the instrumental errors are eliminated, and a very exact latitude 
obtained. 
By north and south stars are meant stars which pass the meridian to 
the north and south of the observer’s zenith. If their altitudes are nearly 
the same the exactitude of the result will be much increased, on account 
of the elimination of errors of refraction. 
Latitudes by stars of the same altitude north and south afford the 
traveller a fair means of ascertaining the centering error of his sextant 
for the altitude observed, which is one half the difference of the latitude 
by the respective stars. When the latitude resulting from the star on the 
equatorial side of the observer is less than that from the star on the polar 
side, the correction for centering error will be minus, and vice versa . 
The following will illustrate the manner in which this observation 
is taken. Suppose that on the 1st of December, 1881, we wished to fix the 
position of the Society’s Observatory in latitude, by north and south stars. 
On looking at the heavens we should see that y Pegasi and y Cephei were 
well situated for that purpose, and with these stars’ right ascensions and 
the sidereal time at mean noon (taken from the f Nautical Almanac ’), we 
should find that y Cephei passed the meridian, to the north , at 6h. 51m. 
24s., and y Pegasi to the south at 7h. 28m. 57s., thus leaving an interval 
of 82m. 33s. between the meridian passages. We should commence 
observing altitudes of y Cephei at 6h. 35m., and continue to do so until 
71i. 5m.; we should then turn to y Pegasi , and continue our observations 
of that star until 7h. 40m. We should then compute the latitude by each 
set of observations, and take the mean of their results as the true latitude. 
