170 
HINTS TO TRAVELLERS, 
appear on the other side of the moon; note this time also. Either of 
these observations is sufficient to determine the G M T,, and thence the 
longitude, in the manner shown in the example. When the star is 
occulted by the moon’s dark limb, the observation will afford most 
decisive results. At or near full moon a star occulted by the bright 
limb is not so easy an observation. The description of a telescope 
suitable for this observation is given on pp. 7, 8. The example given is 
computed by Raper’s rule and tables. It will be observed that several 
of the logs can be taken at one opening of the book, and as only four 
places of decimals are used, the log sines, cosines, &c t , can, in most 
cases, be taken at sight to the nearest 30"; this is not, however, the 
case with the proportional logs; where they occur the strictest accuracy 
must be observed, and the decimals of seconds must not be neglected. 
This remark also applies to the Moon’s Declination, Right Ascension, 
Horizontal Parallax, and Semidiameter. 
This observation is much easier, and more certain in its results, than 
the lunar observation. As the instrument (the telescope) is one that 
every person can use, and is not liable to any error, all that is required 
is that the observer shall be certain that one instant he does see the 
star and that the next instant he does not (with an emersion the exact 
contrary is the case). Neither is there much difficulty in recognising 
the star, as the moon only moves its own diameter among the stars in an 
hour, and there is ample time after the star and moon are in, apparent, 
close proximity to make sure of the star. Before, or immediately after 
this observation, a set of sights should be taken to find the error of the 
watch on apparent or mean time at place, 
