cx 
APPENDIX. 
Compasses ; the Brass Plate and Compass were usually placed on a Copper- 
fastened Stool, 
Unless where a particular notice is made in the Column of Remarks, the 
Variations are the results of Observations at suitable hours of the Magnetic 
Bearing of the Sun’s Centre, (corrected for a small Index Error of the 
Compass, determined for each respectively by Captain Kater himself,) com- 
pared with the true Azimuth deduced either from corresponding Altitudes, 
or from the time of the Observation being noted by ^ Watch, whose Error on 
Mean Time was known. 
The latter method is said to be but rarely practised at Sea ; it may, how- 
ever, frequently be of service when the former and ordinary one is inappli- 
cable ; good opportunities will sometimes occur of observing the Sun’s 
Magnetic Azimuth, when the Horizon is either uncertain from Fog, or inter- 
cepted by Land, and when consequently the Altitude cannot be observed ; 
moreover, in laying down a Coast by Astronomical Bearings whilst running 
along it, a knowledge of the Sun’s Azimuth is frequently desired, not only at 
the times when it cannot be obtained from the Altitude by reason of the 
causes beforementioned, but also, and more commonly, at others when the 
Sun’s Motion in Altitude is too slow to enable the bearing to be deduced from 
it with sufficient correctness ; on such occasions the instant of Observation 
being noted by a Chronometer, or a steady-going Watch, whose Error on 
Time may be ascertained at a more suitable part of the day, (the difference 
of Longitude caused by the Ship’s intermediate run being allowed,) the 
Azimuth may be deduced corresponding to the Observation. 
There are some respects also in which this method is preferable at all times 
to the more usual one ; the Observation is made, generally, in much less 
time ; and the observer is independent of an assistant, since he can note the 
time of his own Observation himself both conveniently and accurately, and 
he has thus the additional advantage of depending on himself alone for the 
correctness of both the observed elements. 
. It appears desirable, therefore, that in works on Practical Navigation^ 
which purpose to contain short and convenient Rules for all such Calculations 
as are most useful at Sea, a method of deducing the Azimuth from the Hour 
Angle should be given, as well as from the Altitude. 
The Mean of several Observations, usually from six to twelve, was taken 
for each Result ; the Compass being removed and directed afresh between 
every Observation. 
