APPENDIX NO. X. 
401 
inclined to the horizon; and the azimuth of any object was determined 
from the transit of the sun’s first and second limb over the vertical 
wires of the instrument. The time being known, the aeimuth of the 
zero of the limb is easily calculated, and nothing remained but to 
measure the horizontal angle between that direction and any object 
the astronomical bearing of which was desired. The azimuth is reckoned 
from north by east round to 3G0°. As objects for azimuthal determi¬ 
nation, well-defined glaciers, bluffs, islands, prominent capes, and the 
most distant headlands, were selected; and, in order to make sure of the 
stability of the instrument during the period of observation, a second 
set of observations of the sun for azimuth of zero of limb was obtained. 
By means of two positions thus determined, a number of objects 
were located by the intersections of the bearings of the known points, 
and whenever practicable a third or check azimuth was obtained; in 
this latter case any discrepancy was properly taken into account accord¬ 
ing to known principles. 
In observing with the sextant for altitude of the sun, the usual 
precautions were taken, and in particular the parallelism of the upper 
and lower surfaces of the covering-glass of the artificial mercurial hori¬ 
zon was tested. An error of ten seconds, it is thought, cannot exist on 
this account, although another roof gave results differing as much as 
fifteen minutes in the direct and reversed position, and consequently 
had to be rejected. 
The sextants used were made by Gambey, and divided to ten seconds * 
They were provided with an astronomical telescope, which has invariably 
been made use of iu connection with the artificial horizon. When 
observing for latitude, multiplied observations were generally taken: 
first, three of the sun’s upper limb; next, three of the lower; and, 
finally, again three of the upper limb. These observations were com¬ 
menced about eight or ten minutes before noon. The corresponding 
index error was always determined. 
Observations for longitude were never made nearer than three hours 
from noon; and, whenever weather and time permitted, corresponding 
observations iu the forenoon and afternoon were secured. On these 
occasions twelve observations, divided into four groups, and an equal 
number for the upper and lower limb, were taken. 
In observing corresponding altitudes, the index was set to an even 
five or ten minutes, and the time noted when the contact was perfect. 
The successive changes of the index were regulated according to the 
sun’s relative changes in altitude. 
To illustrate the above by an example, the following is subjoined:— 
Von . II .—20 
