APPENDIX. 157 
tive nights, either by equal altitudes of a star, or simply by 
noticing the disappearance of any large fixed star from the 
same exact point of view behind the edge of a board fixed at 
some considerable distance in the horizon, and having its 
edge adjusted to a vertical position by a plumb line, the 
interval between two such disappearances being an exact 
sidereal day, or 23h. 56m. 4 sec. mean time. Under the 
head of secondary observing stations, may be classed those 
in which no lunar distances can be got, and when the sights 
for time and meridian altitude can only be superficially and 
imperfectly taken, or one without the other. With the view 
to the connexion of these with the primary station, and to the 
sketching out a chart of the country passed through at every 
primary station, a series of angles should be taken by the 
sextant, between remarkable and well defined points in the 
horizon, dividing the horizon into convenient portions, and 
carrying the angles all round the circle back to the point of 
departure: and in the selection of such points, two ends 
should be kept in view, first the precise identification of the 
point of observation, in case of its being desirable to find it 
again; and secondly, the determination from it of geogra- 
phical points. The first of these purposes will require angles 
to be taken between near, the second, between distant objects. 
For the latter, of course, remarkable mountain peaks will, 
if possible, be chosen. Of such, when once observed, the 
appearances from the place of observation should be pro- 
jected by the camera lucida, and their changes of aspect 
and form, as the expedition advances, should be well and 
carefully noticed, to avoid mistakes. The approximate dis- 
tance of any remarkable object may be had by pacing, or 
otherwise measuring more exactly, a base line of a few hun- 
dred paces, in a direction perpendicular to that in which 
it appears, erecting a staff at each end, and from each 
