the Exjjedition into Central Africa. 263 



adjusted to a vertical position by a plumb-line ; the interval 

 between the two such disappearances being an exact sideral 

 day or 23 h. 50 m. 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 a view to the connec- 

 tion 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 with the Sextant between 

 remarkable and well-defined points in the horizon, dividing the 

 horizon into convenient portions, and cari-ying 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 geographical points. The first of these 

 purposes will recpxire angles to be taken between near, the 

 second between distant objects. For the latter of course re- 

 markable mountain peaks will, if possible, be chosen. Of 

 such, when once observed, the appearances from the place of 

 observation should be projected 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 distance of any remarkable object may be had by 

 pacing or otherwise measuring moi'e exactly, a base line of a 

 few hundred paces, in a direction perpendicular to that in 

 which it appears, erecting a staff at each end, and from each 

 staff measuring the angle between the object and the other staff. 



In this manner the neighbourhood of any station may be 

 mapped down so as to be available for many useful purposes. 

 In all such cases the compass bearings of the most important 

 object in the horizon should be taken, and in the absence of 

 tlie sextant angles, azimuth compass readings of each point 

 may be substituted, though of course with less precision. 



Indications of the progress of the expedition should be left 

 at various points in its course, by making marks on rocks or 

 stones, &c. and by burying documents in bottles. In regard 

 to the latter it will be necessary to deposit them one foot deep 

 at some known distance, say 15 feet from a conspicuous sur- 

 face of stone, on which there is painted a circle containing the 

 distance and bearing by compass of the bottle, from its centre, 

 and that the situation of such places of deposit should also be 

 ascertamed by exact compass bearings of several remarkable 

 points in the horizon, both near and distant, as well as by 

 angles between them, carefully determined with a sextant, and 

 noted down in the journals of the expedition for their own re- 

 ference or that of future travellers. 



