172 



Expedition into Central Africa. 



[Apbsl 



guarded against by making it the daily duty of more than one per- 

 son to remind their beavers to wind them at a stated hour) it will 

 be most convenient in place of setting them, to differ winding them 

 until the hours and minutes come round, at which they may respec- 

 tively have stopped^ as near as may be ascertained from one to the 

 other or from both, to other watches of the party, and such event^ 

 should it take place, should be conspicuously noted in the observation 

 book ; and, as a further and useful precaution,, it is recommended 

 to keep some of the best-going watches belonging to individuals of 

 the expedition, to mean Greenwich time, by frequent comparison 

 with one of the Chrononieters. In every case where time is observ- 

 ed express m.ention should be made of the Chronometer or other 

 watch employed, designating it by the maker's name and number, sx> 

 that no uncertainty may ever arise as to the proper application of 

 the correction for error and rate. 



The rates of the Chronometers should be examined at any station 

 w%ere the expediton may rest two or more consecutive nightSy 

 either by equal altitudes of a star or more simply by noticing the dis- 

 appearance of any large fixed star from the same exact point of view? 

 behind the edge of aboard fixed at some considerable distance in the 

 horizon, and having its edge adjusted to a vertical position 

 by a plumb-line ; the interval between the tw^o such disap- 

 pearances being an exact sideral day or 23 h. 56 m. 4 sec, 

 mean time. Under the head of secondary observing statioris 

 may be classed those in which no lunar distances can be got^ 

 and when the sights for time and meridian altitude can only be su- 

 perficially and imperfectly taken, or one without the other. With a 

 view to the connection of these with the primary station and to the 

 sketching out a Chart of the Country passed through, at every pri- 

 mary station a series of angles should be taken with the Sextant be- 

 tw^een 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^ firsts the precise 

 identification of the point of observation, in case of its being desir- 

 able to find it again ; and, secondly, the determination from it of 

 geographical points. The first of these purposes will require angles 

 to be taken betw^een ?^e«r, the second betw^een 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 projected by the Camera Lucida, and 

 their changes of aspect and form as the expedition advances should 

 be w^ell and carefully noticed, to avoid mistakes. The approximate 

 distance of any remarkable object may be had by pacing or other- 

 wise measuring more 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 bf tw ecu 

 the object and the other staff. 



