the Expedition into Central Africa. 261 



rate observations as can be obtained at the secondary stations 

 themselves. Of course, however, should circumstances permit, 

 the more important in other respects the point which can be. 

 made a primary observing station the better, and the Com- 

 mittee would expressly notice Griqua Town, Lattakoo, Kurre- 

 chane, and Meletta, as points of which the Geographical 

 position should be deternnned with care by observations on the 

 spot, and the observations then made transmitted home along 

 with the latest communications with the Colony. Since, how- 

 ever, the circumstances which may render stations objectionable 

 as primary points are mostly of a moral or political nature, it 

 is expected that no great difficulty Aviil cccur in fixing them at 

 positions of especial geographical interest, as at the confluence 

 of rivers, at the extreme borders or on the culminating points 

 of mounting ranges, on remarkable rocks, 8^c. or at least of 

 determinating their bearings and relative situations with respect 

 to such prominent features, with some degree of exactness. A 

 combination of circumstances of this kind of local interest will 

 of course have its due weight in determining (cateris paribus) 

 the halt of the Expedition. 



At primary stations the Committee recommend the assiduous 

 application of every instrumental means for the determination 

 of the three elements of latitude, longitude, and elevation 

 above the level of tlie sea, and especially, at such stations, as 

 many series of lunar distances as possible should be procured 

 in addition to the usual sights for time, (or observations of the 

 altitudes of heavenly bodies near the prime vertical,) which, 

 together with meridian observations for the latitude, they 

 would recommend to be practised daily as a matter of regular 

 duty, at every station, as well primary as secondary. At 

 primary stations also tlie barometer and thermometer should 

 be observed at regular intervals, and the magnetic variation 

 ascertained bi/ tuhinq llie sun's azimuth immediatebj before and 

 after tlie ot)Servution fur time, {noting the (xnct moments, and 

 thus oblainitig da /a /or interpolating to the time of observation). 

 At such stations likewise a careful investigation of the Index 

 errors of Sextants should be made, the zero points or index 

 corrections of the Sympiesometer should be determined by 

 leisurely comparison with the mountam Barometer (giving time 

 for the instruments to attain the same temperature,) and tlie 

 difference noted in the observation books. The necessity of 

 frequent comparisons of these instruments will be apparent 

 if it be considered that in the event of fracture of the Barome- 

 ter tube, no other means will exist by which the zero point of 

 a new one can be determined. Occultations of stars l)y the 

 moon, and, if possible, eclipses of the satellites of Jupiter 

 should be observed whenever an opportunity may occur. The 

 A>rmer especially, affording the best known method of ascer- 



