1835] 



Expedition into Central Africa. 



m 



three elements of latitude, longitude, and elevation above the level 

 of the 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 seconda- 

 ry. At primary stations also the barometer and thermometer should 

 be observed at regular intervals, and the magnetic variation ascer- 

 tained bij taking the sun's azimuth immediate l// before and after the 

 observation for time, (noting the exact moments, and thus obtaining 

 data for 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 mountain 

 barometer (giving time for the instruments to attain the same tem- 

 perature,) and the difference noted in the observation books. The 

 necessity of frequent comparisons of these instruments will be ap- 

 parent 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 by the moon, and, if 

 possible, eclipses of the satellites of Jupiter should be observed 

 whenever an opportunity may occur. The former especially, afford- 

 ing the best known method of ascertaining the longitude by a single 

 observation should be constantly borne in mind, and the Almanac 

 consulted several days in advance, so that no occultation of a large 

 star certainly identifiable, should be allowed to escape through inad- 

 vertence. 



The Committee especially recommend that every observation 

 made should be registered in a book devoted to that purpose, and 

 preserved m exact terms of the readings off of the instruments 

 and Chronometers, and kept rigorously separate in its statement from 

 any calculation thereon grounded, and that the observed or presum- 

 ed index or zero corrections, whether of Chronometer, Sextant^ 

 Barometer, or other instrument, should be stated separately in every 

 case, and on no account incorporated with observed quantities^ 

 and, moreover, that the observations upon which such index errors 

 have been concluded, should also be preserved. Since however the 

 guidance of the expedition will necessitate the calculation of many 

 observations on the spot, the reiSuUs of such calculations should be 

 entered (as such) beside the observations from which they have 

 been concluded. 



The Committee farther recommend, that the Chronometers with 

 which the expedition has been provided by the liberality of His Ma- 

 jesty's Government should on no account be corrected by moving 

 the hands, however great their errors may become, not even in 

 the extreme case of one or both of them having been allowed to run 

 down. In case of such a misfortune (which (should be most carefully 



