I.— REPORT ON ASTRONOMIC OBSERVATIONS. 



By p. CAJiriui.i. 



The astronomic determinations made by H.R.H. the Duke of the Alutizzi 

 on the route between Entebbe and Ruwenzori are the result of solar 

 observations made with an aluminium sextant, which was constructed in the 

 engineering workshop of the Xaval Hydrographic Institute at Genoa. Its 

 graduated arc has a radius of 14-5 mm. (about 6 inches), being so subdivided 

 as to show the 20 seconds on the vernier. Magnaghi's astronomic circle was used 

 only in the very few cases where, for observations at the meridian or in its 

 neighliourhood, the height of the sun was such as to make the use of the 

 sextant less convenient. 



Of course, all measured heights were duplicated at an artificial mercurial 

 horizon, care being taken to reverse the position of the roof at half of each 

 series of observation;?, in order to lessen to the utmost the influence of errors 

 in case the glasses of the said roof should eventually become prismatically 

 affected. 



The calculations were carried out by means of logarithms of S decimals, 

 tables of 7 decimals being used only in calculating the mean hour at Greenwich 

 at the moment of emersion of B A C 81 from the lunar disk, as observed at 

 midnight between the 11th and 12th July, 1906, at Bujongolo, the last 

 astronomic station in the district nearest to the Ruwenzori uplands. 



The astronomic refraction r, corresponding to the considerable altitudes at 

 which the astronomic observations were made during the journey, was calculated 

 with Bessel's well-known formula : — 



/■ = log ('( tang :) + A (log B + log T) + log y, 



neglecting the factor A, for apparent zenith distances :, under 77^ and the 

 factor A, besides A, for apparent zenith distances less than 45 . The values of 

 the elements contained in the foregoing fornuila wore deduced fiom Albrecht's 

 tables, 1894 edition. But the Table 34/, which gives the value of log B, only 

 comprises barometric pressures between GOO and 7S0 mm. (24 and 31 inches), 



.307 X 2 



