Appendix 13. 



brought with us on these excursions were: — ;i Fortin barometer which hud for 

 a long time been compared with another left at Bujongolo ; an aneroid likewise 

 compared with the two Fortins ; a thermometer for taking the temperature of 

 the air, and two hypsometric thermometers. 



For the more important points, for instance, for nearly all the peaks, and 

 always where possible, the measurements of pressure were made with the 

 mercurial barometer, the aneroid being used only in a few special cases where 

 it would have l>een very difficult to carry, or take measurements with the 

 mercurial barometer, and also for places of secondary importance. The 

 precaution, however, was taken to take down or record the indications of the 

 aneroid even whenever the Fortin was used. 



Not till after 12th July, when the Fortin got damaged, was the hypsometer 

 employed. AVhenever it was possible, the observations were made at the same 

 hours as those of Rujongolo (9, 12, and 17), but occasionally this was not 

 possible, and then we assumed as terms of comparison the data of Bujongolo 

 made at the nearest hours, uidess there were reasons for adopting the mean 

 of two consecutive <lata. 



In connection with this preferable choice of data for the calculation of 

 altitudes it .should be noted that for places for which the daily variation of 

 pressure, temperature, etc., is known, that is to say, where the hour of the 

 ma.xima and minima and the extent of the daily variations are ascertained, it 

 is possible to reduce a determination made at any given hour to another 

 determined hour. But in the present case these fundamental notions are 

 lacking, and for regions such as that under consideration, meteorological studies 

 are too rare to enable us confidently to extend to them those laws that have 

 been estal)lished for regions of the temperate zone.* 



For Bujongolo we should no douljt have some element to establish 

 approximately the daily movement of pressure, temperature, etc., but the same 

 cannot be said for the other places in Ruwenzori, and especially for the peaks. 



For these reasons the heights were calculated with the data, such as they 

 were, without modifying them in any way on the ground of the hours when 

 the observations were niade. 



Another matter, which, however, has no great influence, is that concerned 

 with the humidity, or rather the tension of the aqueous vapour which is always 



* Let one example suffice to show what caution is necessary in this respect. At 

 Bujongolo the mean pressure at 9 o'clock i;* 48887 mm. (see Table VIIT, G) ; at 12, 4S8G7, 

 and at 17, 48808, so that the nuiximum of the morning is reached before middav, and 

 at thi-» hour the barometer is already falling. On the otlier hand on the Santis Peak 

 (2,467 metres), the maximum of the morning is delayed till toward 14 o'clock, and on Mt. Blanc 

 (4,811 metres), till towards 15 (Angot, meteorologie). 



3G6 



