358 APPENDIX A. 



instrumeiit did not exceed 2^ millimetres, we liave the mean 

 pressure 



By Oarstensen's instrument ^= 755'30 mm. 

 „ Beaumier's „ = 760'80 mm. 



The error of the last-mentioned instrument does not probab ly 

 exceed 1 mm. in. excess of the true pressure, and, if the observa- 

 tions of the mercurial barometer were used, it would be 

 expedient to apply to them a correction of + 4'5 mm. But, in 

 addition to the circumstances already mentioned, it must be 

 noted that Mr. Oarstensen's observations extend over but 

 eighteen days ; while M. Beaumier's record covers twenty-six 

 days, from May 11 to June 5 inclusive. For these reasons it 

 has appeared best to make use exclusively of the record sup- 

 plied by M. Beaumier. His instrument was an aneroid 

 barometer of the construction adopted by its maker (Leja), 

 called in Paris barometre holosterique. The readings were 

 recorded daily at 10 a.m. and 4 p.m., and are carried to intervals 

 of the quarter of a millimetre. 



The first questions that arise ia applying observations to 

 the determination of altitudes relate to the corrections appli- 

 cable to each instrument. The corrected readings of Secr6tan's 

 aneroid at Tangiers, and during the voyage between that place 

 and Mogador, varied from 760 mm. to 761 '5 mm., and may 

 safely be assumed to be nearly correct ; but, on arriving at 

 Mogador, they fell considerably. Inasmuch, however, as on 

 comparison with the mercurial barometer at the British Con- 

 sulate the difference was inconsiderable, the fall was attributed 

 to the condition of the weather at that time. It was only on 

 our return to Mogador from the interior, when a direct com- 

 parison between Secr^tan's and Leja's instrviments disclosed a 

 difference of 7 "3 mm. between the readings, and a further com- 

 parison between the recorded observations of the mercurial 

 barometer and M. Beaumier's instrument showed a difference 

 of about 5 "5 mm. between the scales of those instruments, that 

 it became clear that Secr^tan's aneroid had suffered some change 

 at or about the time of landing at Mogador. A careful com- 

 parison of all the observations leaves no ground for supposing 

 that this arose from any gradual process ; and it seems almost 

 certain that by one of those accidents to which the best 



