Corrections to Reduce Monthly Mean Pressures to Annual Means

Jan. Feb. March April May June July
mm. mm. mm. mm. mm. mm. mm.
Siwa− 3.4 − 2.0 − 1.9 + 0.9 + 0.9 + 2.7 + 3.5
El Obeid− 1.2 + 0.7 + 0.3 + 1.2 + 1.0 + 0.6 0.0
Mean− 2.3 − 1.4 − 0.8 + 1.0 + 1.0 + 1.6 + 1.8

A further correction to allow for the distribution of the sea-level isobars over the region traversed was desirable, but no sufficient data exist for estimating its amount; the distribution is, however, probably nearly linear, and such a distribution was approximately allowed for by assuming the previously accepted levels at Siwa (− 17 m.) and El Fasher (793 m.) to be correct and distributing any residual difference found by the otherwise corrected barometer readings between these two places uniformly among the different sections.

The difference of height corresponding to each difference of mean corrected barometer-readings was calculated by the tables of Barometrische Höhenstufen in Jordan’s “Mathematische und Geodätische Hülfstafeln,” for the air-temperature corresponding to the mean of the thermometer-readings at the two ends of the line.

The adopted levels of thirteen principal camps as found in the above manner are tabulated below. It is interesting to note that the residual difference of height which had to be distributed between Siwa and El Fasher, and which is presumably mainly due to systematic pressure-gradient, was sixty-three meters, corresponding to a normal fall of sea-level pressure between the two places of about 5 mm., and this is from other considerations probably very near the truth; also that the resulting adjustment which had to be made in the levels in any single main section of the route did not exceed five meters.

Concluded Altitudes above Sea

Number of observations Mean pressure (corrected) Mean temperature Diff. of height from Jordan’s tables Diff. of height (adjusted) Altitude above sea-level
mm.° C.metersmetersmeters
Siwa4762.612——− 17
Jaghbub50757.715+ 54+ 49+ 32
Jalo18754.717+ 34+ 2961
El Harrash6732.823+ 254+ 249310
Taj31718.519+ 170+ 165475
Arkenu12708.031+ 128+ 123598
Ouenat14706.331+ 21+ 18616
Erdi7683.331+ 295+ 290906
Agah3695.234− 157− 162744
Bao5677.733+ 230+ 225969
Furawia11685.831− 107− 112857
Um Buru8679.530+ 83+ 78935
Kuttum5660.224+ 254+ 2491184
El Fasher5689.731− 386− 391793

After thus determining the levels for the principal camps, those of intermediate camps and other places were computed in a similar manner, adjusting each section to the adopted levels at its terminal points. The maximum adjustment which it was found necessary to apply to the difference of height given by the barometer between points a day’s journey apart was five meters, and the average three meters.

An exception was made in the stretch between Jaghbub and Jalo, where no intermediate levels were adopted for the map, owing to the extremely unstable state of the atmosphere during the journey between these two places; sand-storms of great violence occurred on several days of marching, with such rapid fluctuations of air-pressure that no heights could safely be deduced from the barometer-readings.

As regards the degree of reliability of the deduced levels, some little uncertainty exists in the levels adopted for the terminal points Siwa and El Fasher, while the temperature-compensation of the aneroid has not been tested and may not be quite perfect. Taking everything into account, it may be estimated that the levels of the principal camps are probably correct to within about twenty meters, while those of the intermediate camps and other points, for which only one or two readings of the barometer are available, may possibly be in error by twice that amount.