6. Altitudes above Sea-level
For the barometric estimations of altitudes above sea-level, a two-inch aneroid by Steward was used. This instrument, which was one of two specially made for the expedition, was very carefully compensated for temperature, and provided with a fairly open pressure-scale, a millimeter of pressure being represented by very nearly a millimeter of actual scale-length, so that estimations to half-millimeters of pressure could be readily made.
The aneroid was read morning and evening at each camp, and at numerous other points on the route, the air-temperatures being as a rule recorded at the same time, as given by sling-thermometer. The aneroid behaved very satisfactorily throughout the journey. There had unfortunately been no opportunity of testing the instrument before Hassanein Bey’s departure, but it was in perfect order on his return, and was then tested in the laboratory of the Physical Department at Cairo, when it was found to require the following corrections (at about 25° C.):
| Pressure, mm. | 760 | 750 | 740 | 730 | 720 | 710 | 700 | 690 | 680 | 670 | 660 | 650 |
| Correction, mm. | − 2.3 | − 2.3 | − 2.3 | − 2.1 | − 1.4 | − 1.1 | − 0.1 | + 0.6 | + 1.7 | + 2.0 | + 2.8 | + 2.9 |
That the above corrections had remained sensibly constant throughout the journey is rendered extremely probable by the close agreement noted further on ([p. 329]) between the levels found for Jalo directly from the aneroid readings (corrected, of course, on the assumption of constancy of the table) and those found indirectly from readings of the mercurial barometer in the meteorological station at Siwa.
The first step in the calculation of the barometric levels was to collect the whole of the readings of the barometer and sling-thermometer at each of the nine principal camps, where a halt of at least several days had been made, and a considerable number of readings taken. The means of all the recorded pressures and air-temperatures were taken for each of these principal camps, and the pressure corrected for instrumental error from the table above given. The readings being taken at various times of the day, the diurnal variation of pressure could safely be neglected, as it would most probably disappear on taking the mean of the readings. To allow for the annual variation, the mean pressures were next reduced to the mean of the year by applying a correction based on the mean of the normal annual variations at Siwa and El Obeid, as found from the recent volume of “Climatological Normals” issued by the Physical Department of Egypt and shown in the following table:
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. | meters | meters | meters | ||
| Siwa | 4 | 762.6 | 12 | — | — | − 17 |
| Jaghbub | 50 | 757.7 | 15 | + 54 | + 49 | + 32 |
| Jalo | 18 | 754.7 | 17 | + 34 | + 29 | 61 |
| El Harrash | 6 | 732.8 | 23 | + 254 | + 249 | 310 |
| Taj | 31 | 718.5 | 19 | + 170 | + 165 | 475 |
| Arkenu | 12 | 708.0 | 31 | + 128 | + 123 | 598 |
| Ouenat | 14 | 706.3 | 31 | + 21 | + 18 | 616 |
| Erdi | 7 | 683.3 | 31 | + 295 | + 290 | 906 |
| Agah | 3 | 695.2 | 34 | − 157 | − 162 | 744 |
| Bao | 5 | 677.7 | 33 | + 230 | + 225 | 969 |
| Furawia | 11 | 685.8 | 31 | − 107 | − 112 | 857 |
| Um Buru | 8 | 679.5 | 30 | + 83 | + 78 | 935 |
| Kuttum | 5 | 660.2 | 24 | + 254 | + 249 | 1184 |
| El Fasher | 5 | 689.7 | 31 | − 386 | − 391 | 793 |
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.