Part of my work comprised the precise fixation of the points specified in the Arrêté, with a view to laying down the boundary accurately on a map. As was natural in the case of a boundary settled without careful reference to the ground, certain difficulties arose in the location. In the first place, there are two distinct Gebels Niqrub, and two distinct Gebels Um el Tiur. I assumed the higher mountain of each pair to be the one indicated. Secondly, the precise point of the mountain was unspecified; I assumed the highest peak to be the point referred to. Thirdly, the text of the Arrêté disagrees with the map accompanying it, in that the map shows the line curved to pass close to Gebel Mishbih, which is not mentioned in the Arrêté. I assumed the text of the order to be determinative, and that between the points specified the limit followed great circles on the globe, i.e., practically straight lines on the map. The locality called Deiga was not visited, and can only be approximately fixed; it is said by guides to be a narrow road near Gebel Muqsim, of which mountain several peaks were fixed by triangulation.
The following table gives the positions found for the various points along the boundary from the sea to the meridian of 34° (1) from my survey operations, and (2) as scaled from the map accompanying the Ministerial Arrêté. A comparison of the two sets of positions will show how much the survey has added to our knowledge of the geography of this part of the desert.
| Point. | Latitude N. | Longitude E. | ||
|---|---|---|---|---|
| From my Survey. | From Arrêté Map. | From my Survey. | From Arrêté Map. | |
| Bir Shalatein | 23° 8′ 5″ | 22° 39′ 0″ | 35° 36′ 28″ | 36° 2′30″ |
| Bir Meneiga | 22° 47′ 8″ | 22° 41′ 30″ | 35° 12′ 20″ | 35° 2′30″ |
| Gebel Niqrub (El Foqani) | 22° 51′ 29″ | 22° 48′ 0″ | 34° 56′ 48″ | 34° 51′ 0″ |
| Gebel Um el Tiur (El Foqani) | 22° 17′ 54″ | 22° 18′ 30″ | 34° 41′ 1″ | 34° 32′ 0″ |
| Deiga (approx.) | 22° 10′ 0″ | 22° 9′ 0″ | 34° 1′ 0″ | 34° 3′ 0″ |
Variation of the Compass.
Careful observations were made of the variation of the compass at three stations, viz., Berenice Temple, Abu Saafa Springs, and near Halaib Fort,[80] these three points being selected as being well-known places and at the same time likely to be free from local magnetic disturbance. Berenice is on the coast-plain where only coral and sand occur; Abu Saafa is in a sandstone district about 100 kilometres south-west of Berenice, and Halaib is on the calcareous and gypseous rocks of the coast about 220 kilometres south-east of Abu Saafa.
The instrument used was a five-inch theodolite fitted with a good trough compass. A lens was used to bring the needle accurately to zero, and the sun or a star was employed to find the true meridian. In each case several observations, each with an independent setting of the needle, were made, and the mean taken. The values obtained were:—
| Place. | Date and Time. | Compass-Variation West. |
|---|---|---|
| Berenice Temple | January 6, 1907, 5·30 p.m. | 2° 44′ |
| Abu Saafa Springs | October 25, 1907, 4 p.m. | 2° 37′ |
| Near Halaib Fort | May 7, 1908, 10 a.m. | 2° 18′ |
In order to find to what degree of accuracy the results of these determinations might be relied on, the instrument used was tested against the Kew magnetometer at the Khedivial Observatory, Helwân, after the completion of the work. Three observations for the declination at Helwân gave the westerly variation as
| 2° 41′·2 | |
| 2° 58′·8 | |
| 2° 55′·0 | |
| Mean | 2° 55′·0 |
while the true declination as given by the Kew magnetometer at the same time was 2° 52′ 5″. Thus it appears reasonable to believe the observed values to be within about 5′ of the truth.