Maghagha Railway Station, long. E. of Greenwich30°50′49″
Recorded west departure, Maghagha to Zubbo, 187·77 km.15646
Giving long. of Zubbo28543

II.—Traverse from Minia to Zubbo.

Minia Railway Station, long. E. of Greenwich30°45′39″
Recorded west departure, Minia to Zubbo, 176·32 km.14750
Giving long. of Zubbo285749

III.—Traverse from Zubbo to Minia, via the south end of the Oasis.

Recorded east departure, Zubbo-Minia, 174·87 km.1°46′45″
Minia Railway Station, long. E. of Greenwich304539
Giving long. of Zubbo285854

The arithmetic mean of these three determinations is 28° 56′ 55″; owing, however, to the breakdown of the measuring-wheel during the outward traverse from Maghagha, and the consequent necessity of estimating a part of the distance traversed by the time taken in marching, the different traverses are not equal in value, and the longitude finally adopted by the survey, as the best approximation after investigation of the various sources of error in the measurements, was 28° 58′ 34″. It would thus appear that Jordan’s position may be a little too far east, the difference amounting to 2′ 27″, or about 3½ kilometres, while Cailliaud’s value would place the position at a rather less distance west of that adopted.

The survey observations confirmed the accuracy of Jordan’s latitude (28° 22′ 7″), from which the value found by Cailliaud differs, as already remarked, only slightly.

The surveying operations within the oasis were based on a rapid plane-table triangulation from a measured base line within it, details being simultaneously sketched in, on a scale of ¹⁄₅₀₀₀₀. The site chosen for the base was a level stretch of ground extending between the camp at Zubbo and an isolated clump of date-palms to the north-east; the length of this line was found by repeated wheel-measurement to be 3·88 kilometres. The two parties ran off their triangulations from this base to the surrounding hills, and carried on the mapping southward by plane-table, taking stations chiefly on the hills and prominent points of the scarps, and not meeting again till Ain el Haiss, in the southern part of the oasis, was reached. Here a test was made as to agreement of the two sets of maps, only a small difference being found. The position of Ain el Haiss, as found by taking the mean of the two determinations, is latitude 28° 2′ 11″ N., longitude 28° 39′ 19″ E. of Greenwich; this places the spring about 4′ 18″ east of Jordan’s determination (lat. 28° 1′ 55″ N., long. 28° 13′ 47″ E. of Green.).

The two parties made a third connection at the extreme south end of the oasis-depression. Our observations for this point give its latitude as 27° 48′ 13″ N., and its longitude as 28° 32′ 19″ E. of Greenwich, placing it very near the position shown on Jordan’s map.

The plane-table method making use of the magnetic meridian, it was imperative to determine the amount of declination of the compass. This was done at one point only, viz., at the south end of the oasis, the value found (by observation of the transit of Polaris) being 4° 50′ W. The declination is fairly constant over the entire area, except near the eruptive dolerite masses, the magnetite in which causes a very sensible deflection of the needle; in the neighbourhood of these, however, the surveying was carried on independently of the compass. With regard to the yearly change of declination, we have as data the previous observations of Cailliaud, who found the declination at Zubbo in January, 1820, to be 12° 13′ W.; of Jordan, who obtained the value 6° 56′ W. in March, 1874; and of Capt. Lyons,[12] whose observations with a Bamberg declinatorium at Mandisha in April, 1894, gave the value 5° 8·9′ W. Tabulating these:—