The longitude is of course always the difficult matter in the geographical determination of places not easy of access. Cailliaud’s value rests on the method of distance and azimuths, both roughly determined only; Jordan, on the other hand, used the absolute though not very precise method of lunar distances measured with a sextant. The Survey parties depended entirely on direct measurement by means of measuring-wheels from known points in the Nile Valley; the values obtained in this way are tabulated below[11]. I.—_Traverse from Maghagha to Zubbo._ Maghagha Railway Station, long. E. of Greenwich 30° 50′ 49″ Recorded west departure, Maghagha to Zubbo, 187·77 km. 1 56 46 ------------- Giving long. of Zubbo 28 54 3 II.—_Traverse from Minia to Zubbo._ Minia Railway Station, long. E. of Greenwich 30° 45′ 39″ Recorded west departure, Minia to Zubbo, 176·32 km. 1 47 50 ------------- Giving long. of Zubbo 28 57 49 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 Greenwich 30 45 39 ------------- Giving long. of Zubbo 28 58 54 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.