| ″ | ||||
| Sollum | 4 | nights | Av. dev. | 8 |
| Jaghbub | 5 | “ | “ | 40 |
| Taj | 6 | “ | “ | 12 |
| Arkenu | 2 | “ | “ | 6 |
| Furawia | 2 | “ | “ | 8 |
| Um Buru | 2 | “ | “ | 23 |
| El Fasher | 2 | “ | “ | 6 |
It thus appears unlikely that any observed latitude can be as much as 1′ in error, and consequently in the preparation of the map, Hassanein Bey’s observed latitudes have been accepted as fundamental data for all the places for which no previous determinations are known to exist, viz., El Harrash, Taj, Arkenu, Ouenat, Erdi, Agah, Enebah, and Bao. His latitudes for Jalo (El Erg), Bir Buttafal, and Furawia have also been adopted for the map, the first because it is possibly better than that of Rohlfs, with whose mapped position it is however in very close agreement; the second because while differing by about 2′ from Rohlfs’s value (28° 36′ 22″) it is doubtless more exact, because it checks admirably well with Hassanein Bey’s dead reckoning; and the third because, although the position of Furawia is shown on the Sudan maps, it is outside the present limits of the Sudan triangulation and is possibly liable to some slight error.[1]
4. Observations of Compass-Variations
For finding the pole-star easily when the sky was not very dark or was partially obscured by clouds, and also for obtaining the approximate bearings of time-stars for their subsequent identification, the theodolite was invariably set up in the magnetic meridian by its trough-compass, and the magnetic bearing of the pole-star was read on the horizontal circle after each latitude-observation and the time noted. In this way approximate compass-variation was determined at each camp, with the following results:
Variation of the Compass
| ° | ′ | |||||
| Sollum | Dec., 1922 | 3 | obs. | 2 | 34 | W. |
| Siwa | Jan., 1923 | 1 | “ | 2 | 42 | “ |
| Jaghbub | Feb., 1923 | 5 | “ | 2 | 25 | “ |
| Near Jalo | March, 1923 | 1 | “ | 4 | 12 | “ |
| Jalo (El Erg) | “ “ | 1 | “ | 4 | 5 | “ |
| Buttafal | “ “ | 1 | “ | — | ||
| El Harrash | “ “ | 1 | “ | 3 | 48 | “ |
| Taj | April, 1923 | 6 | “ | 3 | 32 | “ |
| Arkenu | “ “ | 2 | “ | 3 | 25 | “ |
| Ouenat | “ “ | 1 | “ | 3 | 32 | “ |
| Erdi | May, 1923 | 1 | “ | 3 | 57 | “ |
| Agah | “ “ | 1 | “ | 4 | 0 | “ |
| Enebah | “ “ | 1 | “ | 4 | 21 | “ |
| Bao | “ “ | 1 | “ | 4 | 59 | “ |
| Furawia | June, 1923 | 2 | “ | 4 | 32 | “ |
| Um Buru | “ “ | 2 | “ | 3 | 25 | “ |
| Kuttum | “ “ | 1 | “ | 4 | 26 | “ |
| El Fasher | “ “ | 2 | “ | 2 | 51 | “ |
The method of estimating the compass-variation with the theodolite is, of course, only rough; but the values found are probably correct at most places within half a degree, and they serve to show that there is no likelihood of any serious errors of dead-reckoning owing to local irregularities of the compass-variation. They have accordingly been utilized in reduction of the compass-bearings of the traverse to true bearings over the major portion of the route, where no previous determinations exist, and where in consequence the distribution of the isogonic lines is not known with any exactitude.
5. Longitudes
The probability of some watches breaking in the course of seven months’ travel had been foreseen, and from the outset it appeared improbable that any use could be made of the watches in the direct determination of longitude of so long and difficult a journey. We had decided accordingly to depend ultimately on dead-reckoning for longitude, making every effort to obtain an unbroken chain of compass-bearings and estimated distances between Jaghbub and some known place in the Sudan, the bearings to be taken with a good compass with all possible precision, at very frequent intervals, and the estimates of distance to be carefully made each day from the marching time of the baggage-camels, assuming a normal rate of 4 km. per hour over average desert, and making allowance for the variations in speed over ground of varying character. The journey being predominantly north to south, the estimates of distance could mostly be well controlled by the latitudes, while errors of bearing would not be cumulative, and would tend to cancel out over any considerable stretch of route. The primary reason for taking six watches was therefore not to determine longitude, for which at best they could afford only somewhat uncertain values, but to make sure of at least one watch being available throughout the journey for latitude-observations, without which no proper control of the all-important distances could be obtained.
The doubts about the possibility of safely transporting the watches proved justified, for all but one broke down before the end of the journey. But fortunately on the one hand the watch which survived sufficed amply for the determination of latitude (though its rate was insufficiently constant for it to be used uncontrolled in finding longitudes), and on the other hand the program of a continuous chain of very careful bearings and estimation of distances was scrupulously adhered to from the departure of the caravan from Jaghbub, the last known place in Egypt, to Furawia, the first known place in the Sudan—a journey of 2430 kilometers—and from this chain of bearings and estimated distances, combined with the observed latitudes, it was possible to estimate the longitudes of all places on the route with a fairly high probability of accuracy.