(2) A journal giving a continuous record of observed compass-bearings and estimated distances from Siwa Oasis to Lameina Wells, near El Fasher, a distance of about 2430 kilometers.
The journal also contains a considerable number of observed compass-bearings to conspicuous features on either side of the route, with a few rough trigonometrical estimations of the altitudes of mountains passed, a large number of readings of an aneroid barometer and sling thermometer for the estimation of altitudes along the route, daily observations of maximum and minimum temperatures, observations on the character of the country passed through, and notes on the meteorological conditions.
These observational data have been reduced in the Desert Survey Office, Cairo, and utilized in the preparation of the map on a scale of 1:2,000,000 which accompanies Hassanein Bey’s account of his travels. The objects of the present note are, first, to give an account of the critical examination to which the records were subjected in the course of the reduction of the observations, so as to enable an estimate to be formed of the degree of precision which may be fairly assigned to the geographical positions, altitudes, and other results used in the construction of the map; and, secondly, to indicate the additions to existing geographical knowledge concerning a little-known region of northeast Africa which have accrued from the expedition.
2. Astronomical Determination of Local Time
Theodolite observations of altitudes of the sun or of stars were made at all the principal camps for determining the error on local mean time of the half-chronometer watch which was used in the latitude observations. In all, thirty-four complete time determinations were made at seventeen camps. The observations were made with a three-inch Troughton & Simms theodolite, the vertical circle of which could be read by two verniers to 1′, and which was provided with a sensitive level on the microscope arm. The theodolite was invariably set up in the magnetic meridian by its trough compass, and the method used was to note the times of passage of the sun’s limb or of the star over each of the three horizontal wires of the stadia-graticule, reading the level and circle at each pointing, face-right and face-left. In the case of stars, the magnetic bearing of the star was also noted from the horizontal circle, and a note made of the color and brightness of the star, for the identification of the stars subsequently in the office, and thus to free the observer from the necessity of knowing the stars’ names. The barometer and thermometer were carefully read at each observation for calculating the refraction.
No difficulty was experienced in subsequently identifying the stars. In only a single case was it found necessary to reject an observation, and this because the observer had accidentally sighted different stars on the two faces of the instrument. On many days two or more observations were taken at the same place, and comparisons of the results at these places indicated that the observations are remarkably accurate for so small an instrument. In seven cases where the sun has been observed shortly before sunset and a star soon after sunset, for example, the maximum difference between the results given by the two observations is only seven seconds, while the average is under four seconds. It is thus apparent that the precision of the time observations is amply sufficient to insure that no sensible error in the latitudes can be due to errors of the adopted local times.
As the only use made of the time observations in the preparation of the map has been in the determination of latitude, it would serve no useful purpose to give a list of the watch errors. But it may interest future geographers, undertaking long desert journeys, to note some of Hassanein Bey’s experiences in the transport of watches, and the hazard of relying on constancy of rate for long periods with even the best watch.
Of the six watches carried, only a single one remained in going order throughout the journey. The timekeeper which thus successfully resisted the vicissitudes of the seven months of desert travel was fortunately the one with which Hassanein Bey took all his observations, and was carried in his pocket during the entire journey; it was a large-sized half-chronometer “explorer’s” watch of English manufacture, with a dust-proof cap to the winding mechanism. It had been awarded an “especially good” certificate at the National Physical Laboratory of England, and was consequently the most expensive of all the six watches taken on the journey. Even this watch failed to maintain a sufficiently constant rate to be of any service in the determination of longitudes, though it amply sufficed for the latitudes, even in two cases when reliance had to be placed on the constancy of its rate for one or two days owing to latitude alone being observed without corresponding observations for local time. Thus, for instance, the following were the average rates of the principal watch deduced from local time observations at places of previously known latitudes:
Rate of Principal Watch
| Sollum to Siwa | Dec. 29-Jan. 13 | 15 | days | losing | 5.8 | secs. |
| Siwa to Jaghbub | Jan. 13-Jan. 20 | 7 | “ | “ | 0.1 | “ |
| Jaghbub to Furawia | Feb. 14-June 5 | 111 | “ | “ | 7.7 | “ |
| Furawia to Um Buru | June 5-June 8 | 3 | “ | “ | 6.6 | “ |
| Um Buru to El Fasher | June 8-June 26 | 18 | “ | “ | 9.4 | “ |
| El Fasher to El Obeid | June 30-July 15 | 15 | “ | “ | 9.4 | “ |