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 | “ |
The above table fails, however, to bring out fully the actual variations of the watch. So long as the other five watches remained in order, Hassanein Bey made frequent comparisons with his principal watch, and between March 21 and 23 there is strong evidence that the principal watch made an abnormal gain of about fifty seconds. A similar abnormal gain of twenty-nine seconds by the principal watch is evidenced in the twenty-four hours between comparisons of March 24 and 25. Both these abnormalities occurred between Jalo and El Harrash, at an early stage of the journey, while all the watches appeared to be behaving tolerably well, and it is quite possible that other abnormalities occurred at later stages, when, owing to some or all of the other watches having stopped or broken down, no satisfactory control by comparison was possible.
Of the other five watches carried, one was an English half-chronometer, similar to the principal watch but of smaller size; three were high-class Swiss lever (“Peerless”) watches with very tight-fitting cases; and the fifth was a small Swiss lever-watch with luminous dial, carried on the wrist for noting the times of marching. The small half-chronometer stopped on April 3, after going for over four months; and though it was restarted, its rate changed considerably after the stoppage. The three “Peerless” watches, though they failed to continue going to the end of the journey, showed by no means a bad record. One was found stopping and unreliable on May 6, after going for over five months; the two others continued to go for over a month longer; and so far as can be judged from the comparisons made on the route, their variations of rate were about of the same order as those of the half-chronometers. The wrist-watch, from the manner in which it was carried, was of course liable to much greater variations of rate, and was occasionally reset by the principal watch; but it kept going till nearly the end of the journey.
As regards dust-resisting power, which must always be one of the principal aims in selection of watches for desert exploration, there appears to be nothing to choose between good English half-chronometers and the highest class of Swiss watches, the cases of the latter being remarkably close-fitting. The most probable cause either of stoppage or of abnormal changes of rate appears to be the sudden shocks which may arise either to a watch carried on the person when jumping on or off a camel, or to one carried in the baggage by sudden movements of the camel. The most likely explanation of the abnormal gains of the principal watch for short periods on the two occasions above noted would seem to be that, owing to a jerk in mounting or dismounting, two contiguous coils of the hair-spring may have been made to touch each other for a short time, with consequent temporary shortening of the period of vibration of the balance-wheel. It is noteworthy that the watch which remained going throughout the journey was the largest of all those taken, and its greater resisting-power may have been to some extent due to its size, permitting of greater strength in its component parts.