So that the difference of longitude found by the dead-reckoning requires correction by 15′ 48″. This correction, which implies an average error in the observed compass-bearings of less than a degree, and a negligible correction to the adjusted distances, was distributed along the entire traverse in proportion to the difference of latitude between the principal camps, leading to the finally adopted longitudes shown in the following table:
Concluded Longitudes
| Dead-reckoningcorrected by the latitudes | Furthercorrection | Concluded longitude | ||||||
|---|---|---|---|---|---|---|---|---|
| ° | ′ | ″ | ′ | ″ | ° | ′ | ″ | |
| Jalo | — | — | 21 | 28 | 3 E. | |||
| El Harrash | 22 | 15 | 5 E. | 4 | 10 | 22 | 10 | 55 “ |
| Taj | 23 | 29 | 5 “ | 5 | 34 | 23 | 23 | 41 “ |
| Arkenu | 24 | 52 | 10 “ | 7 | 55 | 24 | 44 | 15 “ |
| Ouenat | 25 | 2 | 34 “ | 8 | 18 | 24 | 54 | 16 “ |
| Erdi | 23 | 22 | 34 “ | 12 | 5 | 23 | 10 | 29 “ |
| Agah | 23 | 28 | 49 “ | 12 | 54 | 23 | 15 | 55 “ |
| Enebah | 23 | 25 | 58 “ | 13 | 30 | 23 | 14 | 28 “ |
| Bao | 23 | 16 | 18 “ | 14 | 31 | 23 | 1 | 47 “ |
| Furawia | 23 | 53 | 58 “ | 15 | 48 | 23 | 38 | 10 “ |
In attempting to estimate the probable degree of accuracy of the concluded longitudes, we are faced with the difficulty that while we may be certain that the average error of the compass-bearings was less than 1°, which average error has been allowed for in the adjustment, we have no proof that in some of the individual sections the errors may not have been considerably greater. But in view of the great number of the observed compass-bearings (339) which make up the directional data of the 1754 kilometers of traverse from Jalo to Furawia (an average of thirty-eight observed bearings for each of the nine sections), and bearing in mind also the remarkable accuracy of the estimations of distance as evidenced by the latitude-observations, it would seem unlikely that any of the above-adopted longitudes for the principal camps can be in error by more than three or four miles. This implies a degree of accuracy which it would have been difficult to insure by transport of even a considerable number of chronometers over a land journey lasting more than three months. It may, I think, be concluded that no better values for the longitudes can be obtained without the aid of wireless time-signals.
6. Altitudes above Sea-level
For the barometric estimations of altitudes above sea-level, a two-inch aneroid by Steward was used. This instrument, which was one of two specially made for the expedition, was very carefully compensated for temperature, and provided with a fairly open pressure-scale, a millimeter of pressure being represented by very nearly a millimeter of actual scale-length, so that estimations to half-millimeters of pressure could be readily made.
The aneroid was read morning and evening at each camp, and at numerous other points on the route, the air-temperatures being as a rule recorded at the same time, as given by sling-thermometer. The aneroid behaved very satisfactorily throughout the journey. There had unfortunately been no opportunity of testing the instrument before Hassanein Bey’s departure, but it was in perfect order on his return, and was then tested in the laboratory of the Physical Department at Cairo, when it was found to require the following corrections (at about 25° C.):
| Pressure, mm. | 760 | 750 | 740 | 730 | 720 | 710 | 700 | 690 | 680 | 670 | 660 | 650 |
| Correction, mm. | − 2.3 | − 2.3 | − 2.3 | − 2.1 | − 1.4 | − 1.1 | − 0.1 | + 0.6 | + 1.7 | + 2.0 | + 2.8 | + 2.9 |
That the above corrections had remained sensibly constant throughout the journey is rendered extremely probable by the close agreement noted further on ([p. 329]) between the levels found for Jalo directly from the aneroid readings (corrected, of course, on the assumption of constancy of the table) and those found indirectly from readings of the mercurial barometer in the meteorological station at Siwa.
The first step in the calculation of the barometric levels was to collect the whole of the readings of the barometer and sling-thermometer at each of the nine principal camps, where a halt of at least several days had been made, and a considerable number of readings taken. The means of all the recorded pressures and air-temperatures were taken for each of these principal camps, and the pressure corrected for instrumental error from the table above given. The readings being taken at various times of the day, the diurnal variation of pressure could safely be neglected, as it would most probably disappear on taking the mean of the readings. To allow for the annual variation, the mean pressures were next reduced to the mean of the year by applying a correction based on the mean of the normal annual variations at Siwa and El Obeid, as found from the recent volume of “Climatological Normals” issued by the Physical Department of Egypt and shown in the following table: