It is rather curious to note that the declination observed at Abu Saafa is less than that at Berenice, which lies further east, even if allowance is made for the secular change in the interval. The difference from what one would expect is, however, possibly owing to errors of observation of the magnitude above-mentioned, or it may be due to a slight local influence of magnetic rocks underground at Abu Saafa, where the ground is near to the base of the Nubian sandstone beds, which rest presumably on eruptive and metamorphic rocks.
It is interesting to compare the observations of declination at Berenice and Halaib with those recorded by Rossler[81] at the same places in 1895. Thus we have:—
| Berenice. | Mersa Halaib. | ||
|---|---|---|---|
| Rossler, Nov. 27, 1895 | 3° 54′ | Rossler, Nov. 18, 1895 | 3° 36′ |
| Ball, Jan. 6, 1907 | 2° 44′ | Ball, May 7, 1908 | 2° 18′ |
| Diff. in 11·1 years | 1° 10′ | Diff. in 12·5 years | 1° 18′ |
| Yearly decrease | 6′·3 | Yearly decrease | 6′·3 |
We thus arrive at a yearly secular diminution of declination of 6′ 3″, confirming the value arrived at by Mr. Keeling[82] for the rate of secular change in the Red Sea area from a comparison of other observations. It may be therefore concluded that the rate of 3′ per annum given in the “Admiralty Pilot”[83] is only about half the true value.
Travellers in the Eastern Desert should exercise care in the reliance they place on compass bearings. Bearings taken with a compass are generally normal in granite and sandstone country; but wherever dark igneous and metamorphic rocks abound, disturbances are likely to occur. Local deviations of 5° to 10° are quite common, and in some serpentine areas even 40° of disturbance may be observed. In some places, one can deflect the needle through a large angle by means of a fragment of the rock no larger than a nut, so rich are some of the basic rocks in magnetic minerals. Some lumps of rock even show strong polarity, attracting or repelling the north pole of the needle according as one part or another of the lump is presented to it.
[62]This measurement was made in conjunction with Messrs. Villiers Stuart, Charteris Stewart, and Clarke.
[63]The logarithms A′, B, and C are taken from Merriman’s Precise Surveying and Geodesy, 1899, 255. p.
[64]Chauvenet (Spherical and Practical Astronomy, Vol. I, p. 283) shows that when both latitude and time are required, the three stars should differ in azimuth by 120° to give the most accurate results. But when only the latitude is required the stars are best selected near to the meridian north and south of the zenith. This is evident by considering that if two stars could be observed to culminate at the same altitude on opposite sides of the zenith, the latitude would be precisely determined independently of any reference to time.
[65]I am indebted to Mr. B. H. Wade, of the Survey Department, for drawing my attention to this method.