2 



Capt. H. G. Lyons. 



[June 6, 



The declinatorium used at first consisted of a horizontal circle 

 furnished with two verniers reading to 30", while the magnet was 

 balanced on a vertical steel pivot, in a box which occupied the centre 

 of the horizontal circle. Attached to the magnet was a mirror, in 

 which the reflected image of the cross wires was observed with a small 

 telescope placed excentrically. The same telescope also served to 

 observe the sun or a meridian mark for determining the geographical 

 meridian. 



The geographical positions were taken from Maps Nos. 740, 662, 

 published by the Intelligence Division, War Office, or from astro- 

 nomical observations taken on the spot; the former are indicated by * 

 and the latter by f. 



In each case the time of the observation is given as Cairo mean 

 time, i.e., 2 11 5 m 8 s -9 fast of Greenwich, since all the observations were 

 made before the time of the 30° meridian E. of Greenwich was adopted 

 as civil time for Egypt. 



D'Abbadie's station, beside the Great Pyramid at Giza, was occu- 

 pied on May 10, 1901, to obtain improved values of the secular 

 variation. The observations were made between 1 p.m. and 4 P.M. 

 when the electric tramway was not working. 



Since there are, as yet, no self-registering magnetic instruments in 

 Egypt, it is impossible to reduce the results obtained to a single epoch 

 with any accuracy ; they are, therefore, given as they were originally 

 observed. 



Helwan, 20 kiloms. south of Cairo, was chosen for the observations 

 of 1898 and 1899, since the electric tramways of Cairo render obser- 

 vations impracticable even if the amount of iron in the present 

 observatory building at Abbassia did not vitiate all observations taken 

 there. For this reason the value of 5° 36' west for the declination, 

 given in the £ Bulletin Mensuel ' of the Abbassia Observatory for June, 

 1886, is wholly wrong. 



In observing with the Declinatorium, the feet of the tripod were 

 firmly pressed into the ground, and this was found sufficient for the 

 precision obtainable with the instrument. With the Kew pattern 

 magnetometer, however, wooden pickets were driven firmly into the 

 ground, and the feet of the tripod rested on these, thus avoiding any 

 errors due to the tripod sinking into soft or sandy soil. 



All the observations which follow may be considered as satisfactory 

 ones taken under favourable conditions, since all those which were 

 interfered with by high winds, sand storms, &c, have been omitted. 

 The stations where igneous rocks are known to be near enough to 

 affect the results somewhat are marked with an asterisk on page 10. 



At several places on the Bahr el Abyad granite masses, and occasion- 

 ally basaltic rocks, rise through the sandstone, and the high value for 

 declination obtained at Kenk (page 23) is probably due to this. 



