Comparisons of Magnetic Standards, 1915-21 427 



NO. 7. HELWAN OBSERVATORY, NEAR CAIRO, EGYPT. 



The comparisons during July 12 to 26, 1918, at the Helwan Observatory were 

 secured at the conclusion of Observer H. E. Sawyer's magnetic expedition through 

 Africa. The stations used for the comparisons of 1911 and 1914 1 were reoccupied; 

 these stations were the stone pier in small wooden hut, designated Hut, the north pier 

 in porch or "Absolute Room," designated N (for D and H observations), and the south 

 pier in porch, designated S (for / observations). The Observatory azimuth marks 

 were used, the azimuths as supplied by Mr. Knox-Shaw, Director, being from Hut, 

 336 55'. 9 (in former years value used was 336 55'. 1) west of true south for point midway 

 between two black dots on side of "Upper Office," and from JV, 176 24'.0 west of true 

 south for monument. With reference to the value of azimuth for the station Hut, 

 Mr. Knox-Shaw states that "A revised value of the azimuth of the mark used in the 

 hut has been used. To compare station-difference (N Hut) with values found in 

 former years, a correction of 0'.8 must be applied to former years, i.e., in 1914 

 (N Hut) would become +0'.6." 



The Observatory instruments used in these comparisons were Kew-pattern magnet- 

 ometer Elliott No. 87 (magnets 87A and 88C), and Dover dip circle No. 193 with needles 

 10 and 11. It is to be noted that these are the same instruments as those used by the 

 Observatory for previous comparisons' 1 except that needles 10 and 11 were used with 

 dip circle 193 instead of needles 1 and 2 as in 1911 and needles 3 and 8 as in 1914. The 

 C. I. W. instruments were magnetometer No. 17 (magnets 17L and ITS), and dip circle 

 No. 223 with needles 1 and 3 of circle 223 and 5 and 6 of circle 178. Throughout, the 

 method of comparison by simultaneous observations was employed, the observers ex- 

 changing stations in order to eliminate the station-differences. For the Observatory, 

 all the declination and horizontal-intensity observations and the inclination observa- 

 tions of July 14 were made by Mr. H. Knox-Shaw, and the inclination observations 

 of July 15 and 24 by Mr. P. A. Curry; the C. I. W. observations were all made by Mr. 

 H. E. Sawyer. 



The I. M. S. values given depend upon the constants finally adopted for C. I. W. 

 magnetometer No. 17 and dip circle No. 223. When these instruments were returned 

 from the field in January 1919, after continuous field service since November 1915, it 

 was found that there had been an appreciable decrease in the moment of inertia for 

 magnet 17L and its suspension. Examination of the comparisons with standards for 

 magnetometer No. 17 showed that the decrease had taken place practically as a linear 

 function of the time during which the instrument was in field service. For inclination, 

 the corrections on I. M. S. finally adopted after careful analysis of the entire series of field 

 results obtained during Mr. Sawyer's campaign are for I=+41.l: needle 1 of circle 

 223, -0'.6; needle 3 of circle 223, +0'.2; needle 5 of circle 178, -2'.5; needle 6 of circle 

 178, O'.O. Because of erratic behavior of needle 5 of circle 178, values determined by 

 it have been weighted one-half in the table of results. 



The Observatory results are as supplied by Mr. H. Knox-Shaw, Director of the 

 Meteorological Service. The inclination values by circle 193 and needles 10 and 11 

 are "reduced to Helwan standard," i. e., to the standard of needles 3 and 8 which is 

 essentially the same as that of needles 1 and 2. 



See Res. Dep. Terr. Mag., Vol. II, pp. 23.5-238. 



