Comparisons of Magnetic Standards, 1915-21 



Table 7D. Summary of Corrections on Standards for Helwan Observatory. 



429 



It is to be noted that the results of the several comparisons are in substantial agree- 

 ment with the results of previous comparisons as regards all 3 elements except for the 

 declination-series of 1914 and for the intensity-series of 1908 which are rejected. As 

 regards inclination all the Observatory results apply to the Observatory standard, viz, 

 mean of needles 3 and 8, which is substantially the same as mean of needles 1 and 2 used 

 prior to 1912. The Director of the Meteorological Service states in his letter of Sep- 

 tember 27, 1920: "Our inclination standard since November 1912 is the mean of values 

 given by needles 3 and 8. Prior to that date our standard needles were 1 and 2, and at 

 the time of changing over it was found that the 2 sets of needles gave so nearly identical 

 results that no correction was considered necessary. Since that date several different 

 needles have been in use, and the results given by them have all been reduced to the 

 standard of 3 and 8. The correction applied to the mean of 10 and 1 1 to reduce to stand- 

 ard, the mean of 3 and 8, is 1'.8." 



From Table 7D we obtain weighted mean values as follows: 



(7) I.M. S.-Helwan (Elliott magnetometer No. 87) = +0.5 (1908-1918). 

 (7a) I . M. S. - Helwan (Elliott magnetometer No. 87) = - 0.00049// ( 1 9 1 1 - 1 9 1 8). 

 (76) I.M. S.-Helwan (Dover dip circle No. 193, needles 3 and 8) = + l'.2 (1908-1918). 



NO. 8. HONGKONG OBSERVATORY, CHINA. 



The comparisons of 1915 were obtained by Observers C. K. Edmunds and F. Brown, 

 with the aid during part of the work of Director T. F. Claxton of the Observatory and 

 of his assistants, Messrs. C. W. Jeffries and B. D. Evans. The same stations, viz, 

 A, A', and B, were occupied as for the comparisons of 1911." Stations A and A' are 

 the observing piers in the absolute house used regularly by the Observatory for the 

 magnetometer and dip-circle work respectively; A (north pier) is 2.55 meters from A' 

 (south pier). The station B is an outside or tent station 14.33 meters from A' and is 

 located on the line between the latter and the azimuth mark used by the Observatory; 

 the true bearing of the mark from stations A and B, as supplied by Director Claxton, 

 is south 35751'.3 west. Some buildings were added near the stations subsequent to 

 the observations of 1911; the new buildings may be largely responsible for the dis- 

 agreements between the station-differences obtained in 1911 and in 1915. 



The same observatory instruments, viz, Elliott magnetometer No. 55 and Dover 

 dip circle No. 71 with needles 3, 4, 7, and 8, were used in these comparisons as in 1911. 

 The C. I. W. instruments used were C. I. W. magnetometer No. 9 and Dover dip circle 

 No. 177 with needles 1, 2, 5, and 6. The corrections on International Magnetic Stand- 

 ards 6 applied to results with the C. I. W. instruments are those finally adopted. 



See Res. Dep. Terr. Mag., Vol. II, pp. 238-240. b See Res. Dep. Terr. Mag., Vol. II, pp. 270-278. 



