Comparisons of Magnetic Standards, 1905-14 235 



NO. 12. HELWAN OBSERVATORY. NEAR CAIRO, EGYPT. 



Three series of comparisons have been obtained at the Helwan Observatorj- in the 

 fourse of the field work of the Department of Terrestrial Magnetism, viz : 



Series I by Observer J. C. Pearson, April 21-28, 1908, using C. I. W. magnetometer No. 5 and Dover 



dip circle No. 177 (needles 1, 2, 5, and 6). 

 Series II by Observer W. H. Sligh, May 17-24, 1911, using C. I. W. magnetometer No. 7 and Dover 



dip circle No. 202 (needles 1, 2, 5, and 6). 

 Series III by Observer W. F. Wallis, March 12-14, 1914, using C. I. W. magnetometer No. 10 and 



Dover dip circle No. 202 (needles 1, 2, 5, and 7). 



The Observatory instruments were Kew-pattern magnetometer Elliott No. 87 and 

 Dover dip circle No. 193 (presumably needles 1 and 2). Throughout, the method of com- 

 parison by simultaneous observations was employed, the observers exchanging stations in 

 order to eliminate the station-difTerences. For the Observatory the observations were 

 made in 1908 and 1911 by Mr. Hurst, and in 1914, by Mr. Eckersly. 



The results from Series I were published in detail in ''Terrestrial Magnetism," vol. 16, 

 pp. 145-146. The mean revised results are: 



C.I. W.- Helwan {1908). 



Declination +0'.5 (13 sets). 



Horizontal Intensity +IO.O7 (?) or +0.00033// (?) (6 sets). 



Inclination +0'.6 (5 sets). 



The horizontal-intensity difference is marked doubtful, for it will be noticed that it 

 differs materially from those obtained in 1911 and 1914. Mr. Hurst compared the Helwan 

 magnetometer Xo. 87 at Kew in October 1907, finding that No. 87 was in practical agree- 

 ment with the Kew standard. However, since the publication^ of the observations, Dr. 

 Chree has found it necessar>^ to decrease the Kew values by 6.87 on account of various 

 accumulated errors in the constants of the Kew standard.^ Hence we have, for October 

 1907, (Kew- Helwan) = -6.37= -0.00034i/. 



jMr. Pearson, using the same instnnnents at Kew as at Helwan, foimd in March 

 1908 that (C. I. W.-Kew) was equal to +0.000065//, and in March 1910 equal to 

 +0.000075// (see Table 14 B, p. 241); hence the mean value of (C. I. W.-Kew) was 

 +0.00007//. Combining the results of Messrs. Hurst and Pearson we get, indirect!}^, (C. I. 

 W. Helwan) = 0.00027//, which tends to confirm the directly-observed results at Helwan 

 in 1911 and 1914. The question therefore arises whether there may not possibly be an 

 error in the 1908 result. 



The value of log ir-K at C. for magnet 87^, as determined at Kew (by the Observa- 

 torj' and by Mr. Hurst) in October 1907, was 3.45319; the value as determined by Mr. 

 Hurst at Helwan in December 1907 was 3.45272, the change being ascribed by Mr. Hurst 

 to "a slight shift of the magnet in its stirrup" during transportation between Kew and 

 Helwan.i The 1908 //-values at Helwan, as supplied by the Observatory, were computed with 

 the later value of log tc-K; had they been computed with the Kew value of this constant, 

 they would have been higher by 0.00054//, and the resulting value of (C. I. W. 

 Helwan) would have been 0.00021//, thus agreeing well -ndth that deduced above by a 

 combination of the respective comparisons at Kew of Messrs. Hurst and Pearson, and also 

 corresponding, in general, with the results fi'om the comparisons of 1911 and 1914. 



Looking over the publications of the Observatory it would appear, from various state- 

 ments and from the annual values of //, that more or less difficulty has been encountered 



'Standardization of the Magnetic Instruments at Helwan Observatory during 1907; Survey Departnient paper, No. 

 S, Cairo, 1908. 



''Terr. Mag., vol. 16, 1911, p. 72. 



