436 Special Reports 



determinations. The two Observatory azimuth marks, each consisting of a white line 

 between two black rectangles on an obelisk in the Park, were used, the upper one for the 

 station N m , and the lower one for the station O m ; the true bearings, supplied by Dr. 

 Chree, were 182 06'.3 and 182 48'.7 west of south, respectively. 



The Observatory absolute-instruments used were the Kew unifilar magnetometer 

 by Jones and the Kew dip circle No. 33 with 2 needles by Barrow. These are the same 

 instruments' 1 as those compared previously 6 except for modifications made during 1914 

 in the magnetometer, the horizontal circle having been redivided' and a two-vernier 

 system having been substituted for the three-vernier system. Owing to the develop- 

 ment of a trace of rust on the magnet of the Kew magnetometer in 1914, a redetermina- 

 tion of the moment of inertia was made by the Observatory authorities. The new 

 value of the moment of inertia has been utilized in determining the Kew values given 

 in Table 10B (intensities calculated using the old value of the moment of inertia found 

 in 1910 required to be reduced by 5 gammas when the new value was used). The Kew 

 intensity values depend upon the distribution coefficients finally adopted by Dr. Chree. 

 The C. I. W. instrument used was magnetometer-inductor No. 26. For those compar- 

 isons which are based upon data from the magnetograms the base-line values depend 

 upon absolute observations with the Observatory instruments designated above. The 

 corrections on International Magnetic Standards/ applied to results obtained with 

 magnetometer-inductor No. 26, were those finally adopted for that instrument. 



The electric-tram service affects both the absolute observations and the magnetograph 

 records at the Observatory. The vertical-intensity magnetograms are badly disturbed 

 on this account. The values of (I. M. S. Kew) given in Tables 10A, 10B, and IOC 

 are based on the assumption that the station-differences, if any, are negligible. This 

 assumption is borne out by the simultaneous absolute-observations, involving exchange 

 of stations, which indicated the station-differences were of an order less than that of 

 the errors to be expected in the observations. Furthermore, the effects of any possible 

 station-differences are practically eliminated in the means because exchanges of stations 

 were made in each series. 



It will be noted that the value determined in August and October 1915 for (I. M. S. 

 Kew) = 0.00047# is materially different from the mean value determined in 1908 

 and 1910, viz, +0.00007i/. e That there was a real change of the order shown appears 

 to be indicated through indirect comparisons. 



In February, June, and July 1915 comparisons were made at Kew between the 

 Kew standard and magnetometer No. 83 of the Hongkong Observatory. Magnet- 

 ometer No. 83 had been sent to England in February 1914 and alterations and repairs 

 completed prior to these comparisons. Director Claxton of the Hongkong Observatory, 

 in his report/ for the year 1915, gives mean result (Kew Hongkong No. 83) = +307; 

 this published value, Director Claxton states in his letter of January 19, 1917, must be 

 corrected because of finally adopted distribution coefficients and moment of inertia at 

 Kew (see Series I and II above) by 87, whence we have: 



(a) Kew Hongkong No. 83 = +22 7 (at Kew in 1915);= +0.001 19//. 



After the return of No. 83 to Hongkong, comparisons between Elliott No. 55, the 



Dr. Chree notes, however, in a letter dated August 7, 191o, that the average difference between Barrow circle No !3 

 and modern Dover circles has apparently been a trifle less since he succeeded Mr. Bake!, who retired at the end oi September 

 1912, as principal dip observer. 



Mag., Vol. II, pp. 240-243. 

 c Dr. Chreestatee thai a verj careful examination of both the declination and horizontal-intensity base-values obtained 

 before and after the i edivided led to the conclusion that no appreciable change has been made so far as the numerical 



values of either declination or horizontal intensity are concerned. 

 Di p. T, rr. Mag., Vol. II. pp. 270-278. 

 Has. Dcp. Terr. Mag., Vol. 11, p. 241. 

 ( llaxton, T, F., Report of the Director of the Royal Observatory, Hongkong, for the year 1915, Hongkong, 1910. 



