Comparisons of Magnetic Standards, 1915-21 459 



Assembling the results, we have: 



(17) I. M. S.-Stonyhurst (Jones magnetometer) = 0'.0 (1915). 

 (17a) I. M. S.-Stonyhurst (Jones magnetometer) = +0.00010// (1915). 

 (17ft) I. M. S.-Stonyhurst (Dover dip circle No. 159, needles I, 2)= - 1'. 4 (1915). 



NO. 18. TANANARIVE OBSERVATORY, MADAGASCAR. 



The comparisons of November 12 to 18, 1920, at the Tananarive Observatory of 

 the Reverend E. Colin, S.J., were secured by Observer F. Brown at the beginning of 

 his magnetic survey of Madagascar. Two stations, designated A and B, were occupied. 

 Station A is the Observatory pier regularly used for absolute observations in the south 

 room of the magnetic observatory, a small house of stone and unbaked brick (the mag- 

 netograph room is the second and north room of this building). Station B is 10.62 

 meters west of A in line with A and the Observatory azimuth mark, viz, the summit of 

 Mount Ambohimalaza, 42 kilometers distant. Director Colin supplied the azimuth from 

 A, as determined by the Observatory from 24 zenith-distance observations of the Sun, 

 viz, 96 45'. 6 west of south. At station B the south tower of the Anglican Cathedral 

 was used as a mark, its azimuth as determined by Mr. Brown and referred to the 

 accepted Observatory azimuth for Mount Ambohimalaza being 89 47'.7 west of south. 



The Observatory instruments used in these comparisons were a Brunner magnetom- 

 eter, medium-size model, made in Paris in 1889, and a Brunner dip circle, medium- 

 size model with two needles, made in 1888 with one needle only. The declina- 

 tion and horizontal-intensity comparisons were made simultaneously at A and B with 

 exchange of station made to eliminate station-differences, Director Colin observing with 

 the Observatory instruments. All inclination comparison observations were made at 

 station A by the alternate method, Mr. Brown observing with both instruments and 

 following the Observatory practice with Observatory dip circle using the unnumbered 

 needle regularly used at the Observatory; lack of time prevented Director Colin from 

 making the Observatory observations for inclination. Because of existing known local 

 disturbance care was exercised to have the magnet systems of each magnetometer at 

 the same height above ground when observing at either A or B; the C.I.W. instrument 

 was mounted on a block of wood on the pier at A to accomplish this. For the dip-circle 

 work the magnetic center of the Observatory circle was one centimeter higher than 

 that of the C.I.W. circle. Thus any possible question regarding erroneous station- 

 difference because of variation in amount of local disturbance with height of instrument 

 was eliminated. The magnitude of local disturbance is shown by the large station- 

 differences observed. The C.I.W. instruments used were magnetometer No. 13 and 

 dip circle No. 177 with needles 13X, 14X, 7 of No. 242, and 8 of No. 242. 



The I. M. S. values given depend upon the constants and corrections as determined 

 at Washington in March 1919, and may require modification later upon the return of 

 the outfit in 1922. 



Director Colin emphasizes in his letter of December 10, 1920, relating to the com- 

 parisons, the difficulties and uncertainties introduced by the great existing local dis- 

 turbances at the Observatory site. He states that the intensity constants for his instru- 

 ment have not been redetermined since the original determinations some 30 years ago. 

 It is also difficult under the meteorological conditions in Madagascar to carry out obser- 

 vations of highest precision in a tent because of rapid temperature changes. He sug- 

 gests the desirability of carrying out intercomparisons later by means of alternate obser- 

 vations for declination and horizontal intensity with the 2 instruments at one station 

 only, making certain to have magnetic systems in each case at identical levels. As 

 stated above, every care was exercised by Director Colin and Mr. Brown to have mag- 

 netic systems of the instruments at identical levels for observations at the same station. 



