246 



Reports on Special Researches 



The next comparisons at the Pilar Observatory were obtained by Observer H. F. 

 Johnston in the course of his field work in South America in 1913. He used C. I. W. 

 magnetometer No. 19, an instrument of the universal type (see Terrestrial Magnetism, 

 V. 16, 1911, pp. 9-12). The comparisons were made chiefly for declination and horizontal 

 intensity; those for inclination comprised but two sets, and as they were intended primaril}' 

 to control the corrections of the needles of No. 19, the results are not given here. 



Table 17 C. Results of Inclination Comparisons at the Pilar Ohscrvalory, 101 1. 



'All Viilues are referred to station B; B = No. 1 2'.8; B = C. 

 'These values are the means of observed dips with dip circle No. 216 .ind of 

 preceding and following values with the earth inductor. 



Since the comparisons of 1911, a new Dover-Kew magnetometer, No. 175, has been used 

 as the Observatory standard; the observer for declination and horizontal intensity with this 

 instrument was Director L. G. Schultz. For the inclination observations by the Ob.serva- 

 tory, Dover dip circle 216 (2 needles) and Toepfer earth inductor No. 3 were used, the 

 observers being, for the former, Mr. O. Liitzow-Holm, and for the latter, Mr. S. Stranger. 



The same stations were used as in 1911. Quoting from Director Schultz's report: 



"The absolute building was cleared of all disturbing substances and everything was arranged, 

 as nearly as possible, as for the 1911 comparisons. 



"Heavy wooden (algarroba) piers, 30 cm. section, were planted at stations B and C after tlie 

 1911 comparisons. They e.xtend 2 meters into the ground and nothing liut earth was used to fix 

 them in place. They are provided with copper foot-plates fixed so that when the foot-screws of 

 an instrument are adjusted in the grooves of the plates the center of the instrument lies in the 

 vertical line which coincides with the center of the small pegs put down by the Carnegie observers. 

 The sliglit differences in the azimuths used in these comparisons are due to the fact that the azimuth 

 of the large stone monument (mark 1) has liecn determined from B and C by a large number of star 

 oljservations and the centers of B and C have been brought rigidly into line with the centers of the 

 four piers in the absolute building and the central line of mark 1. 



"A commodious wooden structure, with padded walls, was placed over station B some time 

 before these comparisons; also a simple wooden building over station C; and the utmost care was 

 exercised to avoid modifying the magnetic conditions at both stations." 



The exchange of stations, to eliminate station-differences, was made as shown in the 

 table of results. The station differences, B C, obtained from the present comparisons are 

 practically the same as those for 1911, showing that the structures put over these points 

 since then have not affected the surroundings. However, the station differences, B No. 1 

 and B No. 8, are somewhat different, apparently bearing out the suspicion that in the 191 1 

 work there was at the time some disturbing influence in the absolute building. 



Director Schultz states that "the .scale coefficients of all variometers were determined 

 a few days after these comparisons and the results show no appreciable change in the values 

 since the comparisons of 1911." 



