Compakisons of Magnetic Standakds, 1915-21 403 



NO. 2. CHELTENHAM OBSERVATORY, MARYLAND. 

 Series I, 1915. 



The comparisons of June 10 to 23, 1915, were made by H. W. Fisk at the Cheltenham 

 Observatory of the United States Coast and Geodetic Survey, using C. I. W. magnet- 

 ometer-inductor No. 26. Before taking this instrument to Cheltenham, careful com- 

 parisons were made at the Standardizing Magnetic Observatory of the Department of 

 Terrestrial Magnetism in Washington between it and C. I. W. standard magnetometer 

 No. 3 in declination and horizontal intensity, and between it and C. I. W. standard 

 Schulze earth-inductor No. 48. Upon the conclusion of the work at Cheltenham, 

 similar intercomparisons were again made at Washington. The standards of the 

 Observatory are the large Wild-Edelmann instruments, consisting of declinometer, earth 

 inductor, and magnetometer, No. 26. The declinometer mounted on declinometer 

 pier, with large theodolite for determining declination, and the earth inductor mounted 

 on pier A, with its galvanometer for determining inclination, are in the east wing of the 

 absolute house of the Observatory, while the magnetometer for determining horizontal 

 intensity mounted on pier B is in the west wing. The observations for the three series 

 were carried out between June 2 and July 2, 1915. The I. M. S. values given in the 

 tables are based upon the corrections finally adopted for C. I. W. magnetometer-inductor 

 No. 26. 



The declination comparisons at Cheltenham were made with C.I.W. No. 26 

 mounted on pier B, in the west wing of the absolute observatory, and were strictly 

 simultaneous with observations with the C. & G. S. standard declinometer No. 26 in 

 the east wing. The new Observatory mark from B u a cross painted on chimney of 

 Hill's house, was used; its azimuth as supplied by the Observatory is 68 26'. 7. The 

 mark, cupola on Hill's barn, used in previous comparisons has been altered because of 

 settling. Additional declination comparisons were obtained with C.I.W. No. 26 set up 

 in a tent at an outside station (designated 0), aligned between the theodolite in the east 

 wing and its mark, the cross painted on one of the brick buildings of the House of 

 Reformation. 6 



In horizontal intensity the observations could not be simultaneous owing to the 

 proximity of the pier B used for the C. I. W. instrument, to the pier B on which the 

 Observatory instrument is permanently mounted. Each determination with one 

 instrument was followed immediately by a determination with the other. The cor- 

 rections to the mean times of the C. I. W. observations on account of diurnal variation 

 have been made from the magnetograph traces. In this manner 9 independent H- 

 differences were found, 3 for each foot-screw orientation of C. I. W. No. 26. 



The inclination comparisons were made with C. I. W. No. 26 on pier B { and Wild- 

 Edelmann inductor No. 26 on its own pier A in the east wing, the observations being 

 as nearly simultaneous as the different forms of instrument would permit. During 

 the first part of the inclination comparison the work was not begun until 10:30 a. m. 

 in order that the declination might approximate that for which the Observatory earth- 

 inductor was adjusted. To determine whether any station-difference had developed 

 since the test of 1908, a non-magnetic framework was attached to the pier on which 

 the Observatory instrument is mounted. C. I. W. No. 26 was placed on this frame as 

 close to the coil of the Cheltenham inductor as possible (this station is designated (EI)' 

 in the tabulation of results). Ten sets with each instrument were obtained, as nearly 



" The C. I. W. instrument is also numbered 26, and is distinguished from the Observatory instrument by the designation 

 C. I. W. No. 26. 



6 In azimuth, 245 11'.5; O is 35.9 feet (10.94 meters) nearly due east from the declinometer pier. 



