226 Reports on Special Researches 



NO. 7. CHELTENHAM OBSERVATORY. MARYLAND. 



All the observations at the Cheltenham Observatory of the United States Coast and 

 Geodetic Survey have been made on the extra pier (5,) in the west wing of the absolute 

 house. Careful determinations made by the Observatory authorities indicate that there is 

 no difference between the magnetic elements at this pier and those at the piers upon which 

 the Observatory instruments are mounted. 



The standard instruments at tliis Ol^servatory are the large Wild-Edelmann instru- 

 ments, consisting of declinometer, earth inductor, and magnetometer. No. 26. The decli- 

 nometer with large theodohte, for determining the declination, and the earth inductor with 

 its galvanometer, for determining the inchnation, are in the east wing of the absolute house, 

 while the magnetometer for determining the horizontal intensity is in the west wing. The 

 Cheltenham Observatory is the magnetic base station of the United States Coast and Geo- 

 detic Survey and fixes also the standards for the magnetic observatories of that Bureau at 

 Honolulu, Vieques (Porto Rico), Baldwin (Kansas, now discontinued and superseded by 

 Tucson, Arizona), and Sitka (Alaska). 



The instruments used by the C. I. W. observers are as indicated in the column of 

 remarks in the following tables. 



The Observatory values of D and H for series I, II, and III, as supplied by the Super- 

 intendent of the United States Coast and Geodetic Survey, were scaled from the magneto- 

 grams, tlie base-lines of which were determined by aid of the observations with the standard 

 instruments, made before and after the comparisons. For series IV, the observations were 

 made as follows: The declinations were observed simultaneously with C. I. W. magnetome- 

 ter No. 3 and the Observatory declinometer; for horizontal intensity, the observations with 

 the 2 instruments, C. I. W. No. 3 and the Observatory magnetometer. No. 26, were made 

 alternately, the results with the latter being referred to the mean time of the values with the 

 former by means of the Observatory magnetograms. 



For the inclination comparisons, simultaneous observations were invariably made with 

 the C. I. W. instruments and the Observatory earth inductor. Observer J. C. Pearson, 

 before and after extensive field work, compared Dover dip circle No. 177 (needles 1, 2, 5 

 and 6) with the Cheltenham earth inductor, the correction on C. I. W. standard being 

 determined at Wasliington before and after the comparisons; thus series I and III were 

 obtained. Furthermore, direct comparisons of the C. I. W. standard inclination instru- 

 ment (Schulze earth inductor No. 48 with correction of 0'.5 applied) were obtained by 

 Mr. H. W. Fisk in March and April 1908 (series II), under varied conditions and for 

 various orientations of footscrews of No. 48, and by Dr. H. M. W. Edmonds in November 

 1913. The last series may have been affected by some disturbing cause, or there may 

 have been some instrumental change, the possibility of which is being further investigated.' 



In addition to the comparisons, 1908-1913, some also were secured during the years 

 1905-1907. As the results obtained during that period were intended primarily for stan- 

 dardizing the magnetic instruments used by the Department of Terrestrial Magnetism, 

 before its own standards could be definitely decided upon, they are not given here. 



'As this volume is passing through the press, we have before us the results of additional inclination comi^arisons 

 made in Juno 1915, which confirm the 191.3 results. Some change appears to have taken plai'e with regard to the Chel- 

 tenham earth inductor between 1910 and 1913. 



