Comparisons of Magnetic Standards, 1905-14 249 



Referrinp: tho moan rosults to I. M. S. (see p. 273), we obtain: 



(18) I. M. S.-Pola (Bamberg magnetometer No. 7904)= -0'.4 (1910). 

 (1 8a) I. M. S.-Pola (Bamberg magnetometer No. 7904)= + 0.00039// ( 1 9 1 0). 

 (18A) I. M. S.-Pola (Wild earth inductor) =+ 0'.6 (1910). 



NO. 19.-P0RT0 RICO OBSERVATORY. VIEQUES, PORTO RICO. 



The comparisons at the Porto Rico Magnetic Observatory of the United States Coast 

 and Geodetic Survey were obtained by the observers of the Carnegie. The following 3 

 stations were occupied: the two observing piers in the absolute house, regularly used for 

 the magnetometer and for the earth inductor; and an auxiliary station used for dip only. 

 Tho latter station, designated 1, was distant 24.2 meters in a line 144 12'. 2 southwest of 

 the regular azimuth-station of the Observatory. The Observatory absolute instruments 

 were: for declination and horizontal intensity, Cooke magnetometer No. 31 (India Magnetic 

 Survey pattern); and for inclination, Schulze earth inductor No. 1. The tabulated values 

 of the Observatory are the results from the magnetograms, as standardized before and after 

 the work of the Carnegie observers. The standards of the Observatory are based on those 

 at the Cheltenham Magnetic Observatory of the Coast and Geodetic Survey, the observer- 

 in-charge being at the time Mr. G. Hartnell. The Carnegie observers were Mr. W. J. 

 Peters (Chief of Party), and Messrs. E. Kidson and C. C. Craft; the instruments used by 

 them were: C. I. W. magnetometers Nos. 2 and 4; Dover dip circle No. 201, with needles 

 Nos. 1 and 2. 



The result of the horizontal-intensity comparison showed that a curious change had 

 taken place in the Cooke magnetometer No. 31 in recent j^ears. Thus, quoting from 

 page 83, Appendix No. 3, Report of the United States Coast and Geodetic Survey for 1911 : 



"As a result of comparisons at Cheltenham in 1903 and at the Porto Rico Observatory in 1905, 

 + 0.00095// was adopted as the correction to be applied to horizontal-intensity results with magne- 

 tometer No. 31 to reduce them to the Cheltenham standard. In July 1910 the yacht Carnegie of 

 the Department of Terrestrial Magnetism of the Carnegie Institution of Washington, stopped at 

 ^'ioques on her way to South America, and her instruments were compared with those of the Porto 

 Rico Observatory. These compari.sons indicated that some change had taken place in magnetom- 

 eter No. 31 since the comparisons of 1905. To verify this, magnetometer No. .36 was compared at 

 Cheltenham in Januarj^ 1911; with No. 31 at the Porto Rico Observatory in February and April; 

 and again at Cheltenham upon its return. These comparisons gave for the correction to No. 31 in 

 March 1911, 0.0005//. A careful scrutiny of the Observatory records failed to find any evidence 

 of a sudden change in magnetometer No. 31 which might account for this change in its relation to 

 the standard magnetometer, and it was therefore considered best to assume a gradual change from 

 +0.00095// in January 1907 to -0.0005// in March 1911." 



The //-values for the Porto Rico Observatory, as supphed by the Coast and Geodetic 

 Survey, were corrected as stated in this quotation; as the values thus derived are the ones 

 referred to the Cheltenham standard, the difference given below, namely, C. I. W. Porto 

 Rico (referred to Cheltenham standard) = 0.00082//, is found to be practically the same 

 as the result from the directly-observed quantities at Cheltenham (see p. 228). 



The dechnation correction of magnetometer No. 31 on the Cheltenham standard as 

 applied to the furnished values was O'.O. 



The inclination values as tabulated all refer to the same station, namely, the earth- 

 inductor pier (E. I.) in the absolute house, the earth inductor having been removed during 

 the period of the Carnegie observations. On July 26, dip circle No. 201 was mounted on 

 this dip pier and on July 27 at the auxiliary station No. 1. The station-difference was 

 determined bj' simultaneous observations on July 26 and 27 between the Carnegie dip 

 circles 201 and 189. To the Observatory values a correction of I'.O was applied by the 

 Coast and CJeodetic Survey to refer them to the Cheltenham standard. 



