398 



Special Reports 



When the constants for magnetometer No. 15 were determined in April 1910 by the 

 Department of Terrestrial Magnetism, at Washington, D. C, the following quantities 

 were obtained: 



C.I.W. Meteorological-Service magnetometer No. 15 = +0'.l. 

 C.I. W. Meteorological-Service magnetometer No. 15= +0.000357/, using 

 constants of April 14, 1910. 



Whence (see Vol. II, p. 273) 



I. M.S. Meteorological-Service magnetometer No. 15 = 0'.0 (1910). 



I. M.S. Meteorological-Service magnetometer No. 15 = +0.000207/ (1910). 



The comparisons in April 1910 included an exchange of stations to eliminate 

 .station-difference. During 1910 to 1915 magnetometer No. 15 was used for field work 

 involving unusually severe conditions of travel. In April 1910 the mean difference 

 between readings for magnet 15L erect and inverted was 4'.1, while in December 

 1915 it was only O'.l. The change in the value of the declination-difference on I. M.S. 

 between 1910 and 1915 is possibly because of slight imperfections, or displacements, 

 of the optical parts of the collimating system; a careful examination in 1915 did not 

 reveal any other probable explanation. For horizontal intensity, the values for the 

 difference on I. M.S. obtained in 1910 and 1915 (Table IB) are in good agreement. 



For inclination, the difference on I. M.S. of earth inductor Toepfer No. 89, obtained 

 at Washington in 1915 (Table 1C), supersedes that derived indirectly for 1910 to 1912, 

 as given on pages 216 and 278 of Volume II. The disagreement in the two values is 

 doubtless to be ascribed to the uncertainties in obtaining the earlier value, viz, lack of 

 data for determining a possible station-difference between the large inductor-pier C and 

 the piers D and E of the Agincourt Observatory, and possible changes in the various 

 dip circles involved in the comparisons. 



Table 1A. Results of Declination Comparisons at the Standardizing Magnetic Observatory of the Department of 



Terrestrial Magnetism at Washington, 1915. 



1 All values are referred to station S; S = m +0'.l, as derived from this series. Observers: M. S. No. 15, W. E. \\ . 

 Jackson of the Meteorological Service; C. I. W. No. 3, H. W. Fisk of the Department of Terrestrial Magnetism. 

 > These are values obtained with C. I. W. No. 3 referred to I. M. S.; I. M. S. C. I. W. No. 3 = -O'.l. 

 : This value was corrected for error in setting of toision-head. 

 This value was obtained from 4 magnet-inverted and 2 magnet-erect readings only. 



