DEPARTMENT OF TERRESTRIAL MAGNETISM. 299 



of the Eskdalemuir Observatory during 1913 1 at Greenwich, Kew, Falmouth, 

 Valencia (Cahirciveen), Eskdalemuir, De Bilt, Potsdam, and Val Joyeux, 

 and (&) results reported upon by Dr. L. Palazzo 2 at Pare St. Maur and Kew 

 in 1898, and at Potsdam and Pola in 1902. 



In general, for those observatories where previous comparisons had been 

 obtained, there is a good agreement in the resulting values of corrections on 

 standards. In some cases rather large corrections on standards have been 

 determined; these apparently are caused by changes in the instrumental 

 constants which have developed since the original determinations of the con- 

 stants. 



The report concludes with a general discussion of the results thus far 

 obtained by the Department of Terrestrial Magnetism and as to the accuracy 

 attainable. A discussion of the absolute standard in horizontal intensity, H, 

 as deduced from the results of the extended comparisons between standard 

 magnetometer No. 3 and sine galvanometer No. 1 of the Department of 

 Terrestrial Magnetism follows. It appears that absolute standards for the 

 three magnetic elements are readily attainable with well-designed instruments 

 and carefully determined constants within O.'l or 0/2 in declination and 

 inclination, and within 0.00015H in horizontal intensity. 



Progress report of Committee on Magnetic Standards, and Instruments and Methods for 

 Aerial Magnetic Measurements. 3 J. A. Fleming. 



This report summarized the developments since the last annual meeting of 

 the section in magnetic and electric methods of determining the horizontal 

 intensity of the Earth's magnetic field. Brief accounts were given of electrical 

 instruments designed by Dr. N. Watanabe in Japan, by Sir Arthur Schuster 

 and Mr. F. E. Smith in Great Britain, and by Dr. S. J. Barnett in America, 

 together with accounts of comparisons of these instruments with magneto- 

 meters. The final results of comparisons obtained in Japan and Great Britain 

 have not yet been communicated. The comparisons of the standard in hori- 

 zontal intensity adopted by the Department of Terrestrial Magnetism in 1914, 

 tentatively designated "International Magnetic Standard" and defined by 

 C. I. W. magnetometer No. 3, with that determined by the electric method 

 defined by C. I. W. sine galvanometer No. 1, showed a difference between 

 the two instruments of only 0.00004#, the probable error of the mean differ- 

 ence being ±0.00003//. While the close agreement indicated is partly 

 accidental, the knowledge of the constants of the respective instruments is 

 such as to indicate an agreement of the standard determined by the magnetic 

 method with that determined by the electric method to be well within the allow- 

 able limits of error both for observatory and field use. 



The report abstracted also the results of direct comparisons obtained during 

 1915 to 1921 at 20 observatories, the compilation confirming the provisional 

 International Magnetic Standards of the Carnegie Institution of Washington 

 as being correct well within the magnitude of unavoidable errors of observa- 

 tion and of the determination of constants of the various types of instruments 

 used. The necessity, however, of careful control of constants by comparisons 

 with reliable standards at least once every two or three years was emphasized, 

 as also the necessity of having control observations from time to time on the 

 moment of inertia used for the oscillating magnet of the magnetometer. 



i British Meteorological and Magnetic Yearbook, Part IV, section 2, 1913 (83-84). 



« Misure magnetiche e confronti magnetometrici a Terracina, Ann. Uff. centr. meteor, geodin, 

 vol. 27, Part I, Rome 1920 (1-33). 



3 Membership of committee: C. G. Abbot, J. P. Ault, S J. Barnett, J. A. Fleming, chairman, 

 and J. T. Watkins 



