Procedure at the magnetic observatories of the United States Coast and Geodetic Survey

Abstract

In Nature for March 20, 1919, Dr. Charles Chree, under the title of “New Procedure at American Magnetic Observatories,” reviewed the publications of the results of observations made at the United States Coast and Geodetic Survey magnetic observatories at Sitka and Honolulu for 1915 and 1916 and called attention to some changes in procedure and to the need of more information regarding certain details of the methods employed. As the questions raised must be settled before there can be that uniformity in the methods of reduction and publication which is essential to the most advantageous, use of magnetic observatory results, it is well to give in some detail the considerations which led the Coast and Geodetic Survey to adopt at its five observatories the practices now in use.Up to the end of 1914 each hourly value of declination (D), horizontal intensity (H), and vertical intensity (Z) in the monthly tabulations represented the momentary value of the quantity for the specified hour, local mean time. This was in accordance with the general practice at the time the observatories were established, a practice adopted before continuous photographic registration came into use, when eye readings were made once an hour, and when attention was paid mainly to the local features of the variations of the Earth's magnetism. This method has the disadvantages that only a very small portion of the observational data is utilized, undue weight is given to the portion which happens to occur at the arbitrarily selected time, and the results at one station cannot be compared in detail with those at another, particularly at the time of a disturbance, because the use of local mean time renders the tabulated values non‐simultaneous.