The C. I. W. deflector in use on the Carnegie for determining the magnetic horizontal intensity and the magnetic declination at sea

Early in 1905, in order to supplement the Lloyd‐Creak dip circle for obtaining magnetic intensities at sea, especially in loxv magnetic latitudes, the first of us devised a deflecting apparatus which could be readily attached to an ordinary liquid compass used in navigation, thus permitting the determination of the magnetic horizontal intensity direct, as well as of the magnetic declination. 1 This “deflector” has been used throughout the ocean work accomplished on board the Galilee , 1905–'08, and on the Carnegie , 1909–'13, by the Department of Terrestrial Magnetism of the Carnegie Institution of Washington. The purpose of the present paper is to describe briefly the development of this instrument; it may be recalled that when it was first devised, Bidlingmaier's “double compass” had not yet been perfected and, even if it had been, it would not have answered our special requirements, chief among which was that of simplicity, both from the standpoint of instrumental detail and that of computation and availability of the instrument for both declination and horizontal intensity work.