19

C.—la

Magnetic Storm of 11th to 12th August, 1919. The magnetogram of this storm, which presented the usual " sudden commencement " of large storms, is published herewith. The range of the storm was about 600 y (0-006 C.G.S. unit) in horizontal force, and about 43' in magnetic declination. The " sudden commencement " was an increase in H.F. of 200 y, and commenced at very nearly 6h. 57 m. G.M.C.T. 11th August; but a preliminary slight disturbance in H and D occurs just after 3h. G, and in H a larger preliminary disturbance occurred between sh. 30m. G and the "sudden commencement." The mean H was evidently much less than normal after 8 h. G, and the type of disturbance changed just after 16 h. G, the irregular short-period oscillations after that time being more of the polar auroral type, which is most noticeable between 24 h. on 11th and 4|h. on 12th G. Similar oscillations occurred between 16h. 15m. on 11th G and 17 h. 15 m. G, with a marked break at 16 h. 40 m. It is noteworthy that the time interval between the extreme maximum value of H and the extreme minimum is only about one hour and a half, the whole storm lasting over twentyfour hours. Below is given a table of exact times of the principal turning-points and commencements of some sudden changes :— Table of Greenwich Times of Remarkable Features. h. M. August 11 . . .. .. 6 57-5 Sudden commencement. 7 2 End of sudden commencement. 7 36 Marked minimum in H. 7 54 Marked minimum in H. 7 57 Principal maximum of 11. 9 21 Marked turning-point (major If). 9 27 Principal minimum in H. 9 59 Marked turning-point (major H). 12 8 Marked turning-point. 12 17 Second minimum of H. 1.2 32 Marked turning-point (minor H). II 8 Marked turning-point (minor H). 15 52 Second maximum of 11. August 12 .. .. 8 32 End of a sudden diminution of H. Mean Annual Values, 1905. Magnetic declination .. .. .. . . .. 16° 25-4' E. of N. Magnetic horizontal force .. .. .. .. 0-22628 C.G.S. unit. Magnetic inclination .. .. .. .. .. 67° 45-8' S. Northerly component .. .. .. .. . . 0-21705 C.G.S. unit. Easterly'component .. . . . . .. . . 0-06398 „ Vertical component .. . . .. . . .. 0-55348 ~ Total magnetic force .. .. .. .. .. 0-59794 ~ Henry F. Skey, Director.

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