1921.] 
Departmental Reports. 
201 
9.30 p.m., and thereafter increasing generally to the next day (figs. 6 
and 7). 
The recorded range of the horizontal force between 7h. 30 m. p.m., 
16th May, and 1.30 a.m., 17th May, was 200 y, but it was probably slightly 
larger than this, as the general diminution brought the recording-light 
spot to the limit. About 9.30 a.m., 17th May, a tendency to further 
disturbance was evident. 
With the phenomena in the horizontal force noted at sudden commence¬ 
ment and afterwards in (2) and (3) there are associated phenomena in the 
declination. 
About 6h. 48 m. a.m. on the 14th there is an almost similar oscillation 
to the first sudden commencement on a smaller scale, and it is curiously 
almost absent in the horizontal force. 
With further reference to the above, and the extraordinary nature of 
the magnetic effects, I should like to draw attention to a curious fact, no 
doubt associated with the existence of solar seiches or oscillations in its 
period of rotation—namely, that on the 18th April at 17 h. 35 m. G-.M.C.T. 
{19th April, 5h. 5 m. a.m. N.Z.S.T.) a very pronounced disturbance of 
the horizontal magnetic force occurred—a “ sudden commencement ” quite 
similar to that which occurred on the 13th May at 13 h. 8 m. G-.M.C.T. 
(14th May, 12 h. 38m. a.m. N.Z.S.T.). The interesting point is that the 
interval between these two similar phenomena was 24-94 days, or exactly 
the period of the absolute rotation of the sun’s equatorial belt. 
The Magnetic Storm of May 13-14, 1921 : Observations at 
Samoa Observatory. 
By G. Angenheister and C. J. Westland. 
(Communicated by the Department of External Affairs.) 
Plate II. 
A = 11 h. 27 m. 6 s. W.; </> = 13° 48'S. 
In the following notes Greenwich civil mean time is used. The 
motions of the north end of the magnet to east and downward are 
reckoned positive. A D = V corresponds to 10 y. The mean values of 
the magnetic elements during five quiet days preceding the storms are— 
H = 0-35263, D = 10° 12‘5' E, Z = - 0-20380 (approx.). 
Preliminary Disturbance. 
Gradual increase in H begins. 
Sudden commencement. A H = -f- 20 y. 
Maximum in H. 
Minimum in H. Max. — min. = 100 y. 
First Storm. 
May 13 d. 13 h, 9 m. Sudden commencement. A H = 4- 70 y ; A D = 
+ 1'; A Z = — 20 y. 
13 d. 13h. 9 m. Maximum in H. 
14 d. 5 h. 53 m. Minimum in H. Max. — min. = 210y. 
May 12 d. 6h. 
8 h. 13.5 m. 
8h. 14m. 
12 h. 
