242 ASTRONOMY: SEARES, VAN MAANEN AND ELLERMAN 
DEVIATIONS OF THE SUN'S GENERAL MAGNETIC FIELD FROM 
THAT OF A UNIFORMLY MAGNETIZED SPHERE 
By F. H. Seares, A. van Maanen, and F. Ellerman 
Mount Wilson Observatory, Carnegie Institution of Washington 
Communicated by G. E. Hale. Read before the Academy, April 28, 1919 
A communication to the Academy in 1917^ describes the method used for 
the location of the sun's magnetic axis and gives values for the inclination, 
the period of revolution about the solar axis of rotation, and the epoch when 
the magnetic pole was on the central meridian. These results were based 
upon observations made on 6v3 days between June 8 and September 25, 1914. 
In the meantime the spectrograms of 11 additional days within the limits of 
this interval have been measured and reduced, and the elements defining the 
position of the axis have been revised with the following results (appended 
quantities are probable errors); 
^ '=6°0±0°4, P = 31.52 ± 0.28 days 
/o = 1914, June 25.38 d= 0.42 days, G. M. T. 
k — 0.99 (a constant inversely proportional 
to the polar field-strength) 
The modifications produced by the revision are unimportant, and, so far as 
the 1914 series of observations is concerned, these values may be accepted as 
final. 
The uncertainty in the period is naturally large, for it has been derived from 
data covering less than four complete revolutions of the magnetic axis. More- 
over, this limitation prevents any conclusion as to the constancy of the period. 
An improvement of the results in these particulars therefore requires additional 
observations, which should be distributed. over a long interval. A beginning 
in this direction was made with a short series in September, 1916, which should 
reduce the uncertainty in the period well below a tenth of a day and make it 
possible to carry the longitude of the pole forward, without ambiguity as to the 
number of revolutions, to the coming sun-spot minimum, when further observa- 
tions can be undertaken wihout risk of interference from the magnetic fields of 
spots. 
The 1916 observations are now under discussion, and the indications are that 
the longitudes to be derived from them will agree closely with those calculated 
from the 1914 results. The period given above is therefore probably near the 
true value. 
All discussions thus far described rest upon the assumption that the sun's 
general field is that of a uniformly magnetized sphere. The example of the 
earth, however, suggests that this hypothesis may be justified only to some 
rough degree of approximation, and that the field may not possess the 
uniformity hitherto presupposed. 
