F’. -H. Bigelow—The Earth a Magnetic Shell. 83 
wide, and at the mean distance 34° from the magnetic poles; 
that the southern pole precedes the northern by about 102° ; 
that the synodic rotation period of the sun is 26°68 days, cor- 
responding to the equator, and not tosome mean value derived 
from the angular velocity of the sun spots in latitudes 12° to 
15°. 
There is one more suggestion, not heretofore advanced, to 
be derived from this magnetic system of the sun, namely, 
regarding the trumpet-shaped extensions of the outer corona, 
discovered by Professor Holden, in some photographs of Janu- 
ary, 1889. If we adopt J. J. Thomson’s language regarding 
the formation of a magnetic field by the motion of positive and 
negative Faraday tubes along the corresponding equipotential 
surfaces (see Recent Researches in Electricity and Magnetism, 
pp- 28-33), we have two possible conditions: (1) If the tubes 
move on the equipotential surfaces in circles about the axis 
of the magnetic sphere, then a mechanical force will be gen- 
erated along the curved lines produced by a meridianal section 
of the equipotential system as T T; (2) if the Faraday 
tubes move along the sections T T then the mechanical 
force will be cireuital about the axis. The former type is 
more probable. If this magnetic field is radiating, then the 
electric and the pressure systems are necessary attendants, 
and we shall have such extensive curved radiating lines T T 
as will dispose matter in space in the tubular forms displayed 
in the coronal extensions, and also probably in the two 
branches of the disks of the zodiacal light. At the earth the 
vectors of the electro-magnetic field have been briefly de- 
scribed* as derived from 30 stations. The following diagram 
was made from a photograph of my 30-inch globe model, 
looking directly down upon the north magnetic pole. About 
half the stations are omitted in the drawing to avoid confusion 
of the vector lines. The polar zone and mid-latitude zone are 
seen with their characteristic systems. The trace of tangency 
of the sheet at 8.30 a.m., through the pole, and back along the 
circle of division to 2.30 p.m., and the trace of zero deflection 
in declination at 10.30 a.m., are shown by dotted lines. From 
these vectors, and their variations, are derived the motions of 
the needle in the diurnal and annual periods, these impressed 
forces causing the observed deflections of the normal terres- 
trial magnetic field. These vectors were eliminated from the 
tabulations of observed magnetic forces in absolute measure by 
discussing the hourly variations on the monthly means. 
Besides such impressed forces, it is evident by an inspection of 
the record, that there is also the system of deflecting forces to 
* Astron. and Astrophysics, Oct., 1893. 
