LI’, H. Bigelow—The Earth a Magnetic Shell. 91 
the labor involved, to compute yp definitely. This implies that 
from station to station the internal axis changes its position in 
the sphere, in the passage around the shell, and while we can 
never be absolutely exact in locating it, the attempt will 
be worth making. 
Fig, 4. 
Stream Lines through a Magnetic Shell. 
If the internal paths were definitely known, the value of 
would be most simply found by the continuity of the tangen- 
tial components of the external and internal force, and the 
discontinuity of the normal components in the ratio yp. 
H,,=H., H,..=. Ho, 
I have taken the value w=2 and plotted the internal lines 
by magnetic refraction at the surface. 
1: U.=tan g, : tan gy, 
where ¢,, ¢, are the external and internal angles of the line of 
force with the radius of the sphere at the station. 
