392 TRANSACTIONS OF SECTION A. 
dv an element of volume of dé of time. It may be shown that the action is 
a minimum consistent with the conditions 
dik 
7 DE == O- ee: nas xh RE) 
If we have also 
—=-cCulE DwH=0 ony tf meee? reuai ang Rag (2) 
(1) and (2) are the electro-magnetic equations. And the necessary conditions 
at any boundary of the electro-magnetic field are just those which obtain at a 
perfect. reflector. 
(1) and (2) possess integrals of two different types. The quantity 
{[eas 
is a constant where ds is any element of area of a closed surface. Thus the 
electric induction over any closed surface is constant, and any portion of matter 
therefore carries a constant change which is yet not an electric substance. 
Further, suppose the material surface is multiply connected like an anchor ring. 
The magnetic induction across the aperture is constant, remembering that at 
the material surface the conditions are the same as those at a perfect reflector. 
Hence the anchor ring may behave like a permanent magnet of constant 
moment. ‘lhe external field produces a tension of amount 
L (ew) 
tar 
in unit area of the surface of matter. This tension accounts for all the 
observed magnetic and electric forces. Magnetic induction is now explicable, 
because this tension acting upon matter has replaced the force acting upon 
electric fluid. The molecular magnetic field is not to be explained as due to 
the circulation of electric current sheets. That is merely a mathematical device. 
There remains the question what constraints must be applied to the material 
surfaces that they may be stable. That problem is not solved in any theory; 
it is no more or less soluble in the present theory than in any other. 
DrPARTMENT OF CosmicaL PHysics. 
1. Radial Motion in Sun-spots. By C. E. Sr. Jon. 
This investigation was undertaken in 1910 with the 60-foot Tower telescope 
and the 30-foot spectrograph on Mount Wilson. Mr. Evershed’s announcement of 
the discovery of displacements of the Fraunhofer lines in the penumbre of spots 
was made in 1909 (Bulletin XV., Kodaikanal Observatory), These displacements 
were referred by him to movements of the vapours of the reversing layer out- 
ward from spots and tangential to the solar surface. His observations were 
made by placing the slit of the spectrograph across the spot and coincident with 
the radius of the solar disk passing through the centre of the umbra. Under 
these conditions many solar lines became curved somewhat like the letter S when 
the spot was between a quarter and half-way from the limb to the central 
meridian. ‘The measurements were made by determining the angle through which 
the observed sections of the lines were tilted and calculating the displacement. 
A preliminary investigation at once confirmed Mr. Evershed’s observations, but 
it seemed desirable to have a method of measuring the displacements directly. 
An occulting arrangement was devised by which the spectra of the outer edges of 
the penumbra directed towards the limb and the centre of the disk respectively 
were obtained side by side upon the photographic plate. By this arrangement 
the displacements to be measured were doubled, and the measurements could be 
made directly with a high degree of accuracy. An extended region of the 
spectrum was examined, and some 500 lines were observed. When the data were 
