2 9 o SCIENCE PROGRESS 



hand on the coil in the direction of the flowing current, the 

 extended thumb points to the north pole of the magnetic 

 lines of force. It would seem, then, that a sun-spot, and the 

 vapours constituting it, is essentially a vortex, or tornado, or 

 whirl of electrons or electric particles, which give rise to the 

 magnetic field of force. In the case of a current flowing in a 

 helical wire a reversal in the direction of the current would 

 cause a reversal in polarity. In a laboratory, when a polari- 

 scope is so adjusted as to transmit one component of a line 

 doubled by a magnetic field, when observed along the direction 

 of the lines of force, this disappears and is replaced by the 

 other component when the current is reversed. This test also 

 Prof. Hale applied to sun-spots, and found that in typical 

 sun-spot groups of leader and follower spots, the two spots had 

 opposite polarity. The intensity of the magnetic field in sun- 

 spots is sometimes as high as 4,500 units, or about 9,000 times 

 the intensity of the earth's magnetic field. This magnetic 

 field in sun-spots diminishes very rapidly in passing upwards 

 from the sun's surface, and it is probable that the vortex 

 producing the field is at a comparatively low level. 



Prof. Hale's discovery was suggested by the whirls observed 

 on the Ha line in spectro-heliographic photographs of the flocculi 

 in the environs of sun-spots. The different behaviour of the 

 spectrum lines in a spot when observed in a polariscopic spectro- 

 scope, when near the limb, and when approaching the sun's 

 central meridian, is also a crucial test of the truth of the theory. 

 In the one case the line of sight is directed at right angles, and 

 in the other, along the lines of force. It demands, however, 

 a gyratory motion in the vapours in the environments of sun- 

 spots. 



Yet it is doubtful whether the umbra and penumbra of the 

 spots themselves partake in this gyratory motion. For Mr. 

 Evershed finds that if a sun-spot be observed with the spectro- 

 scope slit placed across it diagonally from the sun's centre, 

 and when the spot is between 30 degrees and 50 degrees from 

 the central meridian, the lines of the spot spectrum are displaced 

 in the penumbra in opposite directions. If the spot is east of 

 the central meridian, the displacements indicate a motion of 

 approach on the side of the spot nearer the sun's centre, and a 

 motion of recession on the side of the spot nearer the sun's 

 limb, the reverse being the case when the spot is west of the 



