386 
ASTRONOMY: HALE AND LUCKEY 
thus set up with the hehx as a core, which wiU remain nearly vertical 
as long as its angular velocity is low. But if the speed of the motor is 
increased, the lower end of the helix will gradually move toward the sur- 
face, giving rise to the successive forms shown^ in figures 1 to 6. The 
semi-circular vortex ring finally produced, like an ordinary smoke ring, 
will have a motion of translation at right angles to its plane, in a direc- 
tion corresponding to the direction of rotation of its inner edge. It is 
interesting to observe the depressions and whirls in the water when the 
end of the helix is approaching the surface. These represent in the hy- 
pothesis the incipient spots which often appear and disappear near one 
extremity of a bipolar group. 
An instructive control of this experiment may be readily appHed. 
The water in the tank is set into rotation (by an auxiliary paddle) about 
the axis of the helix before the motor is started. After the helix is set 
spinning it will turn up much more rapidly than in quiet water if the 
direction of the preliminary rotation is opposed to that of spin, i.e., if 
the water is whirling in the same direction as the second 'spot' of the 
group. If, however, the whole body of water is rotating in the same di- 
rection as the helix, the latter will turn up with difficulty, or remain in 
the vertical position without turning up at all. 
Thus a sun-spot vortex, if produced within a region already rotating 
