436 THE CHEMISTRY OF THE SUN. [CHAP. 



We have seen in (13) how the hypothesis suggests to us that 

 the sun-spot period is a direct effect of the atmospheric circula- 

 tion, and that the latitudes at which the spots commence to form 

 at the minimum, which they occupy chiefly at the maximum, 

 and at which they die out at the end of one period in one 

 hemisphere, after they have commenced to form a second 

 one, in the same or the other one or in both, are a direct result 

 of the local heating produced by the fall of matter from 

 above descending to the photosphere, and perhaps piercing 

 it. The results of this piercing, are, the liberation of heat from 

 below, and various explosive effects, which, acting along the line 

 of least resistance, give, as a return current, incandescent vapours 

 ascending at a rate which may be taken as a maximum at 250 

 miles a second, a velocity sufficient to carry them to very 

 considerable heights. 



Next we considered the evidence showing that the atmo- 

 spheric circulation is kept up by upper outflows from the poles 

 towards the equatorial regions. In these outflows a particle 

 constantly travels, so that its latitude decreases and its height 

 increases, the true solar atmosphere then resembles the flattened 

 globe in Plateau's well-known experiment. 



These currents, as they exist in the higher regions of the 

 atmosphere, carry and gather the condensing and condensed 

 materials towards the equator. 



We know that when the solar forces are weakest, slight descents 

 of material take place all over the sun, because at that time the 

 spectrum of the corona ; instead of being chiefly that of hydrogen, 

 is one of a most complex nature, so complex that before 1882 it 

 was regarded by everybody as a pure continuous spectrum, such 

 as is given by the limelight. 



When the fall of spot material begins in earnest we get a 

 return up-current in the shape of active metallic prominences, and 

 the production of cones and horns which probably represent the 

 highest states of incandescence over large areas and extending 



