

xxviii.] NORTH AND SOUTH ZONES. 429 



That there are no large spots either at the poles or the 

 equator, is one of the best known facts of solar physics. 



11. By the hypothesis the falls at any one time, especially when 

 the sun is least disturbed, should occur in about the same latitude 

 on either side of the equator, thus forming two zones, one north, 

 the other south. 



This follows naturally from the symmetry of the atmosphere, 

 in the main, producing equal thickness of atmosphere in the 

 same latitudes N. and S. 



The facts are quite in accord. As a rule, whatever be the 

 latitude of the spots north of the equator at any one time, those 

 on the south follow suit. 



12. By the hypothesis the latitudes of the north and south 

 zones of falls cannot remain constant. 



The falls will increase the temperature of the atmosphere 

 over the spots and prominences produced by them (see 5), and 

 they will end by perhaps increasing the height of the atmo- 

 sphere as well ; the particles then will descend effectively with 

 greater difficulty from both these causes, and their effects will 

 cease. 



There is ample evidence in eclipse photographs of these local 

 heatings to great heights, and there is also the fact that from 

 the first appearance of spots after a state of quiescence (a sun- 

 spot minimum), they continually change their latitude. 



This is a question of the highest importance, for the fact that 

 the falls continue at all, shows that so far we have only 

 considered one factor, and that there is another. To this we 

 must next refer. 



