PAST CHANGES IN SUN-SPOTS 555 



the maximum of 1787 to that of 1801 is 17 years and from 1870 to 1883 

 is L'3 years, the average of the two being 15 years. While the solar 

 activity remains low, the length of the intervals from maximum to maxi- 

 mum remains long. For instance, the five cycles between maxima which 

 ha\'e sun-spot numbers of less than 95 have an average length of 12.2 

 years. The remaining cycles — that is, the seven which precede sun-spot 

 maxima of 95 or higher — have an average length of 9.4 years. From this 

 we see that in a period of about 100 years the sun-spots pass through a 

 great cycle which begins with a very long minor cycle and a low maxi- 

 mum. iVfter this the leng-th of the minor cycle decreases and the maxima 

 rise higher and higher. Manifestly, as our diagram shows, the great or 

 major cycle is quite as real as the so-called 11-year or minor cycle, which 

 really varies from 7 to 17 years. The more we study sun-spots the more 

 we see that they are characterized by great irregularity. All attempts to 

 find a definite period have broken down. There may be a distinct pe- 

 riodicity for a few cycles, but it soon changes. If sun-spots can vary in a 

 minor cycle having a length from 7 to 17 years, in a larger cycle which 

 perhaps has a length of about 50 years, as made out by Fritz, and in a 

 still larger one, which has a length of perhaps a century, it would seem 

 probable that they can also vary in cycles of very much greater length 

 and greater intensity. 



THE NATURE OF 'SUN-SPOTS 



At this point a word may well be added as to the nature of sun-spots. 

 Recent studies seem to indicate that they are probably cyclonic vortices 

 which partake somewhat of the nature of volcanoes and somewhat of the 

 nature of cyclonic storms. Of course, in a highly heated body like the 

 sun, where everything, on the surface at least, is in a gaseous condition, 

 we can not expect any exact analogy with our solid earth. Nevertheless 

 the resemblances are striking. Like both volcanoes and cyclones, the sun- 

 spots appear to carry material from lower to higher levels. It is thought 

 by Hale and others that the material which is carried out is cooled some- 

 what, and that this cooling may cause it to act like a cloud and thus ap- 

 pear dark. Humphreys has suggested that perhaps a certain amount of 

 solar radiation is actually cut off in this way. Another respect in which 

 sun-spots resemble cyclones is that they seem to have a spiral motion re- 

 sembling that of the inblowing winds of our terrestrial storms. They do 

 not travel rapidly like cyclones, but they are not stationary like volcanoes. 

 Another inlportant characteristic is that they are highly electric in their 

 constitution, and in this quality they resemble most volcanic discharges 

 and many cyclonic storms. Still another characteristic is that they seem 



