210 THE SUN-SPOTS OF THE PAST FEW TEAKS. 



Prof. Wolf, of Zurich, who is well versed in the history 

 of astronomy, has made a study of ancient records, 

 going back as far as 1615. He regards the ancient, and, 

 therefore, comparatively unreliable, series as extending 

 from 1615 to 1760. In this series he supposes the dates 

 of maximum to be known within a limit of two years. 

 In the modern series they are known within a limit of 

 half a year. From his investigations Wolf has arrived 

 at a mean period of 11.1 years. Other astronomers agree 

 nearly with him in dates of maximum. Thus Wolf 

 makes maximum epochs of 1837.2, 1848.6, 1860.2, and 

 De la Eue of 1836.97, 1847.89, and 1859.67. Spoerer 

 makes the period 11.3 years. 



The maximum follows minimum by a less time than 

 it preceeds the succeeding minimum. The ordinate of 

 the curve of variability increases through (Wolf) 3.07 

 years, and decreases through 7.04 years. Rue makes 

 the figures 3.52 and 7.55 years. This is a close approach, 

 as Wolf reckoned by number and Rue by area. 

 Spoerer differs considerably from both, giving the num- 

 bers 4.78 and 6.55. 



In the diagram constructed by Prof. Wolf to represent 

 the periodic changes since 1615, there is evident, beside 

 the well-recognized period of eleven years, another less- 

 marked rise and fall along the whole length of the curve. 

 Wolf has himself deduced a second long period of about 

 fifty-five years. That such a semi secular period exists 

 most astronomers are inclined to admit, but they do not 

 yet possess material enough to judge with any accuracy 

 of its length. The superposition of this period on the 

 shorter one would account for many of the irregularities 

 of the latter. 



The present maximum was looked for as early as 

 1882. The last minimum occurred about 1878.8. In 

 the last two periods the intervals between minimum and 

 maximum were 4.4 years and 3.6 years. Applying the 



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