132 ANIs'UAL OF SCIENTIFIC DISCOVERY 



author some time ago to regard it as highly probable that the light of 

 the sun, and also it.s magnetic and heating powers, might be subject 

 to changes of a more complicated nature than has hitherto been sup- 

 posed, and that, besides the changes which are indicated by the great- 

 er or less frequency of solar spots, other changes of a minor charac- 

 ter, and occurring in shorter periods, might also taice place. la the 

 hope of detecting these supposed changes, the author resolved t.> 

 undertake the discussion of a series of magnetical observations, and 

 for this purpose he selected the observations made at the laiperial 

 Observatory of St Petersburg, the most northern station at wiuoli 

 hourly magnetic observations have been made for any lengthened 

 period. Commencing, therefore, with the year 1848, the greatest 

 and least values of the magnetic declination for every day were 

 extracted from the observations ; and, taking the differences and 

 arranging them in order, it was found, on a careful examination, that 

 they indicated changes of activity taking place in a period of 31 days. 

 The daily oscillations were then arranged in a table, when it was 

 found that out of 17 consecutive days, the amount of oscillation, on 

 range of the magnetic needle, was above the mean on 13 days and be- 

 low the mean on only four days ; while of the remaining 14 days, 

 the range was below the mean on 13 days, and above on one day 

 only. The total amount of the differences for the 17 days of maxi- 

 mum was -^ per day ; and for the 14 days of minimum -^g-. 



On proceeding to examine the observations for the succeeding 

 years it was found that they could not be represented by a period of 

 31 days. It appeared, therefore, at first sight, that the period which 

 had been obtained for 1848 was merely accidental ; but, guided 

 partly by conclusions drawn from his variable-star investigations, and 

 partly by the high degree of improbability that the results for 1848 

 could be due to mere accident, the author was led to think that the 

 period he had found for 1848 might be variable, gradually diminishing 

 for a series of years, and afterward gradually increasing, to diminish 

 again when it had completed its cycle of change. Assuming, there- 

 fore, that in every year periodic changes took place in the magnetic 

 activity of the sun, the author proceeded to determine for each year 

 the most probable approximate value of the period, and he obtained a 

 series of values gradually diminishing till 1856, when the period was 

 only about 23 days, and afterward rapidly increasing until, in 1851), 

 it amounted to about 32 days. A glance at these results at once sug- 

 gested the idea that the variable period thus found was in some way 

 connected with, and dependent upon, the great solar-spot period, 

 the minimum value occurring in the year of minimum frequency of the 

 solar spots, and the maximum values in the years when the spots 

 were most numerous. 



Several series of thermometrical observations were now examined 

 for indications of periodical changes in the element of mean daily 

 temperature, and it was found that they exhibited, with unexpected 

 distinctness, changes in this element occurring also in a variable 

 period, the range of variation being, however, somewhat less than in 

 the case of the magnetic element, although the times of maximum and 

 minimum were' almost exactly the same. The maximum and minimum 

 values were respectively 31 and 23d days. 



