253 
minimum to the mean value, are therefore as 1 to 1(K)9, 
and as 1 to 8.34. 
On proceeding to examine the observations for the 
succeeding years it was found that they could not he 
represented by a period of 31 days. It appeared, therefore, 
at first sight that the period which had been obtained for 
1818 was merely accidental; but, guided partly by con- 
clusions 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 afterwards 
gradually increasing, to diminish again when it had com- 
pleted its cycle of change. Assuming, therefore, 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 afterwards 
rapidly increasing, until in 1859, it amounted to about 32 
days. A glance at these results at once suggested the idea 
that the variable period thus found was in some way con- 
nected 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 23£ days. 
