254 
A table is given showing the number of days included in 
maximum and the minimum portions of each mean period 
for the years 1848 to 1859, and the number of exceptional 
days, or those on which during the maximum part of the 
period the temperature was below , and during the minimum 
part above, the mean value. From this table it appears that 
out of a total number of 165 days of maximum only 14 were 
exceptional ; and out of a total of 1644 days of minimum 
the number of exceptional days was only 16. The mean 
gives a ratio almost exactly as 1 to 11. Considering that 
the values of the period in the different years are only 
approximate, this result may be regarded as affording 
satisfactory proof of the existence of a variable period of 
temperature ; but a comparison of the total amount of the 
differences of temperature from the mean, and the amount of 
exceptional differences which is given in another table, is 
much more striking and conclusive. From this comparison 
it appears that against a total amount of 255'61° of plus 
differences on maximum days, there were only 8T7° of minus 
differences ; and against 258 , 6T of minus differences on 
minimum days there were only 1 l - 79° of plus differences, the 
mean ratio of the amount of exceptional differences to the 
total amount being therefore as 1 to 25*7. 
At St. I’etersburg the average temperature of the warmer 
half of the period is not less than 3° greater than that of the 
cooler half, and as this difference of temperature is repeated at 
least twelve times in every year, it must necessarily exercise a 
powerful modifying influence over many meteorological 
phenomena. 
With reference to the differences between the maximum 
and minimum values of the magnetic and the corresponding 
values of the temperature period, it is remarked that owing 
to occasional interruptions of the magnet ical observations and 
to the enormous extent of the oscillations of the needle on 
particular days, the values of the magnetic period given in 
