DECENNIAL PERIOD OF MAGNETIC VARIATIONS, ETC. 573 
20. Summary of the Epochs if Maxima and Minima, deduced from the 
Magnetic Observations.—These are as follow :— 








Epochs of Interval Epochs of Interval 
Maxima. Years. Minima. Years. 
BeeSrifonte 
1870°85 9°20 
10°55 1866:50 
1860°30 10°20 
11°40 1856°30 
1848-90 12°45 
11°40 1843°85 
1837°50 10°35 
7°80 1833°50? 
1829°70 9:70 
11°50 1824:20 
USMS E Oba inter dri: or disweevrart eleerntrlaitd sscene 2 
Ceti 2 taeenene 
SaeiesS 2 nace 
thst 2 1800°5 
1797-7 5:5 
10°45 1795°0 
LSE] D TE Re dee Ci Se. | srr) ee al ee7r]| feat ihieas Y 
2 


It will be seen that the maximum interval occurred between the minima of — 
1843 and 1856, being 12°45 years; the diminution in the intervals afterwards 
induced me to conclude in September 1875, that we were approaching a 
minimum period like that experienced between 1829-1837.* ‘This is more 
evident when the intervals are arranged in succession as below. 
Interval of Minima, 9°20 ? years. Interval of Maxima, 11°40 years, 
5 Maxima, 10°55 i, Minima, 10°35 __s,, 
5 Minima, 10°20 =, 3 Maxima, 7-80: ©, 
ny Maxima, 11°40 sz, A Minima, 9°30 ,, 
- Minima, 12°45 __—s«, 3 Maxima, 11°50 so, 
21. Should the next maximum occur about 1879°5, the series will be 
completed within nearly 42 years. Thus we have the intervals— 
1875°7 ?—1833°5? = 42°20 years? 1866°50 —1824:20 = 42°30 years, 
1870°85—1829°70 = 4115, 1860°30—1818-20 = 42:10, 
22. If with an approximate interval of 42 years, we seek to determine the 
epochs before 1818, we shall find them as follows :— 
1, 1856°3 —42°0 = 18143 Minimum. 6. 1829'70—42°0 = 1787°7 Maximum. 
2. 1848°9 —42°0 = 1806°9 Maximum. 7. 1824:20—42'0 = 1783°2 Minimum. 
3. 1843°85—42'0 = 1801°85 Minimum. 8. 18182 —42:0 = 17762 Maximum. 
4, 1837:50—42°0 = 1795°5 Maximum. 9. 1814:°32—42°0 = 1772°3? Minimum, 
5. 1833°50—42°0 = 1791°5 Minimum. 
* Comptes Rendus, t. Ixxxi. p. 752, 26th Oct. 1875. 
