DECENNIAL PERIOD OF MAGNETIC VARIATIONS, ETC. 565 
of London the coincidence between the epochs of minimum and maximum mag- 
netic disturbance and diurnal range of the magnetic oscillations in 1844 and 
1848, deduced by him from the colonial observations, as well as of the epochs 
obtained by Dr Lamont for the diurnal range, with those which ScuwaBeE had 
previously discovered in his persevering observations of sun-spot frequency.* 
The coincidence in ScowaBe’s and Lamont’s decennial periods was also 
remarked independently in the same year by Dr R. Wo tr of Bern (now of 
Zurich)+ and M. Gautier of Geneva.t The former, who has devoted himself 
with great zeal for years to the collection and discussion of observations on 
sun-spots made during the last two centuries, has deduced a mean period of 114 
years, differing widely from 10°43 years, the mean interval last obtained by Dr 
Lamont.§ The care bestowed on this investigation, and the wide interval 
covered by it, have given to Dr Wo Lr’s result so great a weight that it has been 
accepted by many men of science as the true duration of this inequality. 
While there can be little doubt that continuous series of magnetical obser- 
vations are better fitted for determining the true epochs of maxima and 
minima than observations of sun-spots, which cannot always be made, and 
which before this century were noted by different observers without system, 
yet when no magnetic observations have been obtained, any epochs of sun-spot 
frequency which Dr WoLr may have shown to rest upon sufficient data should — 
have a great value in this investigation. 
The first part of this paper is occupied in the determination of the mean 
duration and variable length of the ‘decennial period” from the earliest 
systematic observations of the magnetic needle till now, employing for this end 
a somewhat more exact method than has been used hitherto. The second part 
is devoted to the decennial period of magnetic disturbance of the magnetic 
declination (1854-64) at Trevandrum, and the relation of the changes of mag 
netic disturbance to those of sun-spot area, as determined by Messrs DE La 
RveE, Stewart, and Lawy. 
7. Trevandrum Observations—The following table contains the ranges of 
the monthly mean diurnal variations of magnetic declination for each year 
from 1858 to 1875; these are obtained from hourly observations from February 
1853 to February 1865, and from observations made during the following years 
eight times daily at the hours—64, 74, 103, 114 a.m., and 04, 23, 44, and 5} 
pM. The ranges were also deduced from the observations made at these 
hours in the years 1853 to 1865, and found to be on the average 003 less than 
from the whole series of 24 hourly observations ; this quantity was therefore 
added to the ranges after February 1865. 
* On the Periodical Laws, &c., received Mar. 18; read May 6, 1852—Phil. Trans., p. 183, 1852. 
+ Berner Mittheilungen, No. 245, 1852. $ Bibliotheque Universelle, Juillet et Aofit, 1852. 
| § Ueber die zehnjiihrige Periode; Sitz. b. der k. Akad. z. Miinchen, 1862, Bd. ii. Heft 2. 
