584 : MR J. A. BROUN ON THE 
In the interval 1850-61, the spot area remains nearly constant from .1859-4 
to 1861°5, while the magnetic oscillation attains a sharply marked maximum 
at 1860°25. 
In the next period the spot area diminishes with a series of secondary 
maxima and minima, which are also shown, but not simultaneously in ‘the 
magnetic oscillations. Dr Wo tr’s relative numbers have also been projected 
from 1860 to 1874,* as the spot areas are given only to 1867. In this period 
the sun-spot frequency curve shows a sharp maximum, while the magnetic 
oscillation maximum is much flatter. 
39. Though the differences between the curves for spot-area, or frequency, 
and for the diurnal oscillation, are, in several cases, strongly marked, there are 
also occasions on which inflexions in the one curve find corresponding in- 
flexions in the other. Obviously, the observations of sun-spots are imperfect 
and incomplete. We know also that the decennial variation of the diurnal 
oscillation is not exactly the same for different stations. Making every allow- 
ance for these two causes of difference, it still appears most probable that 
neither the area nor frequency of sun-spots is an exact measure of the mag- 
netic action, but that each is a distinct result due tothe same cause.t We may 
also conclude that any attempt to determine the amount of the diurnal oscillation 
of the needle at any place, by means of equations depending on the spot fre- 
quency, can only give such approximations as follow from the general agreement 
of the two phenomena within certain limits; thus the very rough parallelism of 
the curves of spot-frequency and of diurnal oscillation at Trevandrum from 1860 
to 1869 is very widely departed from thereafter. 
40. Decennial Period of Magnetic Disturbance at Te pede um, 1854°5 to 
1864:°5.—When we determine the mean diurnal variation of the needle for a 
month, the mean position of the needle for every hour is obtained ; the observed 
positions for a given hour on different days are generally east or west of the 
mean. These deviations are due to various causes; to variation of the diurnal 
law with season, to the sun’s rotation, the moon’s revolution, and the cause 
producing the secular and other variations of long period. In order to dimi- 
nish the effects of these various causes, the four-weekly mean diurnal variation, 
corresponding to the middle of each week in each of the 11 years, was com- 
puted. The observations in each week were then compared with their corre- 
sponding hourly means, and the differences were taken; these differences obey 
two diurnal laws, one depending on the moon’s, the other on the sun’s hour 
angle. The latter, due to deviations from the mean solar law, are called solar 
disturbances. The lunar observations have been shown to depend also indi- 
* Astron. Metth. xxxviii. S. 385, July 1875. 
+ This follows from the conclusion, Art. 36, sun-spots appearing only when the magnetic action 
exceeds a given value. 


