158 Proceedings of the Royal Society 
year, is different for each hour, while the maximum and minimum 
do not happen at exactly the same time for all hours of the day. 
Secondary maxima and minima are also shown, which vary in 
their epochs gradually from midnight to noon. 
No clear law appears to connect the amount of the maximum 
disturbance for any hour with that of the minimum for the same 
hour in the 11 years ; the ratio of the first to the second is least 
for the hours near noon, and greatest for those near midnight. It 
is found, however, that the maximum and minimum mean dis- 
turbance in the diurnal variation for each year, as well as in the 
decennial variation for each hour, are connected by the following 
relations : — D m being the maximum and D 0 the minimum dis- 
turbance. 
^/D m — JD 0 = Constant. 
The monthly mean disturbance at Trevandrum in each of the 
years 1854 to 1864 is compared with the monthly mean sun-spot 
areas deduced by Messrs De La Hue, Stewart, and Loewy, from 
Carrington’s and the Kew Observations, with the following re- 
sult : — The monthly mean disturbance in the years 1854-56 had 
a considerable value, and marked variations when there were few 
or no sun-spots. In 1857 to 1862 there are found several maxima 
and minima of disturbance and sun-spots which occur simulta- 
neously. In some cases, and especially in June 1862, there is a 
well-marked sun-spot area maximum without any corresponding 
change of magnetic disturbance. The cause of the solar disturb- 
ance did not then extend its action to the earth at that time. 
The author concludes with a notice of the hypotheses proposed 
to explain the decennial period of magnetic variations and of sun- 
spot frequency, as well as of the cause of sun-spots. It is pointed 
out that no theory of sun-spot formation can be accepted which 
does not explain their non- (or very rare) appearance every 10 or 
11 years, and therefore the cause of the decennial period is bound 
up with this explanation. A planetary action which disturbs the 
equilibrium of the solar gases has been proposed ; no other seems 
to present itself, and this the author believes will be found ulti- 
mately to be in question ; and though he has not himself been able 
to find any satisfactory evidence in its favour, yet remarkable 
results have been obtained by Messrs De De Eue, Stewart, and 
Loewy. 
