586 MR J, A. BROUN ON THE 
43, Comparison of the Monthly Mean Sun-Spot Area and Monthly Mean 
Disturbance.—The annual law of variation of mean disturbance of magnetic 
declination for high latitudes, first deduced from the Makerstoun observations, 
and since confirmed by the results of many observatories, is, that there are 
maxima at the equinoxes, and minima at the solstices. This law, how- 
ever, does not hold near the magnetic equator. An examination of the monthly 
mean sun-spot area also has shown no similar law. The annual law of dis- 
turbance of magnetic declination at Trevandrum is a maximum in January, and 
a minimum in June.* This result is derived from the mean of eleven years’ 
observations, and different years show marked deviations from this law,t As I 
had deduced a somewhat similar mean law for the sun-spot area, I have sought 
to compare the monthly means of the two variations for the years 1854 to 1864. 
Both are projected in the lower curves (Plate XL). The following are the 
conclusions from these curves. 
44. 1st, During the period in which there were few spots, 1854 to 1856, 
there were large variations of the monthly mean disturbance, this difference 
is very marked in 1855. The minimum disturbance occurred evidently in 
March to June 1856; the sun-spot area was nearly zero from March 1855 to 
March 1857. 
2d, In August 1857 a slight increase of sun-spot area is shown with a cor- 
portance to know the ratio of the mean difference (or disturbance) for different parts of the earth’s 
surface; unfortunately this quantity has not been sought in general. Sir E. Sasrne has made his 
valuable investigations on disturbances above certain limits, which vary with the station, so that no 
exact comparison can be made. The only means from howrly observations, in which all the disturbances 
are included with which I am acquainted, are those for Makerstoun in the years 1844 and 1845; the 
minimum yearly mean value of the disturbance was that for 1845°4 = 1°66; if we compare this with 
the minimum at Trevandrum for 1859°7, we have 
Makerstoun 1°66 _ 5G 
Trevandrum’ 0298 ‘ 

If we compare, in like manner, the minimum yearly mean range of the monthly mean diurnal variations 
for the two places, we find 

Makerstoun 1844:25 786 _ rei e 
Trevandrum’ 1856°3 ’ 1°88 _ ? 
a less ratio than for disturbanees. 
Dr Luoyp has given the mean of all the disturbances for the years 1841 to 1850 (Dublin Mag. 
Obs. vol. ii. p. 88); they are derived, however, from two-hourly observations in 1841-43, and from 
six three-hourly observations in the following years; and the means are not strictly comparable with 
those at Trevandrum, we find, however, the ratio of the maximum to the minimum yearly mean dis- 
turbance from the three-hourly observations, 
1848'1 =. 2’"39 
1845-75? 1750 — 199: 
which is slightly greater than the ratio for the ranges (Art. 33). 
* Trev. Mag. Obs. vol. i. p. 142, 
+ There is one remarkably uniform result breaking the regularity of the mean law. The mean 
disturbance for November in eight years, and for October in the three remaining years is less than that 
for the months immediately preceding and following. 



