582 MR J. A. BROUN ON THE 
We find also from the Dublin observations between 7 A.M. and 1 P.M. for 
1849°50 and 1844:0:— 
12’-28 
SG ee 
which is nearly the same ratio as that already obtained (83) from all the obser- 
vation hours.* 
36. If we represent the mean diurnal range of the magnetic needle at any 
place in years of maximum and minimum disturbance by 7,, and 7 respectively, 
and at any other place by 7’,, and 7%, we have seen that for stations in the 
northern and southern hemispheres, and near the equator. 
Ul 
Vm ae ip ™m 
%) ro 
There is every reason to believe that the cases for which slightly different 
ratios have been found during the same period can be explained by the super- 
posed variations, due to local laws, of the magnetic SES EE Snes as they affect 
the mean position of the needle at different hours. 
Tf, then, the diurnal oscillations increase from the minimum value to the 
maximum in a constant ratio for different stations, or if 
We arrive at the very probable conclusion that the varying amount of the 
oscillation is due to the variation of intensity of the same cause; or that the 
cause of the diurnal movement of the magnetic needle is the same in years when 
the sun is without spots, as in the years when the spots are most numerous.t 
* The generality of this result may also be shown by the equations representing the diurnal varia- 
tions in different years. These for Hobartoun in the minimum year 1843-5 (January to December 
1843), and the maximum (observed) year 1848°25 (October 1847 to September 1848) are as follow :— 
1848-25. y=1'48 Sin (6 + 200°) + 1°30 Sin (20+ 0°) + 0’'56 (Sin 30+ 201°) + 0°20 (Sin 40+ 8°) 
18435. y=0"94 Sin (6+ 209°) +095 Sin (204 4°) +0'37 (Sin 30+ 203°) + 0°12 (Sin 40+ 14°) 
When we remember the greater amount of irregularity produced in the diurnal variations by the dis- 
turbances in 1848, it will be seen that the equations for both years show as nearly the same law as 
could be expected with a cause whose increased action in producing the diurnal variations is accom- 
panied with increased irregularities. 
+ It is not to be forgotten that near the equator the diurnal variation of the magnetic needle 
during the equinoctial months is nearly annihilated by the action of two opposing laws, that of the 
northern and that of the southern hemisphere, one of‘ which prevails more or less in the other seasons. 
Since both the southern and northern forces vary alike (or nearly so) during the decennial period, it is 
only the difference of the increments or decrements which are shown at Trevandrum; and it is one of 
the results most confirmatory of the preceding conclusion that the increase from the minimum 
to the maximum year bears still the same ratio to the whole diurnal collection at the minimum as for 
Hobarton, Dublin, and Toronto, where one of the forces always prevails. 

