IN THE MEAN EFFECTS OF THE LARGER MAGNETIC DISTURBANCES. 361 
which the same labour has been bestowed as on those of the Horizontal and Vertical 
Forces^ so as to cause them to include all the observations which differ less, and exclude 
all those which differ more than a certain fixed value from themselves. In this revi- 
sion also the fixed or standard measure of a large disturbance has been increased from 
five scale-divisions (3'‘6 of arc) to seven scale-divisions (or 5'"0 of arc) ; the experi- 
ence gained in the first or experimental examination having led to the belief that the 
higher standard is on the whole to be preferred. 
General Conclusions. 
Decenjiial Period. — -In a communication made to the Royal Society on the 18th of 
March, 1852, it was shown that the larger disturbances of the Declination, both at 
Toronto and at Hobarton, indicated by the variation in their numbers and aggre- 
gate values in different years, the existence of a periodical inequality., of which the 
extreme and opposite phases were five years distant from each other, and the years 
1843 and 1848 were respectively the epochs of minimum and maximum. The 
examination which led to this conclusion comprehended the years from 1841 to 1848 
inclusive, and was definite in respect to 1843 as the year of minimum, inasmuch as 
1841, 1842, and 1843 showed a progressive decrease in the number and aggregate 
value of the larger disturbances in each year, whilst from 1843 to 1848 there was a 
progressive increase of both in successive years. It was noticed in the same com- 
munication, that the regular diurnal variation of each of the three magnetic obser- 
vation-elements at Toronto and Hobarton, the Declination, the Horizontal Force and 
the VerticalForce, and of each of the two derived elements, the Inclination and theTotal 
Force, exhibited evidences of a corresponding periodical inequality in the amplitude 
or extent of the diurnal variation, the years 1843 and 1848 being also epochs re- 
spectively of minimum and maximum. The observations of one of the elements (the 
Declination), extending uninterruptedly at both stations over eleven entire years, or 
from 1841 to 1851 inclusive, distinctly pointed out 1848 as the year of maximum as 
well as 1843 as the year of minimum. 
This discovery of the existence of a periodical inequality, common to the magnetic 
disturbances of larger amount, and to those more regular diurnal magnetic variations 
of which the sun has been long recognized as the primary cause (inasmuch as they 
conform systematically to laws depending on solar time), was regarded as affording 
presumptive evidence of the subsistence of a causal connexion common to those two 
classes of phenomena, which presumption was corroborated by facts adduced in the 
same communication, proving that the disturbances are themselves subject, on the 
average, to a well-marked diurnal period, which is also regulated by solar time. 
The periodical inequality thus manifested, having its opposite phases of maximum 
and minimum separated by an interval of five years, and of which the cycle might 
therefore be conceived to include about ten of our solar years, did not appear to 
connect itself with any of those divisions of time with which we are conversant as 
