IN THE MEAN EFFECTS OF THE LARGER MAGNETIC DISTURBANCES. 371 
the circumstance, that though the disturbance was manifested by the Declination at 
one station, no indication of it was shown by the contemporaneous observations of 
the Declination at another and a distant station. Now, simultaneity at stations sepa- 
rated by considerable intervals of longitude implies a difference in the solar hours; 
and the observations at Toronto show that a difference in the solar hour may deter- 
mine the question, whether a disturbance, which may nevertheless be common to 
both stations, may or may not be traceable at both by simultaneous observations of a 
single element only. Towards the attainment of a just conclusion, therefore, in 
regard to a possible local origin, it is indispensable that a more extensive general- 
ization should be made, and that contemporaneous affections of the three elements 
should be brought into the comparison. Nor can this condition of the inquiry be 
dispensed with even in comparing the phenomena at stations under the same meri- 
dian, but separated by large intervals of latitude, unless it be first shown that the 
same law of solar hours prevails at both stations in regard to the occurrence of the 
disturbances of each particular element. It need scarcely be said that the general 
simultaneity of the disturbances has a very important bearing upon their theory, 
inasmuch as it militates decidedly against the supposition of their originating in 
atmospherical peculiarities, and tends to assign them with far greater probability to 
a cosmical source. That some disturbances may have a local origin, is undoubtedly 
possible, but no such case has yet, I believe, been established on adequate evidence. 
For the purpose of viewing in its simplest form and expressed in numerical value 
the influence which, on a daily average, the larger disturbances exercise on the 
Declination, Inclination and Total Force, we must revert to the aggregate values in 
the five years which supplied the ratios of disturbance at the different hours in each 
of the six classes of phenomena contained in Table IV. From these values we 
obtain readily and immediately for each hour the excess in the aggregate amount of 
easterly over westerly, or of westerly over easterly deflection, and of disturbances 
which increase or decrease the Inclination or the Total Force over those which 
respectively decrease or increase those elements. Flence we easily form a table con- 
taining, for each of the elements at every hour, the numerical excess in the aggregate 
values of whichever kind of disturbance predominates at that hour; and by dividing 
the excess by 1550 , which is the number of days of observation in the five years, we 
have the mean daily effect corresponding to the values of the larger disturbances of 
each of the elements at the different hours; or the average diurnal variation of each 
element due to the larger disturbances. This is shown in Table V., in which the 
diurnal variation of the Declination and Inclination is expressed in decimals of a 
minute of arc, and that of the Total Force in parts of the Total Force at Toronto, 
which in absolute value, and employing British units, may be taken with sufficient 
approximation at 13 ' 9 . 
