Horizontal Component of Magnetic Foece. 



367 



TABLE XXXIII. — Differences between the Hourly Means of Bifilar Scale Readings for the whole Series 

 in each Month, and those for the selected Five Days ; or Table XXVI. minus Table XXIX. 



Considering, first, the differences of the hourly means for the year, as obtained from a comparison of the 

 whole observations with the 60-day series of observations, we obtain the following results : — 



The mean effect of disturbances upon the hourly mean of the horizontal component is a positive maximum 

 at 4'' 10™ P.M. ; it is a negative maximum about 12'' 30™, or near midnight; there is the appearance of a 

 secondary positive maximum at &^ 10™ a.m., and minimum at lO"" 10'" a.m. ; the effect is zero about S^, 7*", 

 and lO"" Q™ a.m., and at 8*^ 20"' p.m. The comparison of the whole series with the 120-day series, gives 

 almost exactly the same result. It will be perceived that the epochs for the effect of disturbance on the 

 horizontal component, are considerably different from those for the magnetic declination : the effect on the 

 latter is zero when the effect on the former is a maximum, and vice versa. The maximum effect of 

 disturbance in diminishing the horizontal component, occurs about two or three hours after the corresponding 

 epoch for the magnetic declination, the time for the latter being about 10'^ p.m. 



The gi-eatest effects of distui-bance in increasing and diminishing the hourly mean of the horizontal 

 component, as deduced from the comparison of the whole series with the 60-day series of observations, are as 

 follow, the whole horizontal component being unity : — 



Maximum effect in increasing the horizontal component (4'* 10"" p.m.) = 0000300 



dimmishing {IV" 10"" p.m. and 2^ 10"" a.m.) = 0-000228 



The effect of disturbances on the hourly means of the horizontal component is therefore less in compari- 

 son with the diurnal range, than in the case of the magnetic declination ; in the latter case the maximum effect 

 is between \ and J of the range of the mean diurnal variation, whereas in the case of the horizontal component 

 it is only between }- and \ of the diurnal range. 



An examination of the differences for each month, will shew that the law is somewhat variable ; the 

 secondary maximum about G^ a.m., and minimum about 10'' a.m, are most distinct in some montlis, especiallv 

 in the winter. 



