Total Magnetic Force. 399 



Ibtteemittent Disturbances. 



Effect of DisUtrbances on the Yearly Mean for the Total Magnetic Force. — We obtain the following results 

 from the mean values of the horizontal and vertical components for the year, pages 365 and 384 : ■ 



The mean total force, as deduced from the whole series of hourly observations 1 120-day series, by 0'000038 

 for the year, is less than that deduced from the J 60-day series, by 0-000039 



Effect of Disturbances on the Monthly Mean of the Total Force. — Using the corrections for the two com- 

 ponents of force, pages 365 and 384, we obtain the following quantities, corrections of the means of total force 

 for each month obtained from the 10-day and 5-day series, to tlie means from the complete series : 



Jan. Feb. March. April. May June. Jnlj. Aug. Sept. Oct. Nov. Dec. 



10 days, -0000046 +0-000050 -O'OOO | 152 030 067 026 030 029 049 091 01'' I +0-00002'' 



6 days, -0-000037 +0000052 -0-000 | 163 062 044 083 028 015 039 079 028 | +0-000034 



From these quantities the effect of disturbance is in general to diminish the value of the force, the gi-eatest 

 diminutions occurring in March and October ; the diminution is least near the summer solstice, and the effect 

 is to increase the force in December and February. Portion of these effects is due to consecutive disturbance 

 such as secular change, but the elimination of that portion would not affect the generality of this conclusion. 

 By subtracting the previous quantities from the monthly mean variations for the total force, p. 395, we would 

 obtain the monthly means as deduced from the 10-day and 5-day series ; the resulting means give the same 

 law as that obtained from the means for the whole series. 



Effect of Disturbances on the Hourly Means of the Total Magnetic Force. — The following are the differ- 

 ences of the hourly means of the total force, or the means as deduced from the whole series minus the means 

 as deduced from the 120-day and 60-day series of observations; each series having the same mean value 

 See Table LVIII., and page 396. 



Whole aeries 



Minus. A.M. 12'* 



120-day series, -0000 1 121 

 60-day series, -0 000 | 146 



P.M 



Oh 



120-day series, +0-000 I 073 

 eo-day scries, +0-000 \ 077 



These quantities give the same law of variation. The positive effect of disturbance upon the mean of the 

 total force, is a maximum at S*" 10™ p.m.; the negative effect is a maximum at 2'' 10™ a.m. The effect is 

 zero at lOi" 20™ a.m., and at 9"^ 10" p.m. 



The effect of disturbance in diminishing the westerly declination, and in increasing the dip, is a maximum 

 at 9^ P.M., when the effect on the total force is zero ; the effect of disturbance in increasing the westerly 

 declination is a maximum at 10'' a.m. ; at the same hour there is a secondai-y maximum of effect in increasing 

 the dip, and at the same hour the effect on the total force is zero. On the contrary, when the effect of 

 disturbances upon the mean total force is a positive maximum, — namely, at 5'' p.m., the effect upon the mag- 

 netic declination and dip is zero ; and when the effect upon the total force is a negative maximum at 2'' a.m. 

 the effect upon the magnetic declination and dip is nearly zero ; so that, ivhen the effect of disturbance upon the 

 direction of the magnetic force is a maximum, the effect upon its intensity is sero, and -vice versa. 



It is remarkable, even merely as a coincidence, that the effect of disturbance upon the direction of the 

 magnetic force is a maximum when the sun is on or near the magnetic meridian, and zero when nearly at right 

 angles to that plane ; while the effect of disturbances upon the intensity of the magnetic force is zero when the 

 sun is near the plane of the magnetic meridian, and a maximum when in the plane nearly at right angles to 

 it, for the effect of disturbance differs little at 4'' a.m. from that at 2'' a.m. when it is a maximum. 



