Annual Variations of the Total Magnetic Force. Ixv 



The winter lunations for 1844 agree with this result, in having a minimum immediately after the inferior 

 transit, but not otherwise : this diflference, it is considered, is due to disturbances. (See No. 80.) 



Total Magnetic Force. 



133. Absolute Value of the Total Magnetic Force. — The absolute value of the total magnetic force de- 

 duced from the value of the horizontal component, and the magnetic dip, as in No. 81, is as follows : — 



Total magnetic force at Makerstoun for the mean epoch 1845 = 10-5267. 



1 34. Secular Change of the Total Magnetic Force. — The determination of this depends chiefly on the balance 

 magnetometer, and it is probable that the secular change from that instrument is not to be trusted (No. 84) ; 

 indeed it is probable that the total force remains nearly constant, and this is the more likely the nearer the 

 secular change deduced for the magnetic dip is considered to be to the truth. (See No. 116.) 



135. Effect of Disturbance on the Yearly Mean Value of the Total Magnetic Force. — By Nos. 54 and 85, 

 we find that the yearly mean deduced from the selected undisturbed days, is greater than that obtained from all the 

 observations by 0-000045, the total force here and in the following discussions being considered equal to unity. 



136. Annual Period of the Total Magnetic Force. — This result depends chiefly on that for the vertical com- 

 ponent and is entitled to the same weight. The following are the variations of the monthly means of the total 

 Ibrce, deduced from the observations of the balance and bifilar magnetometers for the four years 1843-6. 

 (See Nos. 56 and 87.) 



Prefix. 



Jan. 



Feb. 



March. 



April. 



May. 



Juue. 



July. 



Aug. 



Sept. 



Oct. 



Nov. 



Dec. 



0-000 



+ 099 



-013 



-074 



-075 



-034 



+ 120 



+ 052 



-001 



-049 



-087 



+ 011 



+ 048 



From these numbers, the total magnetic force at Makerstoun is a maximum about the solstices, and a minimum 

 immediateli/ after the equinoxes (See Plate VI.) 



137. The monthly means deduced from all the hourly observations in 1844 and 1845, were greater ( + ) or 

 less ( — ) than those obtained from the days selected as nearly free from disturbance by the following quantities, 

 (See Nos. 57 and 89.) 



Prefix. 



Jan. 



Feb. 



March. 



April. 



May. 



June. 



July. 



Aug. 



Sept. 



Oct. 



Nov. 



Dec. 



0000 



-069 



+ 001 



-106 



-060 



-062 



-003 



-037 



-061 



-052 



-078 



-008 



+ 002 



The eflect of disturbance on the monthly mean was nearly zero in the months of February, June, and 

 December, and it was greatest in March and October. When these numbers are subtracted from those in 

 No. 136 it is found that the annual period obtained from the undisturbed days in each month, has the same 

 epochs as that obtained from all the days. 



138. Annual Variation of the Manges of the Monthly Mean Diurnal Variation of the Total Magnetic 

 Force. The following are the ranges of the mean diurnal variation for each month, as obtained from Table 62, 

 deduced from four years' observations. 



Prefix. 



Jan. 



Feb. 



March. 



April. 



May. 



June. 



July. 



Aug. 



Sept. 



Oct. 



Nov. 



Dec. 



0-000 



319 



291 



612 



821 



701 



546 



663 



738 



715 



521 



443 



298 



The diurnal range of the total force was least in the three months, December, January, and February, and it 

 was greatest in April and August : the ranges for March and September were greater than the range for June. 

 The following are the ranges of the mean variations obtained from the selected days of 1844 and 1845. 



Prefix. 



Jan. 



Feb. 



March. 



April. 



May. 



June. 



July. 



Aug. 



Sept. 



Oct. 



Nov. 



Dec. 



O'OOO 



112 



151 



277 



398 



489 



449 



448 



389 



409 



321 



176 



154 



The range of the nearly undisturbed mean diurnal variation was least in January 1844-5, and it was 

 greatest in May, but it is probable that the range is nearly constant while the sun is north of the equator, and 

 that the differences exhibited here are due to the greater or less amount of disturbance remaining in the 

 selected days. (See Nos. 92 and 93.) 



139. Variations of the Daily Mean Total Magnetic Force, with Reference to the Moon's Age. — The fol- 



