Diurnal Variations for the Vertical Component of Magnetic Force. li 



97. From Table 46 we conclude, 



1st, That the diurnal range of the vertical component is greatest immediately after opposition, and that 

 it is least about conjunction ; there is the appearance of a secondary maximum at conjunction in three of the 

 years, and in the means of all. 



2d, That the diurnal range is a minimum when the moon is farthest south, and also when it is farthest 

 north, and that it is a maximum when the moon is north of the equator. 



Table 47. — Mean Difference of a Single Observation of the Vertical Component of Magnetic Force, 

 from the Monthly Mean at the corresponding Hour, with reference to the Moon's Age and 

 Declination. 













After 











Moon's 









Varia- 



Moon 









Varia- 



Age. 



1844. 



1845. 



Mean. 



tions. 



farthest 

 North. 



1844. 



1845. 



Mean. 



tions. 



d. d. 



O'OO 



0-00 



O'OO 



0-00 



d. d. 



0-00 



0-00 



0-00 



0-00 



14—16 



0219 



0143 



0181 



+ 0006 



27— 1 



0171 



0148 



0159 



-0016 



17—20 



0229 



0152 



0190 



+ 0015 



2— 5 



0204 



0178 



0191 



+ 0016 



21—24 



0156 



0173 



0164 



-0011 



6— 8 



0176 



0166 



0171 



-0004 



25—28 



0151 



0186 



0168 



-0007 



9—12 



0158 



0182 



0170 



-0005 



29— 1 



0173 



0188 



0180 



+ 0005 



13—15 



0162 



0147 



0154 



-0021 



2— 5 



0134 



0163 



0148 



-0027 



16—19 



0206 



0193 



0199 



+ 0024 



6— 9 



0185 



0166 



0175 



0000 



20—22 



0181 



0150 



0165 



-0010 



10—13 



0240J 



0157 



0198 



+ 0023 



23—26 



0220 



0156 



0188 



+ 0013 



98. Variation of the Mean Difference of a Single Observation from the Monthly Mean for the corresponding 

 Hour, with reference to the Moon's Age and Declination, — Table 47 has been formed from Table 41, p. 386, 

 1844, and Table XLII., p. 23, of this volume. The conclusions from Table 47 are nearly the same as those 

 already made for the diurnal range, No. 97, they are as follow : — 



\st. The mean difference of an observation of the vertical component from the monthly mean for the 

 corresponding hour is a maximum about the time of opposition, and a minimum before and after conjunction, 

 a secondary maximum occurring at conjunction. 



2d, The mean difference is a minimum when the moon is farthest north, and also when farthest south, 

 maxima occurring between these epochs. 



The differences of the results for the single years from those for the mean of both are not greater than 

 might be expected in such an investigation ; the general agreement of the results, however, with those for the 

 diurnal ranges deduced from four years' observations is a confirmation of their accuracy. 



Diurnal Variations for the Vertical Component of Magnetic Force. 



Diurnal Variation of the Vertical Component. — The following Table has been formed in the manner 

 already described for the magnetic declination. No. 26 ; the means from which it has been formed will be 

 found in the previous volumes, and in this volume, pages 20 and 37- 



99. The following are the approximate epochs of the maxima and minima in apparent time, as deduced 

 from Table 48. 





Jan. 



Feb. 



March. 



April. 



May. 



June. 



July. 



Aug. 



Sept. 



Oct. 



Nov. 



Dec. 





h. m. 



h. m. 



h. ni. 



h. m. 



h. m. 



h. m. 



h. m. 



h. m. 



h. m. 



h. m. 



h. m. 



h. in. 



Max. 



+ 7 5 



+ 5 25 



4- 5 10 



+ 60 



+ 60 



+ 6 20 



+ 5 20 



+ 60 



+ 55 



+ 40 



-1- 6 15 



+ 6 45 



Min. 



-14 



-14 



-14 



-12 15 



-13 45 



13 45 



-14 10 



-13 15 



-13 15 



-16 30 



-13 



-16 20 



Max. 







20 15 



21 



19 50 



20 



20 30 



20 40 



22 



? 







Min. 











15 







- 10 



23 55 



23 40 



23 30 



? 







The principal maximum occurs between 4*^ p.m. and 7^ p.m. in each month of the year ; it occurs earliest in 

 February and March of the first six months of the year, and in October and September of the remaining 

 months ; it occurs latest in January and December ; and later in June than in the immediately preceding and 

 succeeding months. A minimum occurs between midnight and 4*^ a.m. throughout the year, which is the prin- 

 cipal minimum excepting in June. 



Only one maximum and minimum occur in the diurnal variation for the four winter months, November to 

 February ; in the other months a secondary minimum occurs about noon, which becomes more distinct the nearer 

 the time is to the summer solstice, when that minimum is better marked than the other near midnight. 



MAG. AND MET. OBS. 1845 AND 1846. n 



