Total Magnetic Force. 397 



Diurnal Variation of the Total Magnetic Force. — In the mean for the year the total force is a principal 

 maximum at 5'' 20™ p.m. and a principal minimum at 2'' 10" a.m. ; it is a secondary maximum at T*" 10™ a.m. 

 and a secondary minimum at lO'' 10™ a.m. The range from the principal maximum to the principal mini- 

 mum is 0-000490 ; the range from the secondary maximum to the secondary minimum is 0-000075, the total 

 force heing unity. 



In the months of June and July the principal minimum occurs between 10'* and 1 1'' a.m. and the secondary 

 minimum about 2^ a.m. ; the two minima have nearly equal values in the months of May and August ; the 

 minimum about 2'' a.m. is best marked in all the remaining months of the year. The principal maximum occurs 

 between 4'> p.m and 7'' p.m. in each month of the year, and earlier, on the whole, in the equinoctial than in the 

 solstitial months. 



Leaving the more minute considerations with respect to the variation of the diurnal law with season to be 

 made in connection with another year's observations, we shall only farther at present consider the diurnal 

 variation for the year as obtained iVom the observations upon the selected 120 and 60 days fi-ee from disturb- 

 ances. The variations for these series deduced from the last columns of Tables XXVIII., XXIX., XLIV., 

 and XL v., and reduced so that the mean for each series equals the mean for the whole series in the last column 

 of Table LVIII., are as follow : — 



Series. a.m. | 121» 1>> 21> S' 4l> Sh I!'' 7^ Sh 9l> lO'^ Hl'l \0>- 1^ 2>' Sh t'' 51' Gk 7l' at 9l> lOl" ll'' | p.m. 

 120-day, 0-000 ( 144 135 140 149 158 172 174 164 133 083 038 033 062 138 201 250 285 306 307 298 283 256 222 180 

 60-day, 0-000 | 171 159 156 158 161 175 176 164 140 086 037 022 058 130 199 242 269 288 288 286 270 249 222 194 



Both series give nearly the same result ; the epochs of maxima and minima are as follow : — 



120-day series, Principal Max. 5'' 50" P.M. Secondary Min. l' 30" a.m. Secondary Max. 5' 50" a.m. Principal Jlin. 10'' 50" A.M. 

 60-day series, S' 50" p.m 2'' 10" a.m S' 50" a.m 10' 50" a.m. 



The principal difference between the results for the complete and undisturbed series consists in the trans- 

 ference of the epochs of principal and secondai-y minimum ; in the complete series the principal minimum 

 occurs at 2^ a.m., in the undisturbed series a slightly-marked secondary minimum occurs at that hour, but the 

 principal minimum occurs about IP a.m., nearly the time of the secondai-y minimum for the complete series. 

 The range of the variations is much diminished in the selected series, the whole range for the 120-day series 

 being 0-000274, and for the 60-day series only 0-000266, little more than half the range for the whole series. 

 The range from the secondary minimum to the secondary maximum in the 60-day series is only 0-000020, so 

 that the selection of days nearly altogether free from disturbance has the effect here also of nearly obliterating 

 the secondary minimum and maximum. 



Ranges of the Mean Diurnal Variation of the Total Magnetic Force. — The ranges of the diurnal variation 

 for each month, as deduced from the whole series of observations, are as follow : — 



Jan. Feb. March. April. May. June. 



0-000 I 233 298 782 784 679 489 



The diurnal range of the total force is a principal minimum in January, and a secondary minimum in 

 June ; it is a maximum at the equinoxes. There is no doubt, however, that if the diurnal variations for each 

 month were deduced from the selected series of observations, the ranges would be found to obey the same law 

 as that already obtained for the vertical component from the same series. See p. 381. 



Diurnal Variation of the Total Magnetic Force with reference to the Moon's Hour-Angle. — The following 

 Table contains the variations of the total force, as deduced from the last three columns of Tables XXXI. and 

 XL VII. 



MAG. AND MET. OBS. 1844. 5 H 



