Veetical Component of Magnetic Force. 



381 



TABLE XLVI. — Epochs of Diurnal Maxima and Minima of the Vertical Component of Magnetic 

 Force, obtained from the selected series of observations, 1844. 



The principal maximum is indicated by + and the principal minimum by — ; the differences, however, 

 between the values of the principal and secondary points are small, excepting perhaps for the months of 

 December, January, and February. The epochs for the 10-day series are the most consistent, 5 days are pro- 

 bably too few for consistent results, especially in the present case, where the days were selected chiefly as being 

 free from irregularities for the magnetic declination. 



Ranges ■ of the 3Iean Diurnal Variations. — The follomng are the ranges of the monthly mean diurnal 

 variations, as deduced from all the hourly observations, and from the hourly observations on the 10 days and 

 on the 5 days selected as least affected by disturbances : — 



Feb. JIareh. April. May. June. July. Aug. Sept. 



273 688 705 516 233 298 587 616 



124 222 201 201 211 204 194 229 



137 140 262 188 258 206 233 177 



Oct. Nov. Dec. Year. 



702, 588 292 419 



159 140 084 124 



135 171 083 111 



The range of the diurnal variation from all the observations is a minimum at the solstices, and it is a 

 maximum at the equinoxes, being three times gi-eater at the latter than at the summer solstice. Wlien we 

 consider the range of the diurnal variation nearly unaffected by intermittent disturbances, as deduced from 10 

 days selected, we find a totally different resxdt, as in the case of the other elements discussed the diurnal range 

 has a nearly constant value during the months for which the sun is north of the equator. The excess, 

 therefore, of the range at the equinoxes from all the observations, is due solely to disturbances, which, as has 

 been shewn in the previous discussions, are a maximum at these epochs. The ranges from the 5 days selected 

 are perhaps less accurate, owing to the fewness of the observations from which they are obtained. 



MAG. AND MET. OBS. 1844. 



5d 



