264 A. D. Bache on the Horizontal Component of Magnetic Force. 



when required, until all readings differing 33 scale divisions or 

 more had been excluded. Table No. VII, of the full memoir, 

 contains the normals thus found, showing the monthly normals 

 of the bi-hourlj and hourly readings of the bifilar magnetometer, 

 reduced to a normal temperature, and corrected for irregularity 

 in the progressive change. 



The variation in the amplitude of the diurnal motion of the 

 horizontal force is subject to the same inequality of about eleven 

 years as the declination, and the means of investigation will be 

 analogous to those used in part I. of this discussion. For greater 

 convenience, the preceding monthly normals were united into 

 annual means, and the results put into an analytical form, using 

 Bessel's function applicable to periodical phenomena, and deter- 

 mining the numerical quantities by the application of the method 

 of least squares. 



Table No. VIII, of the full memoir, shows the regular solar 

 diurnal variation of the horizontal magnetic force for each year 

 of observation, expressed in scale divisions of the instrument. 



These numbers being used in the analytical form, a very close 

 coincidence between the computed and observed results is ob- 

 tained, the differences, using three terms in the equations, being 

 within the uncertainty of the observed values. The probable 

 error of a single representation for a given hour is ±0-6 scale 

 divisions, or ±0-00009 of the absolute force. 



The curves in the accompanying diagram, fig. A, show the 

 results of the corrected means and of the equations. The cor- 

 rected means are given by dots. 



The curves show a double progression in the daily motion, 

 with a principal maximum of horizontal force in the morning, a 

 principal minimum before noon, and a secondary maximum in 

 the afternoon; the precise epochs (to the nearest 5 minutes) and 

 extreme values were computed by means of the preceding 

 formula : 



-TITLS^S^S- 



D.u™a,rao.ein \^^"X^ro.V 



Ju^Mo Epoch. 

 i84o-4.i ?-45 



iiil 



1 Mc„. ,-5^ 



mm 



.fe 



i 



o-:^8|X57 



-i: 



lis 



a.3-5: 



784-0 

 Si, 



