218 



REPORT — 1895. 



Table VI. 



. conspicuously greater in the case of the horizontal force than in that of 

 the declination. 



There is, it will be noticed, a close coincidence between the time, 

 ^ hr. 38 min., of the minimum in summer of the horizontal force term 

 whose period is twenty-four hours and the time, 9 hr. 31 min., of the 

 first zero in summer of the corresponding declination term. The fact, 

 however, that no such coincidence presents itself between the correspond- 

 ing times in winter would suggest the phenomenon was in part at least 

 accidental. It can hardly be connected with the fact — shown conspicu- 

 ously by Tables III. and IV., or still better by curves 1 to 10— that the 

 time of the principal minimum of the total horizontal force inequality, 

 from 10 to 11 A.M., is nearly coincident with a time at which the total 

 declination inequality vanishes. Papers dealing with a theoretical con- 

 nection of the sort, by Professor A. Schuster and Mr. C. Chambers, will 

 be found in the ' Phil. Mag.' for April and May 1886, and in ' Proc. Lit. 

 and Phil. Soc.,' Manchester, Session 1886-87, pp. 23-46. 



liesultant of Cyclic Horizontal Disturbing Forces producing the 

 Diurnal Inequality. 



§ 11. The importance attaching to variations in declination suggests 

 most naturally the resolution of the horizontal disturbing force, to which 

 the diurnal inequalities just considered are due, into components ^X and 

 ?Y respectively in and perpendicular to the magnetic meridian. The 

 preceding results show that, H denoting as usual the total horizontal 

 force, cX/H and oY/H are at most small quantities of the order 1/500. 



Thus to a very close approximation we have 



aY=HtD ) 



(4) 



where ciH and cD are the inequalities of horizontal force and declina- 

 tion, while H is the mean value of the horizontal force for the time 

 under consideration. 



