OF THE MAGNETIC FORCE. 



47 



For July, 

 For August, 



A„= 704^7 + 17.4 sin 



+ 0.7 sin 

 A„= 703''.3 + 17.1 sin 



+ 0.5 sin 

 For September, a„ = 708''. 8 + 14.3 sin 



+ 0.3 sin 

 For October, a„ = 691*. + 6.1 sin 



+ 1.1 sin 

 For November, a„ = 689''.7 + 4.4 sin 



+ 0.0 

 For December, a, = 701''.2+ 4.5 sin 



+ 1.0 sin 



We have also for summer half year 

 half year (October to March inclusive 

 expressions for the diurnal variation : — 



For summer, 

 For winter, 

 For year, 



A„= 700". 1 + 14.6 sin 

 + 0.5 sin 



A„= 702''.0 + 5.1 sin 

 + 0.0 



A„= 701''.0 + 9.7 sin 

 + 0.2 sire 



9 + 86° 30') + 2.6 sin (2 9 + 174° 16') 

 39 + 300°) 



8 + 81° 10') + 3.7 sin (2 9 + 215° 50') 

 39 + 75°) 



9 + 73° 57') + 2.9 sin (2 9 + 210° 24') 

 39 + 165°) 



9 + 119° 48') + 3.1 sire (2 9 + 236° 28') 

 39 + 210°) 

 9 + 83° 33') + 3.0 sin (2 9 + 254° 00') 



9 + 133° 49') + 4.3 sm (2 9 + 231° 57') 

 39 + 63°) 



April to September inclusive), for winter 

 , and for the whole year the following 



9 + 83° 40') + 2.5 sin (2 9 + 191° 01') 

 39 + 255°) , 

 9 + 108° 54') + 4.0 sin (2 + 229° 58') 



9 + 90° 17') + 3.0 sin (2 9 + 216° 22') 

 3e + 255°) 



The following comparison may serve to show the general representation of the 

 observations by the analytical expressions : — 





Comparison 



FOE August. 







Observed. 



Computed. 



Difference. 



0" 20". 6 



718 



718.2 







2 " 



715 



715.1 







4 " 



712 



711.3 



+1 



6 « 



706 



707.0 



—1 



8 " 



700 



699.9 







10 « 



691 



690.0 



+1 



12 " 



681 



683.0 



—2 



14 « 



686 



684.3 



+2 



16 " 



693 



693.5 







18 « 



704 



705.0 



—1 



20 " 



715 



713.9 



+1 



22 " 



718 



718.4 







The summer months are better represented than the winter months ; in May the 

 difference is below half a scale division ; in the winter season in several instances 

 it rises to 3, and in one case to 4 scale divisions. 



Diagram C exhibits the diurnal variation, observed and computed, for the six 

 summer months. Diagram D the same for the six winter months. 

 7 



