OF THE MAGNETIC FORCE. 



47 



For July, A, = 704". 7 + 17.4 sin (9 + 86 30') -f 2.6 sin (29 + 174 16') 



+ O.T sin (39 + 300) 



For August, A, = 703 d .3 + 17.1 sin (9 + 81 10') + 3.7 sin (20 + 215 50') 



+ 0.5 sin (39 + 75) 



For September, A, = 708 d .8 + 14.3 sin (0 + 73 57') + 2.9 sin (20 + 210 24') 



+ 0.3 sin (39 + 165) 



For October, A, = 691 d .O + 6.1 sin (9 + 119 48') + 3.1 sin (20 + 236 28') 



+ 1.1 sin (30 + 210) 



For November, A, = 689 d .7 + 4.4 sin ( + 83 33') + 3.0 sin (20 + 254 00') 



+ 0.0 



For December, A, = 701 d .2 + 4.5 sin ( + 133 49') + 4.3 sin (20 + 231 57') 



+ LO sin (30 + 63) 



We have also for summer half year (April to September inclusive), for winter 

 half year (October to March inclusive), and for the whole year the following 

 expressions for the diurnal variation : 



For summer, A, = 700 d .l + 14.6 sin (0 + 83 40') + 2.5 sin (20 + 191 01') 



+ 0.5 sin (30 + 255) 



For winter, A, = 702 d .O + 5.1 sin ( + 108 54') + 4.0 sin (20 + 229 58') 



+ 0.0 



For year, A, = 701 d .O + 9.7 sin (0 + 90 17') + 3.0 sin (2 + 216 22') 



+ 0.2 sin (30 + 255) 



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

 observations by the analytical expressions : 



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 



