468 





Mr 



. S. G. Starlin 



g on 



the Equilibrium of 







9. 



60°. 



70°. 



i 

 80°. 



85°. 







X. 





X. 





X. 





X. 



</>'• 



0. 





</>• 





<p. 





(p. 









N. 



S. 







N. 

 66 



S. 

 66 





N. 



S. 





N. 



S. 











64 



64 







67 



67 







67 



67 



10 



19* 



82* 



45* 



27 



92 



40 



45 



112 



23 



64 



131 



3 



20 



36 



100 



31 



47 



113 



21 



64 



132 



5 



77 



145 



- 9 



30 



49 



113 



20 



59 



126 



11 



73 



142 



-3 



81 



150 



-11 



40 



59 



125 



13 



68 



137 



5 



78 



150 



-6 



84 



155* 



-12 



50 



67 



136 



8 



74 



146 



2 



82 



156 



-7 



86 



160* 



-11 



60 



74 



147 



41 



79 



154 







84 



161 



-5 



87 



164* 



- 9 



70 



80 



158 



2 



83 



163 



-1 



86 



167 



-4 



88 



169 



- 6 



80 



85 



160* 



1 



86 



171 







88 



173 



-2 



89 



175 



- 3 



85 



87 



174 



1 



88 



175 







89 



177 



-1 



m 



177£ 



- 1 



The values of x, the deviation of the compass from true 

 magnetic north as seen upon the card, are now plotted against 

 4>' the true compass-course of the aeroplane, the values of op 

 when cj)' is greater than 90° being derived from the Tables, 

 as before described. 



Discussion of Curves. — It will be seen (fig. 5) that, as the 

 machine turns from N. to E., the equilibrium position of 

 the compass may be anything from true magnetic N. to true 

 magnetic S., according to the angle of tilt at which the machine 

 takes the curve. For tilt below 23° (that is, the complement 

 of the angle of dip) the error never reaches 90°, but at a tilt 

 of 23° the compass-card is perpendicular to the total earth's 

 magnetic field when the machine is flying E. Consequently, 

 there is no controlling field whatever, and the card will then 

 set anywhere. This vanishing of control is shown by the 

 instability of the compass, as indicated by the fact that the 

 23° curve bends sharply up (or down) to reach the 180° or 

 (0°) point, when the magnetic course reaches 90°. It is a 

 general rule that the steeper the curve at any point, the less 

 will be the stability of the compass-card at that point. 



For tilts above 23°, it is obvious that the compass 

 becomes exactly reversed when flying E., for the direction 

 of the resultant magnetic field of the earth is now pulling 

 the needle into the reverse of its usual direction with respect 

 to the bowl (fig. 6) . This is indicated by the fact that the 

 curves for tilts above 23° run to the 180° point on the course 

 90°. It may also be noticed that as the tilt approaches 90° 

 the error on the 0° course approaches 67°., in fact, at this 

 point the card approaches in action to a dip circle. 



On the courses 180° to 360° the deviation due to tilt is 

 very much less than for the 0° to 180° courses, the reason 



