470 Mr. S. G. Starling on tlie Equilibrium of 



deviation becomes reversed at B and B (fig. 5), for a reason 

 to be explained later. 



Fig. 6. 



In actual flight the positions indicated by the curve will 

 never be actually reached because of the changing course of 

 the machine, but they give an indication of the movement, 

 since they show the point towards which the card tends to 

 move at each course. 



For example, with a tilt of 80° or 85°, there is a powerful 

 couple acting on the needle, tending to reverse its direction, 

 and this acts for a large part of the whole turn. 



Effect of liquid in Bowl. — In the case chosen for a compete 

 circular turn in the direction N.-E.-S.-W.-N., the deviations 

 alternate in direction, being first E., then W., again W., and 

 finally E. If the rotation o£ the machine tends to drag the card 

 after it by reason of the viscosity of the liquid in the bowl, this 

 will sometimes increase the error and sometimes diminish it. 

 Thus on the N. to E. (0° to 90°) turn, the effect of the liquid 

 will be to increase the error, and it might be expected that 

 this is the worst of all the turns — in fact, the card would pro- 

 bably go round faster than the machine if the tilt is great. 



On the E. to S. (90° to 180°) course, the action of viscosity 

 would be to check the motion of the card due to magnetic 

 effects, and this course should not be so bad as the first. Of 

 the last two, the liquid tends to correct the card from 180° 

 to 270° and to assist in disturbing it from 270° to 360°. 



Method of correcting error. — It is clear from the preceding, 

 that the vertical component of the earth's magnetic field (V) 

 is responsible for a large part of the disturbance. If, 

 therefore, this could be balanced, an improvement would be 

 effected. Now, the component of V in the plane of the 

 compass-card is V sin 0, and therefore an arrangement which 

 will produce a field at the card equal and opposite to this is 

 required. A pair of bar magnets NS (fig. 7) attached to a 

 frame, which can rotate about an axis passing through the 

 compass and in the median line of the aeroplane, produce a 

 vertical field, say F, at the compass. On rotating these 

 about the axis through an angle equal to the angles of tilt, a 

 field Fsin0 will be produced in the plane of the card. If 



