462 Mr. S. Gr. Starling on the Equilibrium of 



acceleration when the machine is increasing or decreasing in 

 speed. This causes the card to dip towards the fore part 

 during acceleration and to the aft during retardation. The 

 resulting disturbance of the card will be a maximum when 

 flight is magnetic East or West, and this would only occur 

 during straightforward flight and chiefly near the ground, 

 when the compass readings are not particularly important. 

 By far the most serious errors of the compass are due to 

 the acceleration towards the centre of the circle when a turn 

 is made, and it is with these errors that the present paper is 

 concerned. 



As the aeroplane turns it is tilted towards the centre of 

 the turn exactly as any other vehicle, and the resultant of 

 gravitational and centrifugal forces may still be in the median 

 plane of the machine, so that a plumb-line on it would still 

 be perpendicular to the base-board, or, on the other hand, 

 this resultant may be inclined to it. In the latter case, it 

 would be possible to devise a compensator in the nature of a 

 magnet or electric coil under gravitational control which 

 would produce a magnetic field opposite to that causing the 

 disturbance of the needle. This was tried and found to 

 produce no effect, either of correction or disturbance, and it 

 is therefore concluded that, as the aeroplane turns, the 

 banking is such that the resultant weight of every part of it 

 is still perpendicular to the base-board. Thus the card of 

 the compass, which is free to take its new position under 

 gravitation and centrifugal force, will still remain parallel 

 to the normally horizontal glass of the compass-bowl. The 

 machine then goes round a bend just as a large conical 

 pendulum would do. 



Any disturbances due to motion of the liquid in the bowl 

 and friction at the pivot are now, for the moment, put on 

 one side, and the equilibrium of the needle under magnetic 

 forces alone considered. 



Let H be the horizontal component of the earth's magnetic 



y 

 field, and V its vertical component. Then, tan<fc ^j, where 

 d is the magnetic dip. 



In fig. 1, AB is a truly horizontal plane shown in elevation 

 and EF the compass-card inclined at angle 6 to the horizontal. 

 OP'Q is the plan of the compass-card, and OPQ the card if 

 imagined to be rotated through 6° into the horizontal plane. 

 NS is the magnetic meridian, so that NRQ is the true 

 compass-course (<£'), and S'RQ is the angle between the 

 magnetic meridian and the course RQ, as measured on the 

 card, and is called <f>. 



