256 SYMPOSIUM ON AERONAUTICS. 



the circle described by it, the tilt becoming greater, of course, with 

 the speed of turning or with the decrease of radius of the circle. 

 Everything movable which was at rest in the aeroplane during 

 straight-line uniform flight under the action of gravity alone is still 

 at rest relative to the aeroplane as it tilts on the turn, but now, 

 everything is at rest under the action of the resultant of gravity 

 and centrifugal accelerations. The compass card, which was hori- 

 zontal during rectilinear flight, is now tilted with the aeroplane and, 

 consequently, partly turned in the terrestrial magnetic field. The 

 vertical, component of the earth's magnetic field, which was normal 

 to the card in its level position in rectilinear flight and which, con- 

 sequently, had then no directive effect, now has a component in the 

 plane of the card and normal to the magnetic axis which tends to 

 produce the " dynamic deviation." The horizontal component of 

 the earth's magnetic field also plays a part in this kind of deviation. 



According to some recent investigations in England by S. G. 

 Starling,^ when the angle of tilt of the aeroplane approaches the 

 complement of the magnetic dip, which for Philadelphia would mean 

 a tilt of about 19°, the dynamic deviations of the compass, if, for 

 example, the course steered be an easterly one, may increase to 

 nearly 90°. And if the tilt of the aeroplane exceeded 19° the direc- 

 tion of the compass on the course stated would even he reversed. 



While the dynamic deviations may be large during turns of the 

 aeroplane, yet they disappear, practically, when straight flight is re- 

 sumed. We, therefore, question the desirability of adopting the 

 movable compensating devices, suggested hy Starling, which while 

 eft'ective during aeroplane-turns, might introduce magnetic devia- 

 tions of a more permanent character during the more usual straight 

 flights. If his devices are used, they will require careful control. 



In connection with the use of the compass in aerial navigation, 

 an interesting scientific question comes up as to the change of the 

 earth's magnetic field, or of the magnetic elements with altitude 

 above the surface. Magnetic experiments of this nature were made 

 in balloons by Gay Lussac and Biot in 1804 w^hich were repeated, 

 with more success, a half century later by Glaisher. The available 



1 " The Equilibrium of the Magnetic Compass in Aeroplanes," Phil. Mag., 

 London, Vol. 32, November, 1916 (461-476). 



