1921-22.] Gyroscope and Vertical” Problem on Aircraft. 291 
Consider now the device shown in fig. 20 . This consists of a large 
gyroscope g mounted in a frame / and carried by uprights u, n, after the 
manner already described. Rigidly attached to the pivoted system is a 
small gyroscope g\ the axis of spin of which is perpendicular to that of 
the main gyroscope g. When the pivoted system is upright the axis of g' 
is parallel to ^be direction of the spin of g be clockwise, as 
seen from above, and that of g' be counter-clockwise, as seen from the left- 
hand side of the apparatus (as viewed by the reader). These directions 
of rotation are indicated in the figure by the arrows attached to the 
Fig. 20. 
diagrams of the gyroscopes. Now suppose this device, as described, to 
be set up on a table with the axis of g vertical. If the entire apparatus 
is turned in azimuth at angular speed ip in the clockwise direction, as 
seen from above, the pivoted system will experience a couple tending to 
turn it about the pivots PiPi in the counter-clockwise direction, as viewed 
by the reader. This couple is applied to the gyroscope g, and in conse- 
quence the pivoted system turns on the pivots P2P2 so that g' moves away 
from the reader. 
The applied couple which brings about this result arises from the 
turning of the gyroscope g' in azimuth. If Ia> is the angular momentum 
of g', the moment of the applied couple is Ico?/f. The rate at which the 
pivoted system turns on the pivots P2P2 Is loirpICn, where Qn is the 
angular momentum of the main gyroscope. If the direction of the 
