314 Proceedings of the Boyal Society of Edinburgh. [Sess. 
Sensitiveness of Ball Erectors. 
The ball erectors which have been described, when driven round at 
perfectly uniform speeds (as is the case, for example, when the erector 
spindle is driven directly, through reduction gearing, by the spindle of 
the main gyroscope), depend for their action on the balls moving away 
from their pushers when descending the slope of the track on which they 
Fig. 36. 
move. Such a ball erector, as already stated, is sensitive to within 2 
or 3 minutes of angle. 
To obtain greater delicacy a number of new erectors have been devised 
in which the ball, or balls, in ascending the slope, slow down the erector, 
and thus allow the ball or balls descending the slope to run away from 
their pushers. In some of the latest erectors the balls themselves do not 
contribute the stabilising forces directly, but control the application of 
such forces. 
Consider an erector in which two balls, each provided with a pusher 
and check, are used. Let the balls rotate on a fixed track, being situated 
always on opposite sides of the track, so that when one ball is ascending 
