http://www-sul.stanford.edu/mac/primary/interviews/ideo/preapple.html

excerpt, xerox parc "gimbal"

Floating Ball

The second things was allowing the ball to float. In all the mice before-- in the Xerox PARC mice, Engelbart's mouse didn't even house a ball, but two encoder disks-- you're actually forcing the ball on the table, and the Xerox PARC mouse had a gimbal on top of it that tried to force the ball down. We very quickly realized that you didn't need to do that, that you could get rid of all those parts that were fussy and tended to get dirty and so forth, and that gravity alone would suffice to allow you to do that.