ClipArt ETC
"a, scroll-wheel, intermeshing with the pinion b, which, sliding by a feather on the shaft, c, imparts a gradually decreasing velocity to the latter as b is moved toward the center of a." -Whitney, 1911

Scroll Wheel

"a, scroll-wheel, intermeshing with the pinion b, which, sliding by a feather on the shaft, c, imparts…