... To slow or slacken the speed, loosen the jam nut as before, except that you must run it up a few turns, then taking hold of the thumb nut, turn down slowly until you have the speed required, when you again set the thumb nut secure. In regulating the speed, be careful not to press down on the stem when turning, as this will make the engine run a little slower than it will after the pressure ... — Rough and Tumble Engineering • James H. Maggard