"BRIEF DESCRIPTION OF THE DRAWINGS The nature and advantages of the present invention will be understood more clearly from the following description made with reference to the accompanying drawings, in which: FIG. 1 is a cross section of a speed-shifting device of the first embodiment; FIG. 2 is a front view of the speed-shifting device shown in FIG. 1; FIG. 3 is a perspective view of the cam mecha" . . . . .