between either one of its two operating positions a pushbutton p is provided which is mounted for axial sliding movement in casing a and carries a fork 19 which is engageable between two flange portions of the double sprocket 16 to effect axial displacement thereof if desired a control lever may be coupled with the push button p in addition to being slidably mounted in the casing a the push button p also is slidable axially within a pinion 17 1 which is in mesh with pinion 17 idly mounted on the end of control shaft 2 the pinion 17 1 has a cylindrical extension 17 2 which mounts at one end a further pinion 17 3 which is in mesh with pinion 18 in the position of engagement of sprocket 16 shown in fig1 rotation of the sprocket 16 by one or the other of the hand wheels 23 and 24 causes application of rotation to the control shaft 2 by way of the dog clutch engagement with pinion 17 from pinion 17 to pinion 17 1 and through shaft portion 17 2 to pinion 17 3 and thence ultimately to pinion 18 in the event of depression of the push button p the drive sprocket 16 is moved out of clutching engagement with pinion 17 and into dog clutch engagement with pinion 18 to apply drive directly thereto evidently this will apply a different speed ratio to the control shaft 2 although a drive connection will remain between pinion 18 and pinion 17 3 and ultimately to pinion 17 it will be remembered that pinion 17 is idly rotatable.