The tractor cannot move until the engine is running and delivering power, and it follows, therefore, that it must be possible to disconnect the engine from the driving mechanism in order that it may run independently. This is done by means of a _clutch_, which is a device that connects two shafts, or disconnects them. [Illustration: FIG. 60.—INTERNAL CLUTCH] A clutch must be so made that when it is engaged it takes hold, not suddenly, but gradually. If it took hold suddenly, the tractor would be required to jump at once into full motion; this would cause a severe straining of the parts and probable breakage. The alternative would be the abrupt stopping of the engine, and this would also strain things. By making the clutch in such a way that it slips, and takes hold little by little, the tractor starts slowly, and gradually comes up to speed; the slipping of the clutch then ceases, and it takes hold firmly. All clutches operate by the friction of one surface against another; in some, the surfaces are curved and in others flat, while in still others the clutch is a band around a wheel, or _drum_. A clutch is operated by a hand lever or by a foot pedal. Figure 60 shows a type of clutch that operates inside a drum, which is often the overhanging rim of the flywheel. The shaft in the center is independent of the flywheel, and it is the purpose of the clutch, which is attached to the shaft, to lock the shaft and flywheel together when the tractor is to be started. The brake shoes, which bear against the drum, form the ends of pivoted levers, and are lined with an asbestos material that resists the heat caused by the friction against the drum. A cone-shaped block of steel slides lengthways on the shaft; when it is pushed into position, it forces out the yokes, and thus presses the brake shoes against the drum. A _plate clutch_, or _disk clutch_, is shown in Figure 61. The principle of a plate clutch may be illustrated by placing a half-dollar between two quarters and pinching them with the thumb and forefinger. If they are held loosely, the half-dollar may be turned between the quarters, but if they are pinched tightly, the friction between the coins will be so great that one cannot be turned without turning the others. [Illustration: FIG. 61.—PLATE CLUTCH] Attached to the flywheel are studs, which support a disk, or plate; this plate revolves with the flywheel, and is practically a part of it. On either side of this plate are other plates that are supported on the drive shaft; they revolve with it, but can slide along it. The end of the shaft is square and fits a square hole in a collar, so that while the collar may slide along the shaft, the two must turn together. Cams are mounted on the hub of one of the plates in such a position that they can press the outside plates together and pinch the flywheel plate between. The cams are operated by pressing the collar against them.