The first drawing shows the clutch out, or released; the flywheel may then turn without turning the shaft, for the plates are not in contact. The second drawing shows the clutch in, or engaged. The collar is pressed against the cams, and the plates in turn are drawn together, pinching the flywheel plate between them. The flywheel and the drive shaft then revolve together. Plate clutches are often made with more than three plates; some makes run in a bath of oil, and some are intended to work dry. In a cone clutch, the overhanging rim of the flywheel is funnel-shaped, and into it fits a cone-shaped disk carried on the end of the drive shaft. To engage the clutch, the disk is slid along the shaft against the flywheel, the friction between the two being sufficient to drive the shaft. When a clutch is thrown in it should take hold gradually, slipping at first, but finally having a firm grip. When it is thrown out, it should release instantly and completely. The power delivered by an engine depends on the _bore_ and _stroke_ of the cylinder, and on the speed. The greater the bore, or diameter of the cylinder, and the greater the stroke, or distance the piston moves in a half-revolution of the crank shaft, the larger will be the combustion space, and the larger will be the charge of mixture that it can take in; the larger the charge, the greater will be the power produced when the charge burns. Each cylinder produces power once during every two revolutions of the crank shaft; if the engine runs at 1,000 revolutions per minute there will be twice as many power strokes as there would be if it ran at 500 revolutions per minute, and during that minute it will produce twice as much power. A traction engine is intended to run at a certain speed, at which it will produce its greatest power without overstraining its parts. This _normal speed_ for any particular engine depends on the number of cylinders, their size and design, and other details established by the manufacturer. To get the best from the engine, this is the speed at which it should always be run. The power required to move the tractor depends on various things; the hardness and smoothness of the ground, the grade, the load it is pulling, and so on. The tractor might be running on level ground, pulling so great a load that the engine is called on for all of the power that it can deliver. On coming to a hill, still more power will be required, for now the tractor and its load must be lifted as well as moved forward. The engine, already working at its limit, cannot deliver the extra power needed, and will slow down and stop unless something is done to aid it. In such a case, the change speed gear is used to give the engine a greater leverage on its work, just as a block and tackle gives a greater leverage or purchase to a man who must lift a heavy weight.