Let us say that the normal speed of the engine is 1,000 revolutions per minute, and that it is so connected that it makes 40 revolutions while the driving wheels make 1, the speed of the tractor being 3 miles per hour. If it is a 4-cylinder engine there will thus be 80 power strokes to every revolution of the driving wheels. The engine is delivering its full power and cannot do more should the tractor be called on for an extra exertion, such as climbing a hill or crossing rough ground. By changing the connections between the engine and the driving wheels, the engine can be made to run twice as many revolutions to one turn of the driving wheels, which will give double the number of power strokes; the wheels will thus be turned with twice the force. As no change is made in the speed of the engine, the wheels will now turn at half their former speed, and the tractor will run at 1½ miles per hour. It will, however, have twice the ability to overcome obstacles. This change in the connections between the engine and the drive is performed by the _change speed gear_, which is driven by the engine and which in turn drives the wheels. There are many varieties of change speed gears, but the main principle in them all is the same, for they depend on the action of cog-wheels, or _gears_. When two gears running together, or _in mesh_, have the same number of teeth, they will revolve at the same speed. If one has half as many teeth as the other—10 teeth and 20, let us say—the 10-tooth gear will make two revolutions while the 20-tooth gear is making one. There are two shafts in a change speed gear, one driven by the engine and the other driving the wheels; each carries gears that mesh with gears on the other shaft. These pairs of gears are of different sizes, and any pair may be used; the shaft driven by the engine runs as the engine runs, while the speed of the other shaft depends on the pair of gears that is being used. By changing from one pair of gears to another, the driven shaft, and, consequently, the wheels, may be run at a greater or less number of revolutions, while the speed of the engine and the driving shaft do not change. The number of power strokes that occur during one revolution of the wheels is thus changed, and they turn with more force or with less. _High speed_, or _high gear_, means the combination of gears that gives the greatest speed to the wheels, but the fewest power strokes to each revolution. The combination that gives the slowest speed to the wheels, but the greatest number of power strokes, is called _low speed_, or _low gear_. Many tractors have but two speeds, a low and a high; but others have an intermediate combination for conditions too severe for running on high gear but too easy for low.