The change speed gear mechanism also provides for reversing or backing the tractor. Two gears running together turn in opposite directions, while in a train of three gears the outside gears turn in the same direction. The usual combination in a change speed gear uses two gears for going ahead; to run the driven shaft the other way, which will make the tractor back, a third gear is meshed between the two. The differences between various makes of change speed gears are in the methods used to put into action the desired pair of gears. [Illustration: FIG. 62.—PRINCIPLE OF SLIDING GEAR] Two general plans are used. In one of them, a gear of each pair can slide endways on its shaft, but must revolve with it; thus it can be slid into mesh or out. In the other, the gears of a pair are always in mesh, but one of them is loose on its shaft, so that shaft and gear can revolve independently. To make the pair of gears operate, the loose gear is locked to its shaft. Figure 62 shows the principle of the _sliding gear_ type. One part of the shaft driven by the engine is square, and fits into square holes in its gears, which may thus slide along it, but must revolve with it. Each sliding gear is moved by a shifter block, which is operated by a shift lever. There is a shifter block for each gear, and the shift lever may be moved sideways to operate either one of them. Figure 63 shows the _jaw clutch_ type of change speed gear, in which the gears are in mesh all of the time, but run loose on their shaft when they are not working. The drawing shows _bevel gears_, which are used when the driving and driven shafts are at a right angle. The same principle is used for _spur gears_ on shafts that are parallel, as in Figure 62. [Illustration: FIG. 63.—PRINCIPLE OF JAW CLUTCH CHANGE SPEED GEAR] The center of the shaft is square, and fits a block that can slide endways, but that must revolve with it. The ends of the block have heavy teeth that can mesh with teeth on the hubs of the loose gears; meshing the block with one of the gears forces that gear to revolve with the shaft. The drawing shows only one speed forward; the reverse is obtained by a second gear on the same shaft, which is placed on the opposite side of the center of the driven gear, and turns it in the opposite direction. When a tractor turns, the outside wheel makes a larger circle than the inside wheel, and has a longer path to travel. Both wheels travel their paths in the same time, so it follows that the outside wheel must move faster than the inside wheel, although both are being driven by the engine. This is allowed for by the _differential_, which is driven by the change speed gear, and which in turn drives the wheels; it operates automatically by the difference in the resistance to the rolling of the wheels.