Turning back to our first spoked wheels, it is very evident that we may put a rim over the spokes or even fill in between the spokes and convert the wheels into solid disks that are in frictional engagement with each other without getting away from the fact that we are dealing with levers. Each wheel, then, consists of a continuous revolving lever. Friction gears are used quite commonly in machinery when it is desirable to have the wheels slip if subjected to excessive strain. TOOTHED GEARS By forming teeth on one gear to mesh between similar teeth on the other, we convert the friction gears into a pair of spur gears (Figure 9). We need not go into the intricacies of the form of gear teeth. They are designed to be in continuous rolling contact while they are in mesh. The novice is apt to call all spur gears “cogwheels” and gear-teeth “cogs.” Mechanics, however, recognize a difference between cog wheels and spur wheels. In the former, the teeth, or cogs, are not cast upon or cut out of the wheel body, but are separate pieces fitted to the wheel. Such wheels are found in old water mills. They consist of wooden wheels with iron or steel teeth mortised in the wooden rim of the wheel. In general it is safer to speak of spur gears because there are few cogwheels now in use. [Illustration: FIG. 8.--CROWN AND LANTERN GEAR] When a small gear engages a large one, the former is commonly known as a _pinion_. [Illustration: FIG. 9.--SPUR AND PINION GEAR] [Illustration: FIG. 10.--BEVEL FRICTION GEARS] If two friction wheels are to turn at right angles one to the other, they must have conical bearing surfaces, as in Figure 10. The angle between the shafts of the two gears and the relative size of the gears may be changed as desired, provided each cone surface has its apex at the intersection of the two shafts or axes. It is easy to understand how such conical friction gears may be converted into toothed bevel gears (Figure 11), by forming teeth on the conical surfaces, and it will be evident that the teeth must taper toward the apex of the two cones. Two bevel gears of equal diameter, and with shafts set at right angles one to the other, are known as _miter_ gears. [Illustration: FIG. 11.--TOOTHED BEVEL GEARS]