The function of the muffler is to receive the exhaust gas and permit it to expand as nearly as possible down to atmospheric pressure before delivering it to the air. Fig. 96 shows the simplest form in which it can be made. CONSTRUCTION OF MUFFLER.--The inner pipe A, from the engine exhaust, passes axially through a cylinder B, the pipe, however, being closed at its inner end where it is attached to the head C. Numerous small holes D are formed through this pipe for the escape of the burnt gases. Within the cylinder B is a smaller cylinder E, surrounding the inner tube. This has one end attached to the head C, and its other end is open so as to provide a passage way F from the interior of the cylinder. The discharge ports are at G, through the head C. [Illustration: Fig. 96. Muffler.] Almost any design of muffler is serviceable, if it has sufficient area. However large it may be it is always advisable to have a valve in the pipe A from the engine manifold, so the muffler can be cut out going up steep hills. BALL AND ROLLER BEARINGS.--All running gears are provided with either ball, or roller bearings. For heavy vehicles roller bearings are most serviceable, but for light vehicles and for speed most manufacturers prefer ball-bearings. RACE WAYS.--The object in the use of balls, is to provide two, three, or four points of contact, which should be so arranged as to have the paths of the bearings of equal lengths, as nearly as possible, and thus prevent the balls from wearing by creeping along the contact walls, and also thereby wearing the paths on which they travel. [Illustration: Fig. 97. 3-Point Roller Bearing.] THE THREE-POINT CONTACT.--To understand the full importance of this, examine Fig. 97, in which A is the roller, or shaft, and B the hub having the raceway C designed to hold the balls D, and gives two points of contact, the third point being the shaft A. [Illustration: Fig. 98. Wrong Bearing.] Compare the foregoing figure with the illustration given in Fig. 98, where the contact points A, B, C, represent the three bearing circles, which differ in their circumference, and it is obvious that a ball in traveling around must slip somewhere on one or more of the paths A, B, C. [Illustration: Fig. 99. Improper Alinement.] WRONG CONSTRUCTION.--Another sample of wrong construction is shown in Fig. 99. In this diagram the three bearing points A, B, C, also represent circles of different diameters, which are sure to wear grooves in the three paths made by the balls. [Illustration: Fig. 100. Correct Raceways.] The most ideal form of bearing is shown in Fig. 100, which represents the four-point contact, and this also provides against longitudinal thrust of the shaft or axle.