The end of the engine shaft D, has a tubular housing E, to receive the end of the transmission shaft A. The inner end of this housing embraces the flange F, of a cylindrical shell G, this shell having within a series of disks H, secured to the shell so they will slide longitudinally, but turn therewith, and these disks alternate with the disks on the transmission shaft. It will be observed that the flange F, of the shell G, has a tongue I, which slides within a groove J in the housing, so that the shell G, while turning with the shaft D, may be moved longitudinally on the shaft A, a limited distance. The end of the shaft A, has a collar K, and between this collar and the end of the flange F, is an expansion spring L, so that the normal action of the spring is to push the web of the shell G, toward the disk head M, and thus force all the disks together and produce the friction of a very large surface. In order to release the clutch, it is necessary to draw back the shell G. The mechanical action is merely shown, not the exact structural arrangement. An annular flange N is formed on the head of the shell, and a pair of hook-shaped bars O, pass through the wall of the case, their outer ends being actuated by the foot pedal, in any convenient manner. DISADVANTAGES OF MULTIPLE-DISK CLUTCHES.--These clutches have also their weaknesses. Sometimes they will grip too quickly, if the lubricating oil is too thin, or if there is not enough of it; or, if it becomes very thick and gummy, the disks will not free themselves quickly, and the clutch will drag. CARE OF MULTIPLE-DISKS.--When such is the case, it is better to take out all the lubricant, and thoroughly clean off the disks, and put in a fresh supply. If the case is kept properly closed, so that the oils will not be wasted, and no dust can enter, a light, thin oil, will last for a long time. When the clutch slips, it is due to wear, or to insufficient spring pressure, and a new adjustment is necessary; and it is frequently the case that the rod between the clutch and pedal must be taken up, this being the case, usually, where there is any wear in the clutch itself. The disks are, usually, wholly of metal. Among other materials, cork is used to face friction surfaces of different clutch designs, and a variety of materials are constantly added to the list, which have good wearing qualities. CHAPTER VIII TRANSMISSION, OR CHANGE SPEED GEARS Owing to the peculiar character of Internal Combustion Engines, there is always a certain speed at which it will work more satisfactorily, and with greater economy. In this respect it is unlike the steam engine, which has a much wider range of effectiveness. Since all cars now use internal combustion motors, and throttling is unsatisfactory, as a means of controlling the engine, or changing the speed and power, so as to use it economically, a mechanical speed change system is essential.