It will be seen, therefore, that there are many elements necessary to provide a satisfactory clutch, well adapted for all purposes, and all these factors must be considered and understood by the boy who would be well informed. FRICTIONAL CONTACT.--In any form of automobile clutches, there must be a frictional contact, which means wear, whatever may be the character of the material employed for the surfaces which are in engagement. As a result, clutches are now made which will permit the use of oil. Others dispense with it entirely. Each type has its advantages. The cone clutches usually do not use a lubricant. This is described in the diagram, Fig. 40. CONE CLUTCH.--In the drawing A represents the engine shaft, which has a fly wheel B, and C is the transmission shaft. The engine shaft has a short projecting stem D, which abuts the end of the transmission shaft C. A hollow hub E is loosely journaled on the stem D, and is of sufficient length to extend over and have a bearing on the transmission shaft C, this latter being squared so it will turn with the hub, or it may be provided with a feather to work in a suitable groove in the hub, so that both will turn together, while permitting the hub to move longitudinally. The inner end of the hub E has a web F, with a conical bearing surface G, which engages with an internal cone on the fly wheel. COMPRESSION SPRING IN CLUTCHES.--Within the hollow hub E is a compression spring H, one end of which rests against the inner end of the hub, and the outer end contacts against a collar I, which collar is screwed on the threaded end of the stem D, and by means of which the pressure of the spring may be regulated from time to time. The normal action of the spring is to throw the cone surface G, into engagement, as shown in the diagram, and when the foot presses down the pedal J, the hub is moved back against the tension of the spring, and the clutch released. It is obvious that if oil should find its way between the cone surfaces the grip would be materially lessened, and depending upon the kind of materials used. [Illustration: Fig. 41. Multiple Disk Clutch.] THE MULTIPLE DISK CLUTCH.--A type of clutch which uses oil is shown in Fig. 41. The prominent feature of the multiple disk is the large area of contact surfaces available, and this, together with the comparative freedom from wear, owing to the lubricating material, makes it a favorite structure, especially on account of its gradual engagement which is not easily obtainable with a cone clutch. In the drawing, the transmission shaft A has its ends reduced to receive thereon a set of disks B. The shaft is ribbed along the surface where the disks are located, and the disks B have cut-out portions C, for the ribs, so that, while the disks must turn with the shaft, they are free to move longitudinally.