Lubrication is the application or introduction of some substance that will cling to or flow between two surfaces and thus prevent friction. Bearings and joints of engines and machinery are lubricated to keep the various metal surfaces from coming in direct contact, and thus to prevent excessive friction and consequent heating. (See Fig. 34, page 49.) Perfect lubrication is secured when the surfaces are separated by means of the thinnest possible film that is sufficient to prevent heating. A thick film is harmful because it tends to produce fluid friction.