Lubrication
Lubrication of the ship steam engine crankshaft. The two bottles of lubricant are attached to the piston and move while the engine is operating.
Lubrication is a physical process which allows surfaces to move smoothly against each other. A thin film of lubricant put between the surfaces reduces friction and wear on the surfaces, and keeps temperatures from rising too far. The lubricant film can be a solid/liquid dispersion, a liquid, a liquid-liquid dispersion (a grease) or, exceptionally, a solid or gas.
Examples are the use of oil in machinery, the function of tears in eyes and synovial fluid in joints.