Lubrication. When two solid bodies are rubbed together heat is produced and work wasted. To reduce this waste, rubbing surfaces in machine bearings, etc., are lubricated by inserting a film of fluid between them. This film of fluid prevents the surfaces from actually touching, and the friction between each surface and the fluid is much less than would be the case if the lubricant were not present. The fluid is usually oil, and this should be chosen of such a consistency that the pressure on the bearing will not force it from between the surfaces; heavy bearings must thus have thicker oil than light ones. Tallow and thick vegetable or mineral greases are used for heavy bearings, such as those of railway carriages; petroleums of various degrees of thickness are largely used for general machinery, while refined sperm or neatsfoot oil is used for small work, such as the pivots of sewing machines and clocks.