While fluid friction acts as a retarding force in mechanism it converts the mechanical energy required to produce it into its heat equivalent, thus raising the temperature of the mass in a greater or lesser degree. The resisting property which thus effects this conversion, and which is the cause of fluid friction, is called _viceosity_. It is thus apparent that a _variation of the viceosity_ of the oil used on a watch would cause a variation of fluid friction and consequently a variation of the effort (11), _and would seriously interfere with the rate of the watch_. This will be discussed (84) more thoroughly in another paragraph. ~40. The Laws of Fluid Friction~ are: 1. Fluid friction is independent of the pressure between the masses in contact. 2. Fluid friction is directly proportional to the surfaces between which it occurs. 3. This resistance is proportional to the square of the relative velocity at moderate and high speeds, and to the velocity nearly at very low speeds. 4. It is independent of the nature of the surfaces of the solid against which the stream may flow, but it is dependent to some extent upon the degree of roughness of those surfaces. 5. It is proportional to the density of the fluid and is related in some way to its viscosity. ~41. The Compound Friction of Lubricated Surfaces~, as Thurston terms it, or friction due to the action of surfaces of solids partly separated by a fluid, is observed in all cases in which the rubbing surfaces are lubricated. The solids, in such instances, though partly supported by the layer of lubricant which is retained in place by adhesion (21) and cohesion (20), usually rub on each other more or less, as they are usually not completely separated by the liquid film interposed between them. Wear is produced by the rubbing together of the two solids; and the rate at which the lubricant becomes discolored and charged with abraded metal indicates the amount of wear. The journal and bearing are forced into close contact in the case of heavy pressures and slow speeds, as is shown by their worn condition; while the journal floats on the film of fluid which is continually interposed between it and the bearing, in the case of very light pressures, and high velocities; in the latter instance _the friction occurs between two fluid layers_, one moving with each surface. With heavy machinery, as the hardness and degree of polish of the surfaces cannot be increased in proportion to their weight, the solid friction is so great that while the interposition of a lubricant between the surfaces adds fluid friction, it also reduces the solid friction; and as the fluid friction is so insignificant as compared to the solid friction, the former is almost completely masked by the latter. In this case the laws of solid friction are more nearly applicable.