The load-carrying or power-transmitting capacity of many machine parts is decisively affected by the maximum temperatures that occur in the source of frictional heat. In controlling these temperatures, and thus the performance limits concerned, two types of dissipation of frictional heat, which act in series, have to be accounted for: 1. Primary heat-dissipation, which may be defined as “short distance” dissipation within the source of frictional heat and in its immediate vicinity. a. In boundary lubrication the heat source is constituted by the area of contact and so the “solid-frictional” heat is primarily conducted away into the rubbing bodies where they approach closest. b. In hydrodynamic lubrication the heat source is constituted by the fluid film and hence the primary dissipation of “viscous-shearing” heat takes place from point to point within the film, and from the film to its bounding surfaces, i.e. into the rubbing bodies. 2. Secondary heat-dissipation, which may be defined as “long-distance” dissipation of frictional heat from its source towards some cooling surface, which, by emitting heat towards its surroundings, prevents undue accumulation of heat in the machine part as a whole. Secondary heat-dissipation is not typical of rubbing surfaces: in principle it does not differ from that in machines embodying heat-sources not created by frictional heat.