He further states that these "laws" are not absolutely exact, as here stated, so far as they affect the magnitude of frictional resistance. It is found that some evidence exists indicating the continuous nature of the friction of rest and of motion. When the pressure exceeds a certain amount, fixed for each pair of surfaces, abrasion of the softer surface or other change of form takes place, the resistance becomes greater and is no longer wholly frictional. When the pressure falls below a certain other and lower limit the resistance may be principally due to adhesion, an entirely different force, which may enter into the total resistance at all pressures, but which does not always appreciably modify the law at high pressures. This limitation is seldom observable with solid, unlubricated surfaces, but may often be observed with lubricated surfaces, the friction of which, as will presently be seen (41), follows different laws. The upper limit should never be approached in machinery. The coefficient of friction is that quantity which, being multiplied by the total pressure acting normally to the surfaces in contact, will give the measure of the maximum frictional resistance to motion. ~34. Sliding Friction is Proportional to Pressure~ according to the third law quoted above. This is easily demonstrated by ascertaining what force is necessary to produce, or continue, motion in a body lying on a plane surface; double the weight of the body and the force required to produce, or continue, motion, will have to be doubled. The converse is also true (36). ~35. Sliding Friction is Independent of the Area Of Contact~, the pressure remaining the same (law 4, 33). This is accounted for by the fact that if, for example, the area of contact be doubled, though twice the number of asperities will present themselves, each individual retarding force is only half of what it was previously, and the general effect is the same (36). ~36. The Intensity of Sliding Friction is Independent of Velocity.~ (Law 4, 33.) This is explained by the fact that the interlocking of the asperities on each surface has a shorter time to take place in increased speed, and consequently cannot be so effective as with slow speed. But with high speed more asperities are presented than in low speed, so the effect is the same in both cases. _The above (33-36) are not exact, being the statement of experimental laws, and admit of considerable modification when applied in horological science, as will be shown (41-42.)_ [Illustration: Fig. 12.]