_Answer._ It is the proportion which the resistance to sliding motion bears to the force pressing the surfaces together. Thus a smooth, clean and dry cast iron plate loaded with 100 pounds will require a force of about 15 pounds, or fifteen one-hundredths of the weight or pressure of the plates, to slide them on each other. The _co-efficient of friction_ is therefore said to be 0.15, and with any other weight or pressure on the plates we could determine the force required to slide them on each other by multiplying the pressure by the co-efficient of friction. Thus, if the plates were loaded with 250 pounds, the force required to slide the one on the other would be equal to 250 × .15 = 37.5 pounds. The co-efficient of friction, however, varies for different materials. Thus, while the co-efficient of friction between two pieces of smooth, clean and dry cast iron is 0.15, that of a piece of brass on cast iron, under similar conditions, is 0.22, and of two pieces of wood about 0.4. QUESTION 361. _What is the effect of introducing some unguent or lubricating material, such as oil, between the surfaces in contact?_ _Answer._ The co-efficient of friction is very much reduced thereby. Thus the co-efficient of friction of the cast iron plates, if their surfaces are greased with tallow, is 0.1; if lubricated with lard 0.07, with olive oil 0.064, and with lard and plumbago 0.055, thus showing that the amount of the friction depends very much upon the nature of the lubricant which is used, as well as on that of the materials in contact. QUESTION 362. _What effect on the amount of friction has the manner of applying the lubricating material to the surfaces in contact?_ _Answer._ The more perfect the lubrication the less will be the co-efficient of friction. It has, for example, been found by experiments made with cast iron shafts turning on bearings of the same material that when the lubricating material was applied so that the surfaces were only “unctuous,” that is slightly greasy, the co-efficient of friction was very little less than when they were dry, that is when there was no lubricating substance between them, and that when they were greased “from time to time” the co-efficient was reduced to 0.07 and 0.08; but when they were continually oiled it averaged 0.05, and sometimes fell as low as 0.025, showing that with the best lubrication the friction was only one-sixth what it was when the surfaces were only “unctuous.” Between these two limits there is every degree of frictional resistance, according to the condition of lubrication. This shows how important it is that the oiling fixtures should be kept in the most perfect condition and the utmost care be exercised in keeping every part of a locomotive thoroughly lubricated. QUESTION 363. _What effect does the pressure per square inch of the surfaces in contact have upon the lubrication?_