_Answer._ The tendency is, when this pressure becomes excessive, to press out the lubricant which is between the two surfaces, and ordinary experience proves that the greater the weight or the force per square inch with which two bodies are pressed together, the greater is the difficulty of keeping them perfectly lubricated. Thus it is easier to keep the journals of a car well lubricated when it is empty than when it is heavily loaded, and the guide-bars of a locomotive are more liable to be cut when the engine is pulling a heavy load than with a light one. QUESTION 364. _What effect has the velocity of the surfaces in contact on the friction and lubrication?_ _Answer._ With the surfaces in the same condition, the friction is independent of the velocity of motion of the surfaces against each other, but perfect lubrication becomes more difficult as the velocity increases, so that an increase of velocity will often increase indirectly the amount of friction. Thus, taking our previous illustrations, it is more difficult to keep the journals of a car or engine well lubricated when running fast than when running slow, and the same thing is true of the guide-bars. QUESTION 365. _What considerations should govern the proportions of frictional bearings for locomotives and other machines?_ _Answer._ The dimensions to be given them should not be determined from a consideration solely of their resistance to rupture,[84] but they should be made so large that the pressure they must bear will be distributed over so much surface that the proportion borne by each square inch will be comparatively small, thus making good lubrication much less difficult, and consequently reducing the co-efficient of friction. [84] Morin’s Mechanics. QUESTION 366. _Is not the amount of energy required to overcome the friction on a journal of large diameter greater than would be required if the journal was smaller?_