_Answer._ If the co-efficient of friction in the two cases is the same, undoubtedly the large journal will require the greatest expenditure of energy to turn it, because its periphery moves further than that of the small one; but the advantage attributed to large journals is that they can be lubricated more perfectly, because their surfaces being larger the pressure is not so great per square inch, and thus the gain from the reduction of the co-efficient of friction is greater than the loss attributable to the increase of the diameter of the journal. Thus if a car journal is 3¹⁄₄ inches in diameter × 5¹⁄₂ inches long, the available surface exposed to friction is equal to that of a longitudinal section of the journal, or 3¹⁄₄ × 5¹⁄₂ = 17.875 square inches.[85] Supposing now that the journal is loaded with 5,000 pounds, and the average co-efficient of friction is 0.085. In one revolution of the wheel the journal will move 0.85 of a foot, and therefore 5,000 × .085 = 361¹⁄₄ foot-pounds of work. If now the journal is made, as has been proposed, 3³⁄₄ × 7 inches, then its effective surface will be equal to 26¹⁄₄ square inches, but the journal will move 0.98 of a foot in one revolution. If, however, the lubrication is improved by the increased area of the journal so that the co-efficient of friction is reduced from 0.085 to 0.07, then the energy consumed in one revolution will be equal to 5,000 × 0.7 × .98 = 343 foot-pounds, or less than was consumed with the small journals. The co-efficient of friction is assumed, and could only be determined by experiment, but the assumption shows how the resistance of the large journals may be less than that of the small ones. Of course it would be better to give the increased bearing surface by adding to the length of the journal, but nearly all locomotives and car journals must be increased in diameter as well as in length when they are enlarged, in order to have the requisite strength to carry the loads they must bear. [85] The reason for this is that the effective surface of the journal _A_, fig. 209. which resists the pressure of the bearing, is equivalent only to the horizontal area represented by the dotted line _a. b._ just as the surface which resists the pressure inside of a boiler is equivalent to the diameter multiplied by its length, as was explained in answer to Question 99. QUESTION 367. _Is the law that_ FRICTION IS IN PROPORTION TO THE PRESSURE ON EACH OTHER BY THE SURFACES OF CONTACT _true under all circumstances?_ _Answer._ No; there is a limit to the exactness of the above law, when the pressure becomes so intense as to crush or grind the parts of the bodies at and near their surfaces of contact. At and beyond that limit the friction increases more rapidly than the pressure;[86] and the friction then becomes very irregular. [86] Rankine. QUESTION 368. _In what cases is the limit referred to probably reached?_