192 



Prof. 0. Reynolds. 



[Feb. 11, 



may well be a matter of surprise that any general laws should have 

 for so long escaped detection. 



Besides the general experience obtained, the friction of lubricated 

 surfaces has been the subject of much experimental investigation by 

 able and careful experimenters ; but although in many cases empirical 

 laws have been propounded, these fail for the most part to agree with 

 oach other and with the more general experience. 



The most recent investigation is that of Mr. Tower, undertaken at 

 the instance of the Institute of Mechanical Engineers. Mr. Tower's 

 first report was published in November, 1883, and his second in 1884 

 ("Proc. Inst. Mechanical Engineers"). 



In these reports Mr. Tower, making no attempt to formulate, states 

 the results of experiments apparently conducted with extreme care, 

 and under very various and well-chosen circumstances. Those of the 

 results which were obtained under the ordinary conditions of lubrica- 

 tion so far agree with the results of previous investigators as to show 

 a want of any regularity. But one of the causes of this want of 

 regularity, viz., irregularity in the supply of lubricant, appears to 

 have occurred to Mr. Tower early in his investigation, and led him to 

 include amongst his experiments the unusual circumstance of surfaces 

 completely immersed in an oil-bath. This was very fortunate, for 

 not only do the results so obtained show a great degree of regularity, 

 but while making these experiments he was accidentally led to 

 observe a phenomenon which, taken with the results of his experi- 

 ments, amounts to a crucial proof that in these experiments with the 

 oil-bath the surfaces were completely and continuously separated by a 

 film of oil ; this film being maintained by the motion of the 

 journal, although the pressure of the oil at the crown of the bearing 

 was shown by actual measurement to be as much as 625 lbs. per 

 square inch above the pressure in the oil-bath. 



These results with the oil-bath are very important, notwithstanding 

 that the condition is not common in practice. They show that with 

 perfect lubrication a definite law of the variation of the friction with 

 the pressure and the velocity holds for a particular journal and brass. 

 This strongly implies that the irregularity previously found was due 

 to imperfect lubrication. Mr. Tower has brought this out. Substi- 

 tuting for the bath an oily pad of tow pressed against the free part 

 of the journal, and making it so slightly greasy that it was barely 

 perceptible to the touch, he again found considerable regularity in 

 the results. These were, however, very different from those with 

 the bath. Then with intermediate lubrication he obtained intermediate 

 results, of which he says : " Indeed, the results, generally speaking, 

 were so uncertain and irregular that they may be summed up in a 

 few words. The friction depends on the quantity and uniform dis- 

 tribution of the oil, and may be anything between the oil-bath results 



