Recent Studies

the Council of this Institution tell him that he may talk about anything he likes provided it is of interest to mechanical engi-neers. It is clear from the list of past lectures that mechanical engineers are a sort of men whose interests range wide and high, from the machinery of the earth by Professor Gregory to the machinery of the stars by Sir Arthur Eddington, from atomic physics by Lord Rutherford to muscles and nerves by Pro-fessor Hill. My task this evening is to discuss a very simple physical process-the sliding of one solid over another. What is the physics of that process? What is happening at the surface during sliding? It would be difficult to formulate a simpler problem and, to make the task easier still, I propose to talk mainly about some of the recent studies which my colleagues and I have been making. Apart from the general mechanism of the sliding process I would like to discuss its significance in a few selected practical affairs. Some of this work is very recent and has not yet been prepared for publication so I ask your forgiveness for not presenting a smooth and rounded discourse. The first thing one observes of course if one attempts to slide one solid over another is that there is a resistance to the motion, which is called friction. Experiment has shown that this frictional resistance obeys two well-known laws. The first of them says that the friction is independent of the size of the surfaces in contact, and the second that it is proportional to the load. The first formulation of these laws that I have seen is contained in the notebooks of Leonard0 da Vinci (1452-1519)t where he writes ‘The friction made by the same weight will be of equal resistance although the contact may be of different breadths or lengths’, and ‘Friction produces double the amount of effort if the weight be doubled’. Some 200 years later, in 1699, they were reformulated and verified experimentally by the French engineer, Amontons. Many will be familiar with the elegan