130 Professor H, S. Hele Shaw [April 29, 



WEEKLY EVENING MEETING, 

 T^riday, April 29, 1887. 



Heney Pollock, Esq. Treasurer and Vice-President, 

 in the Chair. 



Professor H. S. Hele Shaw. 



The Boiling Contact of Bodies. 



When two solid bodies roll upon each other, points in the surface of 

 one successively come into contact with corresponding points in the 

 surface of the other in a way which differs essentially from that which 

 occurs in sliding contact, and it is the nature of this rolling-contact 

 that the lecturer proposed to discuss in an experimental manner. 



In the first place, it is well to understand clearly the nature of 

 the relative motion of the two points which come into contact when 

 the surfaces are such that no appreciable distortion of them takes 

 place, and for this purpose one of the two bodies must be at rest. 

 These may respectively be taken as the plane surface of the ground 

 and a circular disk rolling upon it. An approximate representation 

 of this motion is given by the end of the spokes of a wheel without 

 its tyre. In this case it is seen that a point of the rolling body, 

 when it is just coming into contact with the fixed surface, does so in 

 a direction at right angles to the surface at rest, and also leaves it in 

 the same direction. This action is very similar in kind to tbat which 

 occurs with the continuous circle formed by the tyre. The path of a 

 point in the rim can be drawn in a way visible to the audience by 

 means of a piece of apparatus consisting of two circular glass plates 

 held together by a hollow brass spindle in which slides an arm carry- 

 in*^ a brush. The brush traces the well-known cycloid, of which the 

 only portion now to be considered is that where it directly approaches 

 the surface beneath. This part is perpendicular to that surface, and 

 when epicycloids are drawn, by rolling the disk upon the arc of a 

 circle, the same fact is brought out. 



One body may, however, not merely roll upon another, and a 

 normal pressure be exerted, but they may exert a tangential force 

 upon each other. It is convenient to keep these two cases separate ; 

 exam^Dles of them being respectively the wheels of a railway carriage 

 and those of the locomotive which draws it along. It is to be noted 

 that the object in the former case is to permit one body to move 

 relatively to another without permitting sliding contact of their sur- 

 faces, whilst, in the latter case, in addition to this, the object is to 

 obtain such motion. There are, however, many cases in which it is 

 merely the motion of a body about one point which is required, such 



