Prof. 0. Reynolds on Rolling-Friction. 75 



The result of the series of experiments made was to obtain for 

 to the value 



cj = (10— 7 )4:-49 eenthnetre-gramme-seconds. 



Its dimensions obviously are the reciprocal of those of current, 

 viz. 



[»[L-*M-iT]. 



If we put our result in a slightly different form we may say that, 



If plane polarized light passes through distilled water, and the 

 magnetic potential of the water at any two points in the path of 

 the ray differs by unity, then the plane of polarization will be 

 rotated between those points 4| ten-millionths of a unit of circular 

 measure. 



Cavendish Laboratory, Cambridge, 

 April 30, 1875. 



" On Boiling-Friction." By Professor Osborne Beynolds. 

 The motion of a roller or wheel on a surface is always attended 

 with resistance. Coulomb made some experiments with wooden 

 rollers on a wooden plane, from which he deduced two laws, viz. 

 that the resistance is proportional to the weight of the roller, and 

 inversely proportional to its diameter. These laws have since been 

 found to apply to other substances, a different coefficient being used 

 in each case. Beyond this, however, nothing appears hitherto to 

 have been ascertained as regards the nature of this resistance to 

 rolling. The source from which it springs does not appear to have 

 been made the subject of investigation. 



Some time ago it occurred to the author that it was probable 

 that the deformation of the surface of the roller and of the plane, 

 which must take place at the point of contact, would affect the 

 distance which the roller would advance in turning through a 

 certain angle *. The pressure of the roller on the plane causes a 

 certain temporary indentation and lateral extension in the latter, 

 so that in passing from one point to another the roller does in 

 truth pass over a greater extent of surface than the distance between 

 these points. A simple experiment was sufficient to verify the 

 truth of this conclusion. An iron roller 18 inches in circumference 

 was found to roll through something like f inch less than a yard 

 in two complete revolutions when rolliug on a plate of india-rubber. 

 The softness of the india-rubber suffered the roller to indent it 

 considerably j and hence it might be expected that the effect would 

 be much more apparent than when the roller was rolling on iron 

 or any hard material. At the same time there is doubtless a certain 

 amount of indentation in this latter case ; and this will probably 

 cause a similar alteration in the distance rolled through, although 

 too small to allow its being measured. 



This falling off from what may be called the geometrical distance, 

 suggested an explanation of the resistance to rolling, namely, that 

 the extension of the surface or surfaces at the point of contact 

 * The Engineer, 27th Nov., 1874. 



