74 VISCOSITY OF GASES 173 



already been recognised by Newton as a kind of pressure 

 which acts on the limiting faces of the two layers. But by 

 this we must not think of a pressure in the hydrostatic 

 sense, which acts equally in all directions, and whose action 

 on a surface is always directed normally to that surface ; just 

 as the elastic forces in a body depend as to their magnitude 

 and direction upon its changes of shape, and therefore on 

 the shifting of its particles, so must the frictional pressure 

 depend as to its magnitude and direction on the magnitude 

 and direction of the motions that take place. Its value, 

 therefore, may be different according to the position of the 

 surface of pressure or friction in respect of the direction of 

 the motion. 



The way in which the magnitude of this pressure de- 

 pends on the velocity that is present cannot be completely 

 determined by theoretical considerations. There is nothing 

 else, therefore, for us but to make up for our defective know- 

 ledge by a hypothesis, and afterwards to make trial of its 

 soundness by comparing the conclusions drawn from it with 

 the results of experiment. 



Only one hypothesis has been recognised as certain, viz. 

 that the amount of the friction exerted between two layers 

 can depend only on their relative motion, and therefore only 

 on the difference between their velocities ; for if both layers 

 move with the same speed in the same direction, neither 

 gives to the other, and no friction occurs. Starting from 

 this consideration, Newton has put forward the hypothesis 

 that the internal friction is directly proportional to the 

 difference in velocity of neighbouring layers, so that, for 

 instance, a doubling of the friction results from a doubling 

 of both velocities. 



The internal friction must further depend on the nature 

 of the medium, so that a more viscous fluid experiences a 

 greater friction and a thinner fluid a less friction. Such a 

 dependence on the matter may be expressed by a numerical 

 factor by which the mathematical expression of the frictional 

 pressure is multiplied. This numerical factor which gives 

 a numerical measure of the viscosity or the friction of the 

 fluid is called the coefficient of friction or of viscosity. 



