ftTRJBSES or RARIFIED GASES 



a much )a aoearate approximation to the actual distribution of velocities in 

 the gM dote to the surface than it is in the interior of the gas. We must, 

 therafatL consider the surface conditions at which we arrive in this way as 

 liable If important corrections when we shall have discovered more powerful 



i<xis of attacking the problem. 



For the present, however, we consider only terms of three dimensions or 

 we find 



)- 



Substituting these expressions in equation (63), and neglecting a 1 in com 

 pmrison with unity, we find 



(2-S)pR0afi+f(2ir)-*pR0a'P + 2f(2ir)-* (1 + $a) W/>u = ...(66). 

 If we write 



and substitute for o^S and a'/8 their values as given in equations (54) and (51), 

 and divide by 2(/>p) i , equation (66) becomes 



, , _^-L~ u = ..(68). 



\dx 2 p0 dxdyj 4 pd dy 



If there is no inequality of temperature, this equation is reduced to 



~dv 



If, therefore, the gas at a finite distance from the surface is moving 

 parallel to the surface, the gas in contact with the surface will be sliding over 

 it with the finite velocity v, and the motion of the gas will be very nearly 

 the same as if the stratum of depth O had been removed from the solid and 

 filled with the gas, there being now no slipping between the new surface of 

 the solid and the gas in contact with it. 



The coefficient G was introduced by Helmholtz and Piotrowski under the 

 of Gleitungs-coefficient, or coefficient of slipping. The dimensions of G are 



