54 THE DYNAMICAL THEORY OF OASES. 



If the plane moves with the mean velocity u of the fluid, the total force 

 exerted on the medium on the positive side by the projection of molecules into 

 it from the negative side will be 



a normal pressure *p in the direction of x, 



a tangential pressure rjp in the direction of y, 



and a tangential pressure gp in the direction of z. 



If X, Y, Z are the components of the pressure on unit of area of a plane 

 whose direction cosines are /, m, n, 



(63). 

 Z= 



When a gas is not in a state of violent motion the pressures in all direc- 

 tions are nearly equal, in which case, if we put 



ep + ^P + Cp = 3p .............................. (64), ^ 



the quantity p will represent the mean pressure at a given point, and ^p, tfp, 

 and f p will differ from p only by small quantities ; i)p, p, and r)p will then 

 be also small quantities with respect to p, 



Energy in the Medium Actual Heat. 



The actual energy of any molecule depends partly on the velocity of its 

 centre of gravity, and partly on its rotation or other internal motion with respect 

 to the centre of gravity. It may be written 



lM{( + ff + (v + tf + (w + Q} + lEM .................. (65), 



where \EM is the internal part of the energy of the molecule, the form of 

 which is at present unknown. Summing for all the molecules in unit of volume, 

 the energy is 



$(u' + v' + iv')p + t(e + r ) > + )p + tEp ..................... (66). 



The first term gives the energy due to the motion of translation of the 

 medium in mass, the second that due to the agitation of the centres of gravity 

 of the molecules, and the third that due to the internal motion of the parts of 

 each molecule. 



